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Archive-name: computer-lang/java/programmers/faq
Posting-Frequency: weekly
Last-modified: 1999/05/19
URL: http://www.afu.com/javafaq.html
Frequently Asked Questions (with answers) for Java programmers
_____________________________________________________
________| |________
\ | Java Programmers FAQ http://www.afu.com | /
\ | Last modified May 19, 1999 Peter van der Linden | /
/ |_____________________________________________________| \
/___________) (__________\
The Java FAQs here are intended for people who already have some programming
experience, though maybe not in Java.
Go to the FAQ home page at http://www.afu.com for other Java information and
downloads, and the most up-to-date copy of the FAQ. Report FAQ updates to
faqidea at the address afu.com.
------------------------------------------------------------------------
The sections of the Java FAQ are:
* 1. Java Book Information
* 2. Getting Started
* 2.5 Portability
* 3. General Information
* 4. Compilers and Tools
* 5. Compiler Messages
* 6. Java Language issues
* 7. I/O
* 8. Core library issues
* 9. Computer Dating
* 10. AWT
* 11. Swing
* 12. Browsers
* 13. Applets
* 14. Multi-Media
* 15. Networking
* 16. Security
* 17. For C, C++ Afficionados
* 18. Java Idioms
* 19. Java Gotcha's
* 20. Further Resources
* 21. Acknowledgements
-------------------------------
1. Java Book Information
1. (sect. 1) How do I choose a Java book?
[*] There is no one right answer to "which is the right Java book for
me?"
A lot depends on what you already know, and how you like to
learn.
If you already know how to program in another language, and Just want
to learn Java, consider Just Java 1.2 from the FAQ author.
Everything in one handy volume (language basics, Swing, networking,
I/O, database access, etc) and it comes with a CD with tons of Java
applets, games, applications, compiler tools, complete with source.
Includes a Java compiler for Windows, the Mac, Linux, and Solaris
(sparc and x86). look at http://www.amazon.com for details
If you or your manager instead want a briefing on Java
technology, thin clients, XML, CORBA, TCP/IP, Java beans,
etc., take a look at Not Just Java from the FAQ author.
[Image]
This book doesn't teach you how to program in Java; it tells
you why you might want to, and what kind of systems are most
suited to Java. It describes E-Commerce and XML. look at
http://www.amazon.com for details
Here are the points to check when evaluating a programming book.
o Above all, make sure that it is a Java book. If it comes with a
CD, check that it has a Java compiler on it, not a J++ compiler.
VJ++ does not implement Java 2. VJ++ is also a different language
in some important ways, and is missing the latest Java libraries
(Swing, Collections, JFC, RMI). If your interest is Java, leave
the downrev VJ++ book back on the shelf.
o Does the book cover the current level of Java, which is Java 1.2
(aka Java 2)? Look up "JApplet" in the index. If it's not there
the book doesn't cover JDK 1.2, and you probably need a more
up-to-date book.
o Check that the book has a reasonable number of figures, diagrams,
and illustrations. It is not possible to explain how to program a
window system without pictures and diagrams. Other topics benefit
from pictures, too.
o Check what the book says about itself. Is it a reference work,
intended for Java-experts to look things up in? This is the role
of "Java in a Nutshell", and "The Java Almanac". Do you need that,
or are you looking for a book that teaches by examples and
explanations?
o Read Peter Norvig's excellent advice on learning programming
languages and being a programmer.
o Appraise your own level of programming knowledge: are you
proficient in some other language, or are you learning programming
as well? Does the book cater to your level?
o Read a section of the book. Does the style keep you interested, as
it educates you? Will you get bored if you read many pages? Is the
book too long for your initial purpose? Browse Amazon online and
see what other readers say about the text.
o If the book comes with a CD, how much other software is on the CD?
You want at least a Java compiler plus all the examples from the
book. Does the Java compiler work on your platform (Mac, Linux,
etc)? Additional software on the CD is a big plus, as we learn the
most from reading other people's code.
Most people buy one book to begin with, then four or five more as they
wish to learn more, and about more specialized topics. The FAQ author
has purchased and read probably 60 Java books in the last three years.
2. (sect. 1) Where can I find a some lists of Java books and book reviews?
[*] Here are some good ones:
http://www.hszk.bme.hu/~werner/Java2BookReviewBETA.html
and
http://www.geocities.com/RainForest/Canopy/4774/Java/education.html
also
http://www.flathill.com/languages/java/
also
http://www.fastgraph.com/books/java.html
also
http://teamjava.com/links/tj-srv.cgi?MUF=0,tj-booklist.muf
also
http://www.javaworld.com/javaworld/books/jw-books-index.html
(an exhaustive list -- takes a long time to load).
-------------------------------
2. Getting Started
1. (Sect. 2) What is the easiest way to get started with Java?
[*] Follow these steps.
1. Look at the books section of the FAQ to see what kind of book will
suit you. There is no one perfect Java book. The right book
depends on the style, pace, and detail that you are comfortable
with. Amazon has good info and reviews on Java books.
2. Download a free Java compiler from http://java.sun.com Service
packs for Solaris (if any are needed) can be found at:
http://www.sun.com/solaris/jdk/download.1.2.1_02/en/sparc/1.2.1_02_sparc_jdk_patches.tar
3. Read the free Java tutorial, at
http://java.sun.com/docs/books/tutorial/index.html
(bookmark it, so you will easily find it again).
4. Avoid Microsoft's J++ product, which is in the words of
Microsoft's own employees "polluted Java". It is designed to
undermine standard Java, and has many deliberate platform-specific
incompatibilities, including new keywords in the language.
5. To get some tips on getting started with a programming homework
assignment, look at http://www.concentric.net/~pats/beginner.html.
6. Search this FAQ when something in Java confuses you. Many people
have trodden this path before you, and the FAQ contains the
accumulated knowledge and pointers to other references.
2. (Sect. 2) Why doesn't my "Hello World!" program compile?
[*] There are three basic possibilities:
1. Are you successfully running the javac compiler?
Try
javac -garbage
to see if it prints out a message about correct usage. If not,
invoke javac using the full pathname, or set your PATH variable to
include the directory that contains javac.
2. Is the CLASSPATH environment variable used correctly?
In JDK 1.0.2, beginners had to set CLASSPATH, and it had to
include both system libraries and your programs.
In JDK 1.2, CLASSPATH is no longer needed for system libraries.
You do want CLASSPATH to point to the directories containing any
"user classes" you're using.
Getting started, you will probably want at a minimum "." (the
current directory) in your CLASSPATH.
When CLASSPATH is wrong, javac will tell you it can't find
definitions for classes you reference in the source file. For
information on setting up the CLASSPATH, see Question 1.3
3. Is the source correct?
Here javac will emit error and warning messages. See the questions
on compiler messages in the next section.
3. (Sect. 2) Why doesn't my "Hello World!" program run?
[*] There are five common mistakes that cause your VM (java or browser)
to be unable to execute your classes
1. First, did you write an applet or an application? If an applet,
you must make sure that you did extend the java.applet.Applet
class. Your starting code should be in the init routine.
If you wrote an application, your starting code should be in a
routine called main(). Don't mix applets with applications until
you have a bit more experience.
2. You must declare your main class as "public". If you don't,
unfortunately some systems will still run the code, while others
won't. The main class is either the one with the main() method in,
or in the case of an Applet, the class that extends Applet.
3. Your class name and the file name must match exactly, even letter
case. If your class is HelloWorld, your source file must be
HelloWorld.java and your class file will be "HelloWorld.class".
4. If an Applet, and you used ftp to transfer the classes to the
server, you must ftp all the classes, and you must use BINARY
transfer not ASCII.
5. Errors in setting the CLASSPATH (and/or codebase in an applet).
Even seasoned programmers do this, pointing inside a package or
mistyping a path delimiter. For information on setting up the
CLASSPATH and codebase, see Question 1.3
If you are running an applet, you should check the following further
points:
1. If your class isn't loading, recheck the HTML applet tag.
2. If you are writing to System.out, the results are displayed in the
browser's java console. You'll have to create a window if you want
one.
3. Make sure your browser is compatible with the Java language
features you are using. Internet Explorer and older versions of
Netscape's browsers have omitted some support for JDK 1.1. Try
your applet in the JDK's appletviewer first.
4. How do I set the CLASSPATH?
[*] The CLASSPATH environment variable tells the VM's class loader
where to find classes that are directly or indirectly invoked,
including system classes. The CLASSPATH variable should
o point to the directory containing the class file, for classes not
in a package.
o point to the package root, for classes in a package. The root is
the parent directory of the highest directory of the package name.
o point directly to the zip or jar file, if the classes are in an
archive file. You may have to list the contents of the archive to
get the correct package/path name for the class.
Separate multiple paths and archives with a platform-specific
separator, ";" for Windows; ":" for Solaris
Also remember that
o Browsers set the CLASSPATH to the directory of the HTML file, plus
the codebase parameter.
o in JDK 1.1 and after, java adds the system classes
(lib/classes.zip), so you don't have to.
o JDK 1.2 versions of java add "." (current directory), so you don't
have to. (But jre doesn't - see below.)
o JDK 1.1 jre tool does not use the CLASSPATH variable or assume the
current directory. (On Solaris, CLASSPATH does work.)
From JDK 1.1.2 on, it is generally an error if the user sets the
CLASSPATH to include classes.zip. But CLASSPATH will need to be set to
o point to the roots of the programmer's own packages, and third
party packages
o use rmic
o use unbundled packages like Swing in JDK 1.1
o point to native code libraries.
For the sake of consistency, the minimal classpath setting should be: "
set CLASSPATH=. "
Below you'll find examples for Windows (basic application class),
Solaris (package class), javac (multiple packages), and browsers
(applet codebase).
-----------------------------
Here's some Windows examples, assuming the application class is
D:\src\tries\HelloWorld.class
## JDK 1.1, no CLASSPATH set
> cd D:\src\tries\
> D:\jdk11\bin\java HelloWorld
# OK: 1.1 implicitly adds classes.zip and current dir
> D:\jdk11\bin\jre HelloWorld
# FAILS: jre does not automatically add . to CLASSPATH
> cd D:\
> D:\jdk11\bin\jre -cp D:\src\tries HelloWorld
# OK: jre adds classes.zip, -cp adds class directory
## JDK 1.1, CLASSPATH set
> set CLASSPATH=D:\src\tries
> D:\jdk11\bin\java HelloWorld
# OK: java using CLASSPATH
> D:\jdk11\bin\jre HelloWorld
# FAILS: jre does not use CLASSPATH (on Windows)
## JDK 1.0.2, CLASSPATH set
> set CLASSPATH=D:\jdk102\lib\classes.zip;D:\src\tries
> D:\jdk102\bin\java HelloWorld
# OK:
> set CLASSPATH=D:\jdk102\lib\classes.zip;D:\src\tries
> D:\jdk11\bin\java HelloWorld
# FAILS: exception in thread NULL - wrong system classes
-----------------------------
Here's some Solaris examples, assuming the application class is
/usr/src/com/devjoes/killer/App.class
and it is in package com.devjoes.killer:
# JDK 1.1, no CLASSPATH set
$ /usr/bin/jdk11/bin/jre -cp /usr/src com.devjoes.killer.App
# OK:
$ cd /usr/src/com/devjoes/killer/
$ /usr/bin/jdk11/bin/java App
# fails: class name and path are wrong
$ CLASSPATH=/usr/src/
$ /usr/bin/jdk11/bin/java App
# fails: class name is com.devjoes.killer.App
$ /usr/bin/jdk11/bin/java com.devjoes.killer.App
# OK:
-----------------------------
Here's some javac examples, for both Solaris and Windows, based on the
following:
Source files package Makes the call
/usr/src/pack/Minimal.java package pack pack.sub.Try.run()
/usr/src/pack/sub/Try.java package pack.sub (nothing)
$ CLASSPATH=""
$ /usr/bin/jdk11/bin/javac /usr/src/pack/sub/Try.java
# OK: works fine
$ /usr/bin/jdk11/bin/javac /usr/src/pack/Minimal.java
# FAILS: can't find pack.sub.Try
$ cd /usr/src
$ /usr/bin/jdk10/bin/javac pack/Minimal.java
# OK: finds pack.sub.Try based on . as package root
$ cd /usr/src/pack
$ CLASSPATH=/usr/src
$ /usr/bin/jdk11/bin/javac Minimal.java
# OK: finds pack.sub.Try based on CLASSPATH
Now assume the killer application class
/usr/src/com/devjoes/killer/FastApp.java
(in package com.devjoes.killer) uses a third-party package in a jar
file
/usr/jars/JShapes.jar
but makes no other reference to other classes. The following works
fine:
$ CLASSPATH=/usr/jars/JShapes.jar
$ cd /usr/src/com/devjoes
$ /usr/bin/jdk11/bin/javac killer/FastApp.java
Finally, some applet examples. Many applets only use one class, in the
same directory as the html file:
<applet code=ArcTest.class height=400 width=400>
To use classes in subdirectory, use the codebase parameter:
<applet codebase="mysubdir/" code=ArcTest.class ..
To use classes in an archive, use the archive parameter:
<applet archive="applets.jar" code=ArcTest.class ..
See also: JDK 1.1 ReadMe
Solaris JDK 1.1 tool documentation
Win32 JDK 1.1 tool documentation
5. (Sect. 2) How do I do keyboard (interactive) I/O in Java?
[*] Interactive I/O in Java is very poorly supported. Programmers must
piece together several library classes in non-obvious ways to get the
required functionality. See the answer to Question 7.1.
6. (Sect. 2) How do I do file I/O in an applet?
[*] By default, an applet can read files on the server, but not write
them, and has no access to the client. This is for reasons of security.
It would be very unsafe to let any old applet that you downloaded from
an unknown origin on the Internet read/write your files. It would be as
unwise as allowing this kind of access to an ActiveX control (which is
one reason ActiveX is dead on the Internet).
There are several different ways to relax the default rules. See the
answer to Question 7.8.
7. (Sect. 2) How do I do I/O to the serial port on my computer?
[*] Java 1.0 and 1.1 do not have a serial port API. There are several
commercially-available libraries that supply the needed functionality.
JDK 1.2 introduces access to the serial and parallel ports as an
extension (optional extra) library. See also the answer to Question
6.3.
8. (Sect. 2) How do I do formatted I/O like printf and scanf in C/C++?
[*] The java.text package introduced with Java 1.1 supports formatted
I/O. See also the answer to questions 7.11, 7.12, and 17.7.
9. (Sect. 2) I have spent more debugging time finding case (upper vs
lower) typos than everything else put together and squared!
[*] Do not forget that your remark must be phrased in the form of a
question to win on FAQ Jeopardy. In any event, it is worth reminding
those new to Java that letter case really matters in Java, and that the
names of public classes should exactly match (including case) the names
of the files they live in. See also the answer to Question 1.1.2
10. (Sect. 2) Why do I get this compiler error: "Can't make static
reference to method..."?
[*] Your code probably looks something like this:
class myclass {
public static void main(String args[]) {
myMethod();
}
public void myMethod() {
//some code
}
}
The issue is this: a static method means it belongs to the class, not
each individual object. If you leave the static keyword off (the usual
case) as is done here with the method "myMethod()", it means that
method can only be invoked on an object. But your call from main() has
not told myMethod() which object it is to be invoked on. Inside a
non-static method, you don't have to provide this information, as it
assumes you mean the same object on which it was invoked. But when
calling from a static method, you must provide the information, and you
haven't - hence the error message.
A common fix is to instantiate a member of the class, on which to
invoke myMethod(), like this:
public static void main(String args[]) {
myclass m = new myclass();
m.myMethod();
}
This problem is especially common when you are writing code that you
want to run as an applet and as an application. Naturally, you call
init() and start() from main. What you really need to do is:
public static void main(String[] args) {
Applet ma = new myApplet();
ma.init();
ma.start();
}
11. (Sect. 2) Why can't I do myArray.length() ? Arrays are just objects,
right?
[*] Yes, the Java specification says that arrays are object references
[JLS 10.2] just like classes are. However, arrays cannot contain
methods. Instead you have to use myArray.length, which is a data item
(not a method) called "length", belonging to myArray.
12. (Sect. 2) How do I close a Java window by using the icon in the upper
right hand corner of a window?
[*] Create an event handler class to extend WindowAdapter. Then
override WindowAdapter's windowClosing() to do the actions you want
when a window's "close" action is clicked. Then add that to the
listeners for that window.
import java.awt.*;
import java.awt.event.*;
public class MyFrame extends Frame {
public MyFrame(String s) {super(s);}
public class WL extends WindowAdapter {
public void windowClosing(WindowEvent e) {System.exit(0);}
}
// do your other Frame stuff
}
Somewhere in your initialization code, put:
f1.addWindowListener( f1. new WL() );
This last syntax is not commonly known to many people yet, it's another
wacky artifact of inner classes.
Alternatively, combining the inner class and setting the handler in one
go, you could do this:
MyFrame f1 = new f("wave");
f1.addWindowListener( new WindowAdapter() {
public void windowClosing(WindowEvent e) {
// and/or setVisible(false) and/or dispose()
System.exit(0); }
});
See also the answer to questions 1.0.19, 1.0.30 and 15.7.
13. (Sect. 2) Why is b+=100; OK, but b = b+100; fails to compile?
[*] You have code like this
byte b = 0;
Incompatible type for =. Explicit cast needed to convert int to byte.
b = b + 100; // compiler error message
^
b += 100; // works OK
The reason is "promotion". Arithmetic expressions are promoted to the
type of the longest, floatiest operand in them, or at least to int. The
first statement involves the assignment of an expression. The
expression is promoted to 32 bits, and must be cast back down to 8
bits, like this "b = (byte) (b+100);". The second is an operator
assignment, and the cast back to byte takes place automatically. The
Java Specification says:
"A compound assignment expression of the form E1 op= E2 is
equivalent to E1 = (typecast)((E1) op (E2)), where "typecast" is
the type of E1, except that E1 is evaluated only once" [JLS
15.25.2]
The compile-time narrowing of constants means that code such as:
byte theAnswer = 42;
is allowed, with no cast necessary. [JLS 5.2]
14. (Sect. 2) How do I add two Float objects together?
[*] You want to write code like this
Float One;
Float Two;
Float Hard = One + Two;
but the compiler does not allow it.
Java has two separate ways of representing a 32 bit floating point
number, Float and float. Float is a class, that whose sole purpose is
to "wrap" a floating point number so it can be treated as an object.
The class does not support floating point arithmetic, because the
performance would be too slow. float is a primitive type (like int)
that is used for floating point arithmetic.
You choose one or the other depending on your predominant use. If all
you need of your floating point numbers is arithmetic, declare them to
be "float". If you need to use them as objects, for example to place
them in a Vector, declare them as "Float".
If you need both, tough. You have to declare them one way and convert
whenever you need the capabilities of the other. Your specific code can
be written as:
Float One = new Float(1.0);
Float Two = new Float(2.0);
Float Hard = new Float(One.floatValue() + Two.floatValue());
See also questions 6.n , 5.1, and 10.1.
15. (Sect. 2) How can I put all my classes and resources into one file and
have java run it?
[*] Use a JAR file. Put all the files in a JAR, then run the app like
this:
java -jar [-options] jarfile Mainclass [args...]
16. (Sect. 2) How can I see line numbers in a stack trace using JDK 1.1.6?
[*] From JDK 1.1.5 on (i.e. JDK 1.1.6, JDK 1.2) the stack trace of an
uncaught exception no longer has source code line numbers. It only says
"(Compiled Code)".
To see the line numbers where your program throws an exception, use the
command line argument in JDK 1.2:
java -Djava.compiler=NONE myapp
This info is well-concealed in the release note at:
http://java.sun.com/products/jdk/1.2/changes.html#aaa24
-----------------------------
Specify standard Java on your new PC!
Your new PC can come with the most up-to-date standard version
of Java, but only if you ask for it! The JavaLobby is asking PC
vendors to support Java, and to ship new machines with the Java Plug-In
pre-installed.
See http://www.javalobby.org/servlet/PetitionServlet/pjpc
Please help the Java Lobby to promote this initiative.
-----------------------------------------------------------------------
Please support Java Portability.
The biggest value of Java is its portability.
o Portability makes it easier for companies to change/upgrade
operating systems and platforms, without losing their investment
in applications software.
o Portability makes it easier for Java programmers to transfer their
skills to new employers.
o Portability makes a wider variety of software available on all
computers.
Software portability is very much in the interest of most software
developers and customers. Even if you only use Windows 95, portability
matters to you. If your software was all written in Java, it would all
just run when you moved from MS-DOS to Windows 3.1 to Windows 95 to
Windows 98 to Windows NT, and even on Windows CE. Instead, you
typically need to buy new applications software all over again when
Windows changes. Portability is not in Microsoft's interest, as it
removes a revenue stream and makes it easy for users to try other
operating systems.
The 1998 anti-monopoly case against Microsoft revealed a Microsoft
internal memo. The memo revealed that Microsoft's "strategic objective"
was to "kill cross-platform Java." by "grow[ing] the polluted Java
market." This is Exhibit 101 (MS7 033448) in the case.
In November 1998, a Federal judge ruled that Microsoft was probably
breaking its written agreement with Sun by distributing incompatible
Java, and that Microsoft had to stop doing that. If portability matters
to you or your users, avoid Java products from Microsoft; it is
deliberately trying to sabotage it. Microsoft's own internal documents
make this goal unambiguous. See
http://www.usdoj.gov/atr/cases/f1700/1762.htm.
See also "The Microsoft Java Dilemma" paper at
http://www.geocities.com/ResearchTriangle/Node/2005/msjava.htm
-----------------------------
Windows-Specific
17. Is there a Java implementation for Windows 3.1?
[*] Yes. See Question 1.6 It's not that great though because Windows
3.1 has inadequate features to support great software.
18. (Sect. 2) I'm using Win95, and my DOS window won't accept filenames
longer than 8.3.
"This program cannot be run in DOS mode"
[*] Both these problems are resolved by the same process. Assuming
you're running the Win95/98 command.com, then you've changed the MS-DOS
Prompt options under the "Properties" menu item. In the Properties
dialog, Program->Advanced gets you a dialog. Here, make sure the
"Prevent MS-DOS-based programs from detecting Windows" checkbox is
UNCHECKED.
If the option is checked you get exactly the kind of behavior you're
seeing. The option is unchecked by default, so it must have been
selected at some time in the past. Change it back to unchecked.
19. (Sect. 2) I'm using Notepad to edit my files, and how can I save them
with the extension ".java"? Also, in notepad some source files have all
the characters on one line. Why is that?
[*] First answer: put the entire filename in quotes in the save dialog.
Once you have created your first Java file, double click on it in
Explorer, select "Notepad" from the "Open with" box, and Notepad will
stop adding the spurious ".txt" to your .java files.
Second answer: Notepad expects to see a "carriage return/line feed"
pair at the end of each line, rather than just the "newline"
(line-feed) commonly used on Unix. Use this program to expand all
newlines,
/*
* Usage: jre crlf file1.java file2.java ... fileN.java
*/
import java.io.*;
class crlf {
public static void main(String s[]){
byte b[]; byte p;
FileInputStream is;
BufferedOutputStream os;
File f;
for (int i=0; i < s.length;i++){
try{
f=new File(s[i]);
b=new byte[(int)f.length()];
is = new FileInputStream(f);
is.read(b); is.close();
os = new BufferedOutputStream(
new FileOutputStream(s[i]),b.length);
p='?';
for(int j=0; j < b.length; j++){
if((p!='\r')&&(b[j]=='\n')) os.write('\r');
p=b[j]; os.write(p);
}
os.flush(); os.close();
}catch(IOException e){
System.err.println(e.toString());
}
}
}
}
The source code is to show new users a way to make a simple program
which can read a file and write it out buffered.
Compile with "javac crlf.java" and run with
java crlf outfile.txt
or just use Wordpad instead of Notepad. Wordpad is under
Start->Programs->Accessories->WordPad
20. (Sect. 2) How do I fix the message about "out of environment space"?
[*] This occurs under Windows when you have long CLASSPATH names. You
need to increase the environment space. On Windows 95,8 put this in
your c:\windows\system.ini
[NonWindowsApp]
CommandEnvSize=4096
On NT you can right-click on My Computer, select System Properties then
go to the Environment tab and then increase COMSPEC to the value you
want.
The previous suggestion to put this in your config.sys:
shell=command /e:4096
apparently causes you to create two copies of command.com which wastes
memory.
-------------------------------
3. General Information
1. (Sect. 3) Is Java "Open" or "Proprietary"?
[*] The Java specification is publicly available, and anyone is free to
do clean-room implementations of the JVM and core Java API's. Sun
includes a perpetual, irrevocable, free and royalty-free license in the
front of the Addison-Wesley books containing the specification.
Sun also provides free access to the Java source. See
http://java.sun.com/communitysource/
Using the Java trademark does requires licensing from Sun. It is not
clear if the Embedded or Personal Java specifications are open, as it
is not clear if a clean-room implementation may be done without
licensing from Sun.
The relative openness of Java contrasts with systems that are only
available from one vendor, whose interfaces are developed in secret,
without an open process for others to participate, whose owners do not
allow competing implementations of the same API, and whose owners
change the APIs as a strategic weapon against competitors. Typically,
such systems also feature "private" APIs that are published late or not
published at all, to allow the single vendor to gain a competitive
advantage for their other products. Typically such proprietary systems
do not make the source code available for inspection by all.
2. (Sect. 3) What is the best way to refer someone to the FAQ when they
ask a question I know is answered there?
[*] The Java Programmers FAQ (at http://www.afu.com) answers your
question in section N.n. ...
This gives them the answer, and shows them where to go for future
questions. It also demonstrates that the FAQ can answer their
questions, supplying an incentive to go there next time. It's regarded
as elementary politeness to look for the FAQ of a newsgroup and read it
before posting any questions.
In general, FAQs for any newsgroup are available by looking at past
postings in a group, or by searching Deja News (see Q 1.4), or via
anonymous FTP at directories under ftp://rtfm.mit.edu. The pathnames
are called things like
/pub/usenet-by-group/comp.lang.java.programmer/Java_Programmers_FAQ
which may help you get to the right one directly, as it takes some time
to get a directory listing there. Alternatively, you can look for
newsgroup names on the same ftp site by going to the same site and
looking under /pub/usenet-by-hierarchy/. That has subdirectories such
as alt/, ba/, ca/, comp/, and subdirectories under them such as
/pub/usenet-by-hierarchy/comp/lang/ and so on. This helps you explore
the world of newsgroups with FAQs.
If you do not have anonymous FTP access, you can access the
rtfm.mit.edu archives by mail server as well. Send an E-mail message to
[email protected] with "help" in the body for more information.
"RTFM" stands for "Read The effing Manual" - you must expect to put in
some time and effort to master a new area of study.
If you want to look at the definition of Internet standards like FTP,
telnet, visit the IETF site at http://www.ietf.org where all the RFC's
(Request For Comments) can be found.
3. (Sect. 3) What if my question is not answered in this FAQ?
[*] Go to http://www.dejanews.com/home_ps.shtml
In May 1999, Dejanews changed their name to Deja.com, and their web
design to a truly horrible garish interface, covered with ads and
harder to search. Consider using these search sites instead:
http://www.exit109.com/~jeremy/news/deja.html
http://www.dogpile.com
http://www.google.com
http://www.hotbot.com
http://www.mamma.com
o Under "Newsgroups" enter "comp.lang.java.programmer" (or whatever)
o Under "Subject" enter "Frotteur" (or other topic you find
pressing)
o Click "Create Filter"
o It will go to a new document, and you should click the link
labeled
nnn Documents (nnn is some number).
The chances are that you will find several answers to your question.
Some may not be complete or completely accurate however. That is the
nature of Usenet, and free information. If you still don't have an
answer, then post your question on the most appropriate of the
newsgroups. Don't spam the newsgroups by posting to multiple groups.
Knowledgeable posters tend to ignore questions like that.
Also look at http://sunsite.unc.edu/java/cgi-bin/query
and look at http://asknpac.npac.syr.edu/ for a Java newsgroup search.
http://www.javaworld.com/search.html can search the Javaworld
newspaper.
4. (Sect. 3) What Java mailing lists are there?
[*] There are quite a few Java mailing lists.
http://java.miningco.com/msub7.htm has a comprehensive list.
5. (Sect. 3) Where can I look at the definitive Java Language
Specification?
[*] This is available online at:
http://java.sun.com/docs/books/jls/html/index.html (Java 1.0)
and the Java 1.1 inner classes document at:
http://java.sun.com/products/jdk/1.1/docs/guide/innerclasses/
spec/innerclasses.doc.html
and the other Java 1.1 update at:
http://java.sun.com/docs/books/jls/html/1.1Update.html
and the Java API is at:
http://java.sun.com/products/jdk/1.1/docs/api
It is also available as a book in printed form (details at website).
Also see the "Clarifications and Amendments"
http://java.sun.com/docs/books/jls/clarify.html.
You can also see the virtual machine (execution environment)
specification at
http://docs.sun.com:80/ab2/coll.127.1/@Ab2CollToc?subject=java
6. (Sect. 3) Where can I find information about future Java APIs?
[*] JavaSoft has followed a policy of creating new APIs in consultation
with leading industry participants, then posting the draft
specification for public review and comments. Check the JavaSoft
roadmap of new APIs and products at
http://java.sun.com:80/products/api-overview/index.html
Also, some APIs that are under consideration, possibly for JDK 1.2 are
at:
http://java.sun.com/products/jdk/preview/docs/
7. (Sect. 3) I'm looking for a Java style guide on naming conventions.
[*] Check out the section "Naming Conventions" in the language
specification
http://java.sun.com/docs/books/jls/html/6.doc.html#11186
Also take a look at Doug Lea's draft coding standard -
http://gee.cs.oswego.edu/dl/html/javaCodingStd.html
See also naming conventions for some basic rules of thumb.
8. (Sect. 3) How do I check on known bugs in the JDK?
[*] Look at the Java Developer Connection at http://java.sun.com/jdc.
All the Java bugs that Sun knows about are listed there, with the
exception of security-related bugs. The legal department in Sun vetoed
the open publication of security bugs. After you have checked that the
bug is not already listed, you can submit a bug report through:
http://java.sun.com:80/cgi-bin/bugreport.cgi
You should check that the bug doesn't already exist for two reasons:
first, you might find the bug with a workaround listed. Second, you
don't want to waste everyone's time with a duplicate bug report.
You can also send in an RFE (Request For Enhancement) or ease-of-use
issue there. You can even vote on the priority you would assign to a
particular bug fix! Join the Java Developer Connection (it's free) by
going to http://java.sun.com/jdc. Then browse
http://developer.javasoft.com/developer/bugParade/#votes
9. (Sect. 3) What computers have Java ports? Is there a port to Win 3.1?
[*] A partial list of JDK ports is available from
http://java.sun.com/cgi-bin/java-ports.cgi
An (impressive) list of the systems that the GPL Kaffee JVM runs on is
at http://www.transvirtual.com/ports.html
A list of the software updates you should install to run Java on
Solaris is at
http://java.sun.com/products/jdk/1.2/install-solaris-patches.html#2.6
There are several Java ports to Win 3.1. IBM's ADK1.02 is available at
the following locations:
o http://ncc.hursley.ibm.com/javainfo/latest/answers/faq0.html
o http://www.alphaworks.ibm.com/formula
IBM offers a port to Linux, as do others. The IBM Jikes port is at
http://www.alphaworks.ibm.com/ There is a large amount of useful
software there, including a profiling tool called jinsight.
Netscape Navigator for Win3.1 has Java support. Java will never be
well-supported under Win3.1 because Win3.1 lacks the basic features
expected of a modern OS (primarily lengthy filenames and multithreading
support).
Also take a look at JavaSoft's JavaPC kit that can switch a PC into a
thin client Java system (and back to Win3.1/DOS when you want). It's
meant for software OEMs and large corporations running lots of older
PCs, but you can use it on the latest Pentium II too. Details are at
http://java.sun.com/products/javapc/index.html. JavaPC is available now
for $100, runs on 486's with 8Mb or more Unlike the 16-bit versions of
Netscape Navigator and Microsoft Internet Explorer, which provide a
Java Virtual Machine that is only compliant with the JDK 1.0.2 API, the
JavaPC software allows IS managers to deploy JDK 1.1-compatible Java
applications on PCs running DOS and Windows 3.x.
10. (Sect. 3) Where can I find information on Java 3D?
[*] The Java 3D FAQ at http://tintoy.ncsa.uiuc.edu/~srp/java3d/faq.html
may have the answers you're looking for. It contains general
information about Java 3D, as well as programming tips.
11. (Sect. 3) Where can I find information about Java Certification?
[*] Sun is sponsoring an examination which programmers worldwide can
take. Those passing can use the designation "Sun Certified Java
Programmer". There is also a second-level test, involving writing a
program to spec, and taking a further test. That results in the
qualification "Sun Certified Java Developer". You can find out all
about the exam at:
http://www.sun.com/service/suned/
and then search for "sun certified java". It costs $150 to sit the Java
Programmer exam. It is not trivial to pass the Java certification exam.
It requires understanding the objectives of the test, and the material
that is tested for. These are given, along with sample questions, at
the URL mentioned above.
There is a Java certification FAQ at: http://www.marcusgreen.co.uk
12. (Sect. 3) How can I find links to recent news about Java?
[*] This site contains links to late-breaking online news stories about
Java. http://www.intelligence.com/java/
Another good Java news source is http://www.nikos.com/javatoys.
This site is a fine site for programmers who want to be well-informed
about computer industry topics. It has a lot of coverage of Linux as
well as more general news. http://slashdot.org Highly recommended.
This site is a source of independent news and commentary on the
computer industry, including Java. http://www.pjprimer.com/media.html.
You have to subscribe ($10/year, 30 day free trial).
13. (Sect. 3) What are the folks at GNU doing with Java?
[*] First note that the URLs in this section change quickly, and soon
become outdated. If you have an update, send it in. There is a Gnu Java
page at http://www.gnu.org/software/java/java.html
Guava (a GPL'd Java compiler) can be found at
ftp://ftp.yggdrasil.com/pub/dist/devel/compilers/guavac/
Alternatively, it may be available at
http://http.cs.berkeley.edu/~engberg/guavac
Work is progressing on the Cygnus Java frontend to gcc. See
http://www.cygnus.com/product/javalang/
Kaffe (a JVM) can be found at
http://www.transvirtual.com This is Tim Wilkenson's company
devoted to commercializing the Kaffe JVM for the embedded systems
market. He also releases a version of it under the GPL. It also
comes with a the beginnings of a class library and the Pizza
compiler.
Classpath is a free implementation of Sun's Java libraries (v1.1),
being developed for the GNU Project ( http://www.gnu.org). Information
regarding classpath is at http://www.classpath.org They aim to develop
a 100% free drop in replacement for Sun's class libraries, targeting
first the Japhar JVM (below). They are always looking for help, so feel
free to stop by and volunteer.
See also http://www.japhar.org This is the Hungry Programmer's JVM.
Currently it is development grade only.
14. (Sect. 3) What is "San Francisco"?
[*] San Francisco is the code name for a very large Java project led by
IBM, and involving other companies. The project is to provide a Java
framework for data processing applications. A large number of classes
are provided for general ledger, sales order, inventory management,
etc., and these classes can be extended and customized for particular
industries (vertical markets). It is a large and ambitious software
project.
IBM's SF project competes with products from companies like SAP and
Baan. Of course, the SF project is multi-platform and uses Java beans
and GUI interfaces. More information on SF is available at
http://www.ibm.com/Java/Sanfrancisco
15. (Sect. 3) What large Office-style or other applications have been
written in Java?
[*] Well, the first one to consider is IBM's San Francisco project,
mentioned above. There is also Lotus's e-suite - a suite of Java
applets and beans including a spreadsheet and a word processor. See
http://esuite.lotus.com. These became available in March 1998.
Another office suite in Java is Applix Anyware at
http://www.applix.com/anyware/index.htm. Applix became available in
downloadable demo form in April 1998.
Yet another is Star Division's Client/Server Office. It is an office
suite with the client part written in Java and able to run on
JavaStations. The server part will run on Solaris, NT, OS/390, and
AS/400. The older (non-Java) version is bundled with all Sun
workstations sold in Germany. The Linux version is freely downloadable
from http://www.stardivision.com.
Another is Digital Harbor's Wav word processor. It supports component
software, and it runs in 1MB, not the 114Mb of the latest MS Word. A
free trial is avilable. See: http://www.digitalharbor.com
Another Java application is Formula One for Java, an Excel-compatible
spreadsheet written in 100% pure Java, and available for all systems
that support Java. It runs as a Java Bean, so can easily be assembled
as one component of a larger system. It also runs as an application,
and as an applet! Formula One is a product of Visual Components, Inc.
See http://www.f1j.com.
Another one is Ncode Research Inc. who write Java viewers for office
suites. They are file-format specialists. Their mission is to make all
popular file formats available for the Java platform. They write 100%
Pure Java viewers for Word, Excel and PowerPoint (including Office 95
and 97 formats). See http://www.ncode.com/
Another company operating in the same space is JSoft, at
http://www.jsoftinc.com
Intentia, the 8th largest ERP vendor (annual sales of $320 million),
has moved their entire suite of applications (Movex - covering 8
markets) from RPG to Java - 20 million lines worth. See their press
release.
However, the industry is seeing few new office productivity
applications written in any language. The niche for single-user office
productivity applications is already dominated by Microsoft products,
and it is unrealistic to think that Java software will unseat
shrink-wrapped software simply because it is written in Java. This is
why Corel replanned its Java rewrite of Corel Office before taking it
to FCS. When Corel did that, it also increased its investment in Java
from 33% of R&D budget to 50%, at the expense of Windows.
Most of Java development is taking place for custom applications
internal to a company. Most programmers of any kind have never worked
on MS Office, but work on internal applications, and so it is with
Java. These projects don't have the high profile of major vendors'
products, but they are the mainstay of the industry. There are many
companies working on Java Beans, like http://www.quadbase.com who have
Java graphing software (see also GraphMaker mentioned later in this
document). EspressChart is a Java Bean that gives you the ability to
add 2D and 3D graphs in your applications/applets. This bean is easy to
use, 100% Java, and runs anywhere.
There are some excellent Java games applets at
http://www.javaarcade.com Check out the pinball -- dig that crazy
rhythm, man.
There are more good Java games applets at
http://www.frontiernet.net/~imaging/java_games.html
and at http://www.gamesdomain.co.uk/GamesArena
If you want to use Java to learn math & computer graphics, visit
http://www.frontiernet.net/~imaging/math_is_a_game.html
ObjectDesign Inc., has an ODBMS written in 100% java. The product is
named PSE Pro for Java. See http://www.objectdesign.com
Another database written in Java is available from
http://www.cloudscape.com.
Another database written in Java is available from
http://www.instantdb.co.uk. InstantDB is available free to
non-commercial organizations, and is very well documented and
maintained. (Recommended!)
Finally, note that Sun's Java compiler is written in Java. This is a
really big application in widespread use on millions of platforms. The
compile command "javac test.java" is equivalent to
java sun.tools.javac.Main test.java
javac has a script wrapper just to set the heap size as a command line
argument, as you can do in your own programs.
16. (Sect 3.) What Java User Groups are there?
[*] There are scores of Java User groups around the world, mostly in
urban areas, and centers of software technology development. A partial
list with contact information can be found at
http://sunsite.unc.edu/javafaq/usergroups.html.
If you can't find a user group in your area/school, it's easy and
satisfying to start one.
17. (Sect 3.) What is a Java Bean?
[*] A Java bean is a Java class that follows some simple conventions.
Because it follows conventions, it can easily be processed by a
software tool that connects Beans together at runtime. Java beans are
reusable software components.
Think of Java beans as being the software equivalent of Lego[tm]
bricks. Instead of plugging together plastic bricks, you can easily
plug together classes, and have them fit and work with each other. See
http://www.jc100.org/
See the Java Bean FAQ at http://java.sun.com/beans/faq/faq.general.html
18. (Sect 3.) Where can I find examples of the use of the Java class
libraries?
[*] The two volumes of "Java Class Libraries" by Chan, Lee and Krama
published by Addison Wesley, have extensive examples of how to use the
standard libraries. One programmer comments "When I need to use an
unfamiliar area of the class libraries one of the first things I do is
read their examples." You can see them online at
http://java.sun.com/docs/books/chanlee/second_edition/vol1/examples.html
and http://java.sun.com/docs/books/chanlee/second_edition/examples.html
19. (Sect 3.) How can I find out exactly what version of Java I have on my
system?
[*] On a Solaris system, you can use the pkginfo command, like this:
pkginfo -l SUNWjvrt
It will give a reply like this:
PKGINST: SUNWjvrt
NAME: JavaVM run time environment
CATEGORY: system
ARCH: sparc
VERSION: 1.1.6,REV=1998.07.30.16.21
BASEDIR: /
VENDOR: Sun Microsystems, Inc.
...etc
You may also try
java -fullversion
Although that's not an officially-supported command option, and has
gone away in JDK 1.2. Try also
java -version
20. (Sect 3.) What Java newsgroups are there?
[*]
comp.lang.java.beans
comp.lang.java.corba
comp.lang.java.databases
comp.lang.java.gui
comp.lang.java.help (it renamed comp.lang.java.setup)
comp.lang.java.machine (it renamed comp.lang.java.tech)
comp.lang.java.programmer (it renamed c.l.j.api and c.l.j.misc)
comp.lang.java.advocacy (argue all day with anonymous shills)
comp.lang.java.softwaretools
alt.comp.lang.java-games
New Java newsgroups are added every so often. Try not to crosspost.
-------------------------------
4. Compilers and Tools
1. (Sect. 4) Is there a lex and yacc or preferably a flex and bison
equivalent for Java?
[*] There is a lex equivalent called JavaLex and a yacc equivalent
called CUP.
LALR(1) parser JavaLex and JavaCup:
http://www.cs.princeton.edu/~appel/modern/java/
LL(k) parser JavaCC: http://www.suntest.com/JavaCC/
LALR(1) parser SableCC from McGill U.
http://www.sable.mcgill.ca/sablecc/index.html is generously made
available under GNU license.
2. (Sect. 4) Where can I find a byte code obfuscator?
[*] Java Obfuscators replace the original class, field and methods
names in the bytecode with meaningless strings. Second generation
obfuscators are now appearing that also obfuscate the control flow and
encrypt String literals. People use obfuscators on their applets if
they want to hide their code from others. Generally, you wouldn't do
this with software that you put on your website for others to enjoy. It
runs counter to the "open source" philosophy of learning from other's
code and allowing them to learn from yours.
Zelix KlassMaster is a commercially supported obfuscator. It has a free
evaluation version at http://www.zelix.com/klassmaster
Another commercially supported obfuscator, with a downloadable free
trial is at http://www.4thpass.com/SourceGuard. There are also some
free works from students and others. http://www.primenet.com/~ej/
http://www.math.gatech.edu/~mladue/HoseMocha.java
Some people have reported problems using these with JDK 1.1.
This obfuscator has been updated to be fully compatible with JDK 1.1:
http://www.monmouth.com/~neil/Obfuscate.html
Obfuscators are intended to foil decompilers. Decompilers translate
byte code back into Java source code. Mocha was the first and most well
known of the decompilers; it's no longer supported. There is a
decompiler (written in C++) at
http://www.geocities.com/SiliconValley/Bridge/8617/jad.html
Because it is in C++, there are different versions for every
architecture (hah!) There are also commercial products, such as
SourceAgain from
http://www.ahpah.com/
There's a very good Java Code Engineering and Reverse Engineering FAQ
page at http://www.meurrens.org/ip-Links/Java/codeEngineering/.
3. (Sect. 4) Which program is used to create .zip files compatible with
the java* programs?
(e.g. classes.zip, moz3_0.zip)
[*] Use the jar-tool from JDK1.1(.1):
jar [ options ] [manifest] destination input-file [input-files]
E.g.:
jar cvf myJarFile.jar *.class
creates a compressed archive. And watch out -- the order of the options
("cvf") determine the order of the arguments!
jar cvfO myJarFile.zip *.class
creates it fullsize (uncompressed) (note the 'O'-option used for
JDK1.0.2)
On Unix you can also use:
zip -rn ".class" my_file.zip *
Info-ZIP home page: http://www.cdrom.com/pub/infozip/
Latest source code: ftp://ftp.uu.net/pub/archiving/zip/src/zip21.zip
Netscape's command line version of its JAR packager and signing tool is
called "zigbert". They also have a signing tool with GUI written in
Java. More info
http://developer.netscape.com/software/signedobj/jarpack.html
If you zip your .class files for JDK 1.0.2 (for 1.1 you'll use a Jar):
1. zip your files uncompressed (can use WinZip 6.2 up);
Unix command:
zip -r0 classes.zip <directories>
2. Make sure the main class has no parent directory inside the
archive, (in other words, don't build an archive with
foo/bar/myMain.class, unless your myMain is in a package called
foo.bar. Instead start it at myMain.class). Your packages must be
placed in the archive using their corresponding filesystem
pathnames.
3. Put the archive in the same directory as the .html page.
4. Put something like the following tag in the .html file:
<APPLET CODEBASE="."
ARCHIVE=my_zip_file.zip,myOtherZip.zip,thirdfile.zip
CODE="my_main_class.class"
WIDTH=600 HEIGHT=250>
</APPLET>
From JDK 1.1 on, an example of the applet tag used with a jar file is
<APPLET ARCHIVE=myfile.jar
CODE=myapplet.class
WIDTH=600 HEIGHT=250>
</APPLET>
These lines will use an applet called myapplet that can be found in the
jarfile myfile.jar. An example applet tag of a jar file used to hold
classes in packages is
<APPLET ARCHIVE="myclasses.jar"
CODE="linden.net.MyApplet.class"
WIDTH=480
HEIGHT=120>
</APPLET>
You can supply several jar filenames in a comma-separated list. Jar
files are in compressed PKZIP format.
4. (Sect. 4) Can I compile a Java program to a binary executable, .exe on
a PC?
[*] Compiling into native code destroys portability, which is one of
the main benefits of Java. If you want to create a native executable
because you wanted to make it easy to distribute and use programs,
consider a Jar file instead.
Some companies make products that do this. See the webpages for
Symantec http://www.symantec.com, Supercede http://www.supercede.com,
and Tower Technology http://www.twr.com. The first two are targeted to
Windows. Tower Technology supports several flavors of Unix.
Also, there is a native Java compiler from IBM, known as the HPJ (High
Performance Java) compiler. One user has reported that it created a 2Mb
executable from a 12K java file, and did not run any faster. See
http://www.alphaworks.ibm.com/
See also Instantiations JOVE http://www.instantiations.com/jove.htm,
the paper about the Toba project
http://research.microsoft.com/research/lt/toddpro/papers/coots97.pdf,
Network World, "Vendors Rush To Speed Java Performance", Feb 9 1998, at
http://www.nwfusion.com/news/0209java.html
Compiling to native code takes away the most significant benefit of
Java: portability of executables. Further, if you want your Java DLL
(or .exe) to interact with C++, you'll have to specify which specific
C++ compiler and/or actually compile some sort of linkage via the
appropriate C++ compiler. C++ does not have a standard ABI, so there is
a big problem with interoperability. Every C++ compiler uses a
different object model, a different way of laying out class members,
and a different way of "mangling" names for the linker.
C is much simpler. The only question here is how structures are
"packed" (i.e., are integers aligned on four-byte bounds?). All the C++
compilers can interact with C code, thanks to 'extern "C"'
declarations.
Consider carefully why you want to compile to a native executable, and
whether there is a Java way to accomplish your goal. There may be a
good reason for compiling to native code, but it needs to be thought
through.
5. (Sect. 4) How can I performance profile my Java code?
[*]java -prof MyClass
produces some basic output in a file called java.prof, showing the
number of times methods were invoked. The output lines are of the form:
# of calls method called called by time spent
On a Unix system, you can sort the file with something like
sort -r +82 <java.prof > java.sort
More and better Java tools are a third party opportunity. One Java
profiler is JProbe Profiler, available from http://www.klg.com/jprobe.
JProbe is said to be easy to use. Another profiler is OptimizeIt,
available from http://www.optimizeit.com. Each of these profilers has
performance tuning, which shows which methods took how much time, and
also memory tuning, which shows what objects are in memory and how they
were allocated. Both are important things to know. The latest version
of the CodeWarrior IDE http://www.metrowerks.com has a time-based
profiler for Java code. Java Workshop from Sun also has a time-based
profiler.
JDK 1.2 comes with some limited profiling capability built-in.
Depending on your needs, it may be all that you need. Execute the
following to get a short summary of what you can do:
java -Xrunhprof:help
For example, you can see which methods are taking the most time to
execute, in the context of particular stack traces.
6. (Sect. 4) When I use javadoc and I click on any java class included in
the JDK why do I get this message?
Netscape is unable to find the file or directory named:
/E|/Jwrkshop/JDK/bin/java.lang.Throwable.html
[*] References to the JDK classes assume that all generated html files
are in the same directory and, in fact, that all files for all classes
referenced are generated at the same time. There is no way to generate
files incrementally and have them all reference each other, as you
would like.
As long as you have source for everything involved (including the JDK
and all third-party classes), you can list all of your packages and all
of the others on the javadoc command line and generate the whole set at
once, but it is burdensome. Of course, if you receive any libraries as
.class files, even this workaround will not suffice.
Also javadoc will not generate the image files - you need to get them
from the images directory under the JDK API documentation files. You
can just copy the entire directory into your own doc directory. javadoc
is a very nice concept, with a few implementation flaws.
7. (Sect. 4) I'm working on a project with lots of classes and I use the
JDK. A recompile from scratch takes forever when I do it a class at a
time. How do I recompile everything?
[*] The first way is
javac *.java
Another way is
javac -depend tip.java
where "tip.java" is a class "at the tip of the iceberg", i.e. that
depends on (uses) all the other classes. Typically, this may be your
main class. However, "-depend" is known to be buggy and cannot be
relied upon. It also doesn't issue compile commands in parallel to make
use of multi-processor systems.
Without the "-depend" option, the standard "javac files" doesn't look
beyond the immediately adjacent dependencies to find classes lower down
the hierarchy where the source has changed.
The -depend options searches recursively for depending classes and
recompiles it. This option doesn't help when you have dynamically
loaded classes whose names cannot be determined by the compiler from
the dependency graph. E.g. you use something like
Class.forName(argv[0]);
The author of the code using those classes should make sure that those
classes are mentioned in a Makefile.
8. (Sect. 4) Why do I get the java.lang.UnsatisfiedLinkError when I run my
Java program containing Native Method invocations?
[*] Your program is not able to find your shared library or DLL.
On Windows 95/NT, make sure that the DLL exists in a path that is
included within the PATH environment variable. (This need is true for
both standard (untrusted) applications and trusted applets. At least,
if you use the Java Plug-in to give yourself standard Java inside a
browser).
On Solaris, make sure that the environment variable LD_LIBRARY_PATH
includes the path of your shared library.
Note that jdb looks for libraries with "_g" appended to their names.
Thus, if you intend to use jdb on a Java application that invokes
native methods, you must ensure that the appropriately named libraries
are in jdb's path. The "debug" nm libraries can simply be renamed
copies of the nondebug libraries.
For example, if your app invokes native methods in a library named
mynm.dll (on Windows) or mynm.so (on Solaris), make a copy in the same
directory and name it mynm_g.dll or mynm_g.so.
9. (Sect. 4) An anonymous class can't seem to access a private outer
method. Why is that?
[*] It's a known bug in the JDK 1.1.4. The code is:
public class MyDialog {
void Setup() {
addWindowListener( new WindowAdapter() {
public void windowClosing(WindowEvent e) {
myCloseWindow(); }
}
); // anon inner class
}
private void myCloseWindow() { // private outer method
dispose();
}
}
This code sends javac into an infinite loop. The workaround is to make
the private method non-private, or to make the inner class a named
class. Sun put a workaround in the compiler to silently set the field
to package access.
10. (Sect. 4) What are the major Java releases and their contents?
[*] There have been three Java releases from Sun so far, plus a number
of bugfix (dot-dot) releases. The releases are:
o JDK 1.0.2
This was the release FCS in May 1996. It had some security fixes
over JDK 1.0.
o JDK 1.1
This release (Feb 1997) introduced a new event model in the window
system. It also made JDBC support and beans support a standard
feature. It changed and standardized the native code interface to
JNI. It also introduced inner classes.
o JDK 1.2
This release (Dec 1998) made the Swing library a standard feature.
Swing is a set of rich platform-independent graphical components.
It also introduced the Collections library, and Java 2D.
11. (Sect. 4) What is the difference between jre and java?
[*] They are functionally equivalent, with minor differences in the
handling of default classpath and options supported. To reduce
confusion, the jre command was removed in JDK 1.2. Instead there is a
"java" command in both bin and jre/bin.
jre.exe is the java launcher that comes with the Java Runtime
Environment. It ignores the CLASSPATH environment setting in favor of
its own internally generated default and whatever is supplied on the
cmd line using -cp or -classpath. It's intended to be a bit simpler for
those who are only ever running Java programs, not developing them.
java.exe is the java launcher that comes with the JDK. It uses the
CLASSPATH environment setting as a starting point and then tacks on its
own internally generated entries.
They both serve the same purpose and that's to start a Java VM, have it
run a Java application, then terminate. The source for jre.exe is
provided in the JDK. The source to java.exe is provided only in the JDK
Source distribution.
12. (Sect. 4) What IDEs (Integrated Development Environments) are there?
[*] Some popular IDEs include: (... if you have info on the platforms
that each of these support, send it in, and I'll add it to the FAQ).
Apptivity (Progress) http://www.apptivity.com
Blue J (free) http://www.sd.monash.edu.au/bluej
Bluette (free) http://www.bluette.com
Chicory (free) http://www.chicory.com
CodeWarrior
Professional http://www.metrowerks.com
Freebuilder (free) http://www.freebuilder.org
GRASP (free) http://www.eng.auburn.edu/grasp
Grinder http://www.tpex.com
Java WorkShop (Sun) http://www.sun.com/workshop/java
Javelin, Visual Object
Development for Java http://www.stepahead.com.au
JBuilder (Inprise) http://www.inprise.com/jbuilder
JDE for emacs http://sunsite.auc.dk/jde/
Jirvana (free) http://www.jirvana.com/Jirvana/Jirvana.html
JPadPro http://www.modelworks.com
Kawa (Webcetera) http://www.tek-tools.com/kawa
Metamata http://www.metamata.com
NetBeans (Swing-based) http://www.netbeans.com
PARTS alpha
(ObjectShare) http://www.objectshare.com
PowerJ (Sybase) http://www.sybase.com/products/powerj
SilverStream http://www.silverstream.com
Simplicity for Java http://www.datarepresentations.com
Super Mojo (Penumbra) http://www.penumbrasoftware.com
SuperCede (Asymetrix) http://www.supercede.com
teikade (PFU Ltd) http://www.pfu.co.jp/teikade
Together/J (Object Intl
Inc.) http://www.oi.com
Visaj (Imperial SW
Tech) http://www.imperial-software-tech.co.uk
VisualAge (IBM) http://www.software.ibm.com/ad/vajava
Visual Cafe (Symantec) http://cafe.symantec.com
Visual J++ (Microsoft) http://msdn.microsoft.com/visualj/default.asp
(not recommended)
Xelfi 0.94 http://www.xelfi.com
13. (Sect. 4) Why is Visual J++ not recommended?
Because Microsoft's strategic objective is "Kill cross-platform Java"
[*] It is not in Microsoft's financial interest to allow users to
easily move software to different platforms. Microsoft is the only
company in the computer industry that is actively trying to undermine
Java. This is not speculation -- the Department of Justice's lawsuit
quoted a Microsoft memo describing the strategic objective to "kill
cross-platform Java by grow[ing] the polluted Java market". See
http://www.usdoj.gov/atr/cases/f1700/1762.htm
VJ++ can be used in a compatible way, but the tool encourages use of
Windows lock-ins. The J++ compiler introduced new keywords and other
deliberate incompatibilities.
Microsoft is being sued because of unauthorized changes it made in
Java. A federal judge issued a preliminary injunction against Microsoft
in March 1998, prohibiting them from labelling their incompatible J++
product as Java. Another injunction was issued against Microsoft in Nov
1998, requiring them to remove some deliberate incompatibilities with
standard Java. (Recall that Microsoft did not create Java, but
contracted with Sun to distribute it).
Microsoft is in a real jam over Java; it contracted with Sun to
distribute standard Java, and then made many changes to its product
to make it deliberately non-portable. Sun sued Microsoft to enforce the
contract, and the preliminary legal decisions have been in Sun's favor.
Some analysts speculate that Microsoft will leave its customers in the
lurch and cancel VJ++. See
http://cnn.com/TECH/computing/9903/23/javajam.idg/index.html
Speak to your management chain - how comfortable do they feel using a
Microsoft product that is embroiled in a legal dispute, that introduces
deliberate incompatibilities, and whose stated goal is to lock you in
to one platform? It is a safer choice to get standard Java from any
other source than Microsoft. You can use these facts to move your
company to standard Java.
As a Java programmer please join the Java Lobby, an independent
organization dedicated to representing independent non-vendor interests
in Java. It's free, and you can sign up by visiting
http://www.javalobby.org for details. Other ways to encourage portable
java:
o Use development environments from other vendors, or convert
Microsoft Visual J++ to use Sun's JDK, following the instructions
at http://www.orbiter.demon.co.uk/
o Use Netscape Communicator (not Internet Explorer)
o If required to use Internet Explorer, use the Java Plug-In to get
a standard Java system inside it.
o Use a standard JVM from GNU, Kaffe, any of the IDE vendors, or Sun
(but not Microsoft's J++ SDK)
o Free standard Java compilers and the Java Plug-In can be
downloaded from http://java.sun.com.
o Free standard Java Virtual Machines can be downloaded from
http://www.kaffe.org, http://www.oryxsoft.com/projects/gvm, and
http://www.redhat.com/linux-info/jolt
o Free Java AWT software can be downloaded from
http://www.biss-net.com/biss-awt.html and the files are all at
ftp.java-linux.org (the linux site) too.
o Free Java software can be downloaded from
http://www.gnu.org/software/java/java.html
Just for the record, the May 1998 federal case against Microsoft has
nothing to do with innovation or product design, as Microsoft
frequently insists. Microsoft is actually charged with
o taking anti-competitive action to exclude competition in browsers,
in order to protect its monopoly in desktop operating systems.
o using its monopoly to impose restrictive agreements that require
PC manufacturers to accept the Microsoft browser with Windows, and
that hinder the promotion of competing browsers.
Many people think that contracts between "A" and "B",
restricting/discouraging "B" from distributing "C's" products are
sleazy. ["A" is Microsoft, "B" is AOL, many ISPs, computer vendors,
etc. "C" is Netscape"]. Such contracts are in restraint of competition
and illegal when used by a monopoly. This is why Microsoft is facing
mounting legal problems in the United States, Italy, Brazil, and the
European Union.
14. (Sect. 4) What language is the Java compiler and JVM written in?
[*] Sun's javac Java compiler is written in Java.
Sun's java JVM interpreter is written in Java with some C, C++ for
platform-specific and low level routines.
Sun's Java Web Server is written in Java.
Other companies have chosen other approaches. IBM's Jikes compiler
(which is praised for its speed) is written in C++, but of course each
version only runs on one platform. Jikes versions have been prepared
for IBM AIX, Linux Intel (glibc), Win95/NT and Solaris Sparc at
http://www.alphaworks.ibm.com/formula/jikes.
15. (Sect. 4) How do I turn off the JIT in the JDK?
[*] The command that works in both JDK 1.1.6 on, and JDK 1.2/2 is
java -Djava.compiler=NONE ...
One reason for turning off the JIT is to get more
information about any exception that is thrown in your code.
But HotSpot is able to produce line numbers in stack traces
even for JIT'd code. HotSpot rocks.
16. (Sect. 4)
How can I store the errors from the javac compiler in a DOS file?
javac foo.java > errorfile doesn't work.
[*]
javac writes errors to stderr,
The problem is that DOS doesn't allow stderr to be redirected
(as command.com is very poor software). So you
have to use a special error redirection mechanism in the compiler:
javac -J-Djavac.pipe.output=true myfile.java > errors.txt
In JDK 1.2, you can use:
javac -Xstdout
You typically use this when a compilation produces a lot of error messages,
and they scroll off the DOS window before you can read them.
Alternatively,
you can get a scollbar to appear on a DOS window by changing the properties
with the "Layout" tab. Change the Screen Buffer Size Height: to some
multiple > 1 of the Window Size Height. E.g. use a buffer height of 100 and
screen height of 25 (the default). This will give you three buffers of
scroll "history."
The DOS limitation is improved in NT, where you can write
javac myfile.java 2> errors.dat
17. (Sect. 4)
How can I pretty-print Java source?
[*]
For tools that reformat code, try:
o
http://www.parallax.co.uk/~rolf/download/jpp.pl
o
http://www.bigfoot.com/~davidsont/jstyle
o
http://www.mmsindia.com/JPretty.html
o
http://www.almide.demon.co.uk/source_code/java/JavaCC/Pretty/Pretty.html
o
http://www.geocities.com/~starkville
For tools that print code neatly, try:
o
http://www.eng.auburn.edu/department/cse/research/grasp/
Some Unix utilities work adequately:
o
cb (very few style choices though)
o
alias printjava 'vgrind -lC++ -t -w \!* | lp' works pretty well too.
Perhaps the best tools are the GNU utilities. Use enscript to
generate postscript files with Java-specific formatting. Then use
GhostScript/GhostView to preview and print the files to a
non-PostScript printer, if necessary. The scripts can be found at:
o
GNU Enscript - pretty
printer and PostScript formatter
o
Ghostscript, Ghostview, and
GSView for Unix
o gsv25w32.zip
- GSView for NT (requires the following Ghostscript files)
o
gs510ini.zip - Ghostscript configuration, initialization, and
example files
o
gs510w32.zip - Ghostscript core binaries for Windows NT
o
gs510fn1.zip - Ghostscript standard fonts
-------------------------------
5. Compiler Messages
Most of the "questions" in this section are diagnostic messages from the
compiler. Each answer explains what the message means, and how to avoid it.
1. (Sect. 5)
Why did I get an OutOfMemory error when porting working code from
JDK 1.0.2 to 1.1?
[*]
The preset memory limit has changed. It went down to 16MB so as not to
penalize low memory machines. You can adjust it with
java -mx128m Frotz # jdk 1.1
java -Xmx128m Frotz # jdk 1.2
to get a 128MB extent.
Also see the Runtime methods freeMemory() and totalMemory().
2. (Sect. 5) Why do I get a "Statement not reached" error from javac for
no apparent reason?
[*] JDK 1.0 has a limit of 63 words of storage for local variables in
any method. longs and doubles require two words of storage, and all
other primitive types and all reference types require one word. If you
assign values to more than 63 words of local variables, you will get a
"Statement not reached" error on the statement after you assign to the
variable that contains the 64th word. In JDK 1.1, the low limit was
removed.
3. (Sect. 5) class MyOrdinaryClass must be declared abstract.
It does not define void actionPerformed(java.awt.event.ActionEvent)
[*] This is one of those error messages where the compiler tries to
guess what you meant, and gives you a message based on a wrong guess!
So the message is confusing.
Your MyOrdinaryClass class implements ActionListener, which means you
must include a definition of the methods from the ActionListener
interface.
But you did not. You either left a method out, or (more likely) you
misspelled its name. Perhaps you wrote "ActionListener" instead of
"actionListener".
So the compiler did not find the method to fulfill the interface. Since
there was a method promised but not supplied, the compiler thinks you
were aiming at an abstract class, and it prints an error message
accordingly.
4. (Sect. 5) Variable may not have been initialized.
URL test;
try {
test = new URL("http://osprey.avs.dec.com");
} catch (MalformedURLException e) {
System.out.println("bad URL:" + e.getMessage());
}
System.out.println("this is url " + test);
[*] The compiler will warn you if you use a variable before it is
certain to have been initialized (not just with the default value)
since this means you probably forgot to set it.
In the case of exceptions, you have to consider that the flow of
control may terminate abruptly, with no operations completed. In the
example above, if an exception is raised in the try clause, variable
test will not be assigned a value, yet you are using it after the catch
clause. One solution would be to declare test with an explicit initial
value of null, but this works only because toString() works on a null
reference. (toString() is invoked implicitly by operator + with String
operand.)
Always initialize to a value that will work notwithstanding exceptions
being thrown.
5. (Sect. 5) No constructor {superclass}()
I extended the class called Frotz, and the compiler is giving me an
error message "No constructor Frotz()" in the child class. Why?
[*] When you define a constructor for a class, the compiler inserts a
call to the superclass' parameterless constructor unless you explicitly
call the superclass' constructor at the start of your constructor. If
the superclass doesn't *have* a parameterless constructor, the compiler
emits a message to that effect. The solution is usually to call the
superclass' constructor at the start of your constructor.
6. (Sect. 5) No constructor matching MyCheckbox(myApplet)
MyApplet.java:11: No constructor matching MyCheckbox(myApplet)
found in class MyCheckbox.
bp1 = new MyCheckbox(this);
^
[*] If a compiler isn't finding a constructor you thought you created,
check whether you gave a return value to the method (remember,
constructors have no return value). E.g.,
public void MyCheckbox( Container parent )
If you did, the compiler will think it is an ordinary method, not a
constructor. This is a common mistake and hard to spot.
7. (Sect. 5) Type expected {public method variable}
public static void main(String[] args) {
^
Statement expected.
public static final float Conversion_Factor = 39.37;
^
Type expected.
[*] Argument and variable declarations inside methods are never public
or static because they are local to a method. (Before JDK 1.1 they
couldn't be final either, but there was no good reason for that
restriction and it was dropped.) If you have public or static
variables, move them outside the method. They are usually put at the
beginning of the class.
8. (Sect. 5) Can't access protected method clone in class java.lang.Object
T.java:96: Can't access protected method clone in
class java.lang.Object. OtherT is not a subclass of
the current class.
[*] Object.clone() is protected because subclasses might want to
restrict access to cloning, and if Object.clone() were declared public,
subclasses could never make it more restrictive. The subclass can make
access to the clone() operation less restrictive.
This means that a method can clone its own objects, but a method cannot
clone objects of another class, unless you do something like:
class SomeObject implements Cloneable {
public Object clone()
throws CloneNotSupportedException {
return super.clone();
}
}
i.e., override clone() to make it public, and call the superclass
clone().
class Foo {
Bar bar;
Foo (Bar b) {
try {bar = (Bar) b.clone();}
catch (Exception e) {}
}
...
class Bar implements Cloneable {
public Object clone()
throws java.lang.CloneNotSupportedException {
return super.clone();
}
}
Another refinement is to note that Object.clone() only throws a
CloneNotSupportedException when the object doesn't implement Cloneable.
Since you control what your classes do and don't implement, you can
ensure that Cloneable classes implement the interface, and you don't
need to make the overridden clone() throw the exception.
public class X implements Cloneable {
public Object clone() { // no throws
try {
// in case members need cloning
X c = (X)super.clone();
return c;
} catch (CloneNotSupportedException e) {
// should not happen, because of Cloneable
throw new InternalError();
}
}
}
9. (Sect. 5) Deprecated methods
What does "deprecated" mean? I got this in a compiler error message.
[*] "Deprecated" means you are using an older API, that Sun has
replaced with a newer one (usually to follow more consistent naming
conventions). Deprecated methods are not recommended for use. They are
supported in the short term, but your code should be updated with the
new. To update your code, compile your old code using javac's
"-deprecation" option to list deprecated methods, then find the
replacement methods in the current HTML API documentation for the old
deprecated methods.
As an example of a deprecated API, Component.size() was replaced by
Component.getSize().
See also
http://java.sun.com/products/jdk/1.1/docs/guide/
misc/deprecation/index.html, "1.1 Deprecated Methods"
and
http://java.sun.com/products/jdk/1.1/docs/guide/
awt/DeprecatedMethods.html, "Deprecated methods in the 1.1 AWT"
10. (Sect. 5) double y = sin(90);
What's wrong? That code provokes compiler messages.
[*] You need to write it this way:
double cvtDegToRad = Math.PI/180;
double x = 90*cvtDegToRad;
double y = Math.sin(x);
sin is a static method of the Math class that takes radians. You need
to use the "Math" classname, e.g. Math.sin instead of plain sin,
because you have to say what class or object these methods belong to.
A very common mistake is to assume that importing a class means that
you don't have to qualify the names of its members. When you call a
method you have to state the name of the class or object it belongs to,
regardless of any imports you have done. (Except inside the class
itself, obviously).
The trig functions are static methods of the Math class, so you give
the name of the class in invoking them. Further, the Math class works
in radians, not degrees. 360 degrees = 2 pi radians, so use a
conversion factor as shown if you are working with degrees.
11. (Sect. 5) Can't make static reference to method...
[*] Your code probably looks something like this:
class myclass {
public static void main(String args[]) {
myMethod();
}
public void myMethod() { //some code
}
}
Static (class) methods can only call without qualification other static
methods, so you either have to qualify the call in (static) main() to
(nonstatic) myMethod() with an object of type myclass, or you have to
make myMethod() static.
People often forget that even though main is "in" myclass, there is no
implicit object when you are in main() because it is static. This
happens especially when writing code to run an applet as an
application, where you want to call init() and start() from main.
public static void main(String[] args) {
Applet ma = new myApplet(); // have to create object
ma.init(); // use to qualify access to non-static methods
ma.start();
}
12. (Sect. 5) Incompatible type for =. Explicit cast needed...
byte b = 0;
Incompatible type for =.
Explicit cast needed to convert int to byte.
b = b + 100; // compiler error message
b += 100; // works OK
[*] Arithmetic expressions are promoted to the type of the longest,
floatiest operand in them, or at least to int. The first statement
involves the assignment of an expression. The expression is promoted to
32 bits, and must be cast back down to 8 bits, like this: b = (byte)
(b+100); The second is an operator assignment, and the cast back to
byte takes place automatically. The Java Language Specification says
that a compound assignment expression of the form E1 op= E2 is
equivalent to E1 = (typecast)((E1) op (E2)), where "typecast" is the
type of E1, except that E1 is evaluated only once. (See JLS 15.25.2
Compound Assignment Operators) The compile-time narrowing of constants
means that code such as:
byte theAnswer = 42;
is allowed, with no cast necessary. (See JLS 5.2 Assignment Conversion)
Other sites:
JLS 5.2 Assignment Conversion
JLS 15.25.2 Compound Assignment Operators
13. (Sect. 5) Class {package}.{class} not found in type declaration.
I am trying to compile file "{class2}.java" in a package, and I get
this compiler error. {class2}.java refers to {package}.{class}, but the
file {class}.java and {class2}.java are in the same {package}
directory, which is the current directory and which is in the CLASSPATH
variable. Both files have "package {package};" at the top of the file.
What's the problem?
[*] When the source refers to classes in packages, the CLASSPATH has to
point to the root of the package/directory hierarchy for a reference to
resolve correctly. This is true even for source files in the same
package (and directory). I.e., assuming {class} and {class2} are both
in {package}, {class} can't make a reference to {class2} unless the
CLASSPATH is set so javac can find {package}/{class2}.java. It should
make no difference what directory you are in when you invoke javac,
unless you are relying on "." in the CLASSPATH to point to the package
root or are specifying the source file with a relative path (e.g.,
{package}/{class}.java).
Some examples, assuming
o - Foo.java and Bar.java are in /java/source/pack/
o - Both have "package pack;" as the first statement
o - Foo.java includes "Bar b = new Bar();"
# solaris ksh
$ alias jc=/java/jdk11/bin/javac
$ CLASSPATH=/java/source/
$ jc /java/source/pack/*.java # works fine
$ cd /java/source/pack
$ CLASSPATH=.
$ jc *.java # fails - Foo.java can't find class Bar
$ cd .. # now . is package root, /java/source/
$ jc pack/*.java # works
14. (Sect. 5) public class "Foo" must be defined in "Foo.java"
I get this message even though it is in Foo.java. What gives?
[*] Javac verifies that a public class is defined in a file of the same
name (e.g., that public class Foo is defined in Foo.java). Two things
you can check:
First, make sure the case matches exactly. public class Foo cannot be
in foo.java; it has to be in Foo.java.
Second, are you using MKS on win32? Javac on win32 assumes you are
using the DOS path separator (\) even though MKS accepts the Unix path
separator (/). When javac tries to parse a your Unix-style path, it
won't produce the correct filename, the match will fail, and it will
emit an error. You have to use the DOS path separator (\), which must
be escaped in MKS - e.g., "javac H:\\source\\package\\Foo.java".
Alternatively, you can traverse to each source directory and avoid
pathnames altogether.
-------------------------------
6. Java Language Issues
How-to
1. (Sect 6.) How do I compare two Strings?
if (s1 == s2)
is giving me funny results.
[*] The comparison using "==" on objects, such as Strings, is asking,
"Do these two objects have the same reference?" Do they have the same
address, and hence are the same object? What you really want to do is
ask, "Do these two Strings have the same *contents*?"
Compare String contents with any of the following:
if (s1.equals(s2) )
if (s1.equalsIgnoreCase(s2) )
if (s1.startsWith(s2) )
if (s1.endsWith(s2) )
if (s1.regionMatches(s1_offset, s2, s2_offset, length) )
if (s1.compareTo(s2) < 0)
(There are other ways, too.)
Note that you can do this with literals:
if ("apple".equals(s2) ) ...
If you compare these the other way round, like this:
if ( s2.equals("apple") ) ...
and s2 is null, you will get a null pointer exception.
2. (Sect. 6) How do you get the code value of a char?
I would like to transform a char into the corresponding int value, that
represents the code value of the char. How?
[*] Like this.
char c = 'A';
int i = c;
Going the other way is just
c = (char) i;
This question crops up so frequently because the BASIC language uses
functions to map characters into ints, ASC( 'A' ) => 65 causing BASIC
programmers to seek the corresponding Java functions. The same is true
for Pascal, Ada, and other languages.
3. (Sect. 6) Why does b >>>= 1 give me the same result as b >>= 1?
[*] ">>" is a "signed" or "arithmetic" shift, namely, it replicates the
sign bit on the left as it shifts.
The ">>>" operator is an "unsigned" or "logical" shift; it does a shift
right and zero fill. However, ">>>" looks like it does a signed shift
with negative bytes and shorts, where int promotion alters the sign.
This occurs when you have a non-canonical type, byte, or short, with a
negative value, e.g.
byte b = -15; // 0xf1
b = (byte) b >>> 4; // why isn't b 0x0f ?
The initial expectation is that an unsigned shift right of 0xf1 would
successively be (in binary)
0111_1000 then
0011_1100 then
0001_1110 then
0000_1111
But that doesn't happen. The rules of arithmetic in Java say that all
operands are converted at least to int before the operation (and
possibly to a more capacious type). That means our byte is promoted to
an int, so instead of shifting 0xf1, we are shifting 0xfffffff1. If you
shift right unsigned 4 places, you get 0x0fffffff. When you cast that
to a byte it becomes 0xff, or -1.
The bottom line is that the final result is the same as if you had
performed the signed shift because the unsigned shift applied to the
intermediate int, not to the original byte. This anomaly means that
">>>" is useless for negative bytes and shorts. It is probably safer
and clearer not to use it at all, but to mask and shift instead:
// not recommended
byte b = -15;
b = (byte) (b>>>4);
System.out.println("b= "+Integer.toHexString(b) );
// recommended
b = -15;
b = (byte) ( (b & 0xFF) >> 4 );
System.out.println("b= "+Integer.toHexString(b) );
4. (Sect. 6) Why does the <unexpected> happen in floating point?
[*] There are several unexpected things that seem to bite programmers
with floating point. This is almost always a result of the programmer
not being fully conversant with floating point arithmetic in general,
rather than a problem relating to Java.
The question of floating point accuracy comes up with Java more than
with C++ (for example) because of Java's decision in the println()
method to print enough digits to distinguish the number from the
next-closest number, rather than rounding it to 6 significant digits by
default as the C/C++ libraries do. [ISO C, Sect 7.9.6.1, line 21 of
"the double argument"]
If you seem to be having problems with floating point, the problem
probably stems from the fact that floating-point arithmetic is
inherently imprecise. You can expect up to 7 digits of precision with
floats and 16 digits with doubles. However, that does not mean that a
number that can be exactly represented in 7 digits decimal or can be
exactly represented as a binary floating point number. On the contrary,
that is usually not the case.
Additionally, when Java converts floating point numbers to a String, as
is done when they are output, enough digits are printed so the number
can be read back in with no loss of precision. For this reason, you may
see more "inaccuracies" in floating point output than you are used to.
You are actually getting more accuracy than on systems (like C,C++)
that suppress the less significant digits.
For more information and detailed specifications on how Java deals with
floating point, see the URLs listed below.
Other sites:
What Every Computer Scientist Should Know About Floating Point.
http://java.sun.com/products/jdk/1.1/compatibility.html#incompatibilities
JLS 4.2.3 Floating-Point Types and Values
JLS 4.2.4 Floating-Point Operations
JLS 3.10.2 Floating-Point Literals
JLS 5.2.3 Narrowing Primitive Conversions
http://gams.nist.gov/javanumerics/jgfnwg-01.html
See also "the guff from Hough" at http://www.validgh.com/java
If you want the rounded floating point output that most languages have,
use the new java.text package of Java 1.1 to limit the number of digits
that are output. If you need more precision than about 16 digits, use
the BigInteger and BigDecimal classes of Java 1.1.
Understanding the Java Language
5. (Sect. 6) How can I program linked lists if Java doesn't have pointers?
[*] Of all the misconceptions about Java, this is the most egregious.
Far from not having pointers, in Java, object-oriented programming is
conducted exclusively with pointers. In other words, objects are only
ever accessed through pointers, never directly. The pointers are termed
"references" and they are automatically dereferenced for you.
Java does not have pointer arithmetic or untyped casting. By removing
the ability for programmers to create and modify pointers in arbitrary
ways, Java makes memory management more reliable, while still allowing
dynamic data structures. Also note that Java has NullPointerException,
not NullReferenceException.
A linked list class in Java might start like this:
public class LinkedList {
public LinkedList head;
public LinkedList next;
public Object data;
public LinkedList advanceToNext(LinkedList current) { ...
}
Another choice for a linked list structure is to use the built-in class
java.util.Vector which accepts and stores arbitrary amounts of Object
data (as a linked list does), and retrieves it by index number on
demand (as an array does). It grows automatically as needed to
accommodate more elements. Insertion at the front of a Vector is a slow
operation compared with insertion in a linked list, but retrieval is
fast. Which is more important in the application you have?
6. (Sect. 6) Are parameters in Java passed by value or by reference?
[*] All parameters (values of primitive types and values that are
references to objects) are passed by value. However this does not tell
the whole story, since objects are always manipulated through reference
variables in Java. Thus one can equally say that objects are passed by
reference (and the reference variable is passed by value). This is a
consequence of the fact that variables do not take on the values of
"objects" but values of "references to objects" as described in the
previous question on linked lists.
Bottom line: The caller's copy of primitive type arguments (int, char,
etc.) _do not_ change when the corresponding parameter is changed.
However, the fields of the caller's object _do_ change when the called
method changes the corresponding fields of the object (reference)
passed as a parameter.
Also in this FAQ:
How can I program linked lists if Java doesn't have pointers?
Other sites:
JLS 8.4.1 Formal Parameters
7. (Sect. 6) What are "class literals"?
[*] A feature introduced in JDK 1.1. They are literals of type "Class"
that hold a value representing any class. There are even values to
represent "void" and an array, like this:
Class myCl1 = Character.class;
Class myCl2 = Void.class;
Class myCl3 = Object.class;
Class myCl4 = String[].class;
Class myCl5 = int[][].class;
You might use it like this:
Class cl = thing.getClass();
if (cl.equals(myCl1))
System.out.println("It's a Character class");
Note that a class literal
Component.class
is the equivalent of
Class.forName("java.awt.Component")
The second can throw an exception, but the first cannot. If you don't
know the name of the class when you write the code, you cannot use the
first form.
8. (Sect. 6) What are the naming conventions in Java?
[*] The naming conventions are straightforward:
1. Package names are guaranteed uniqueness by using the Internet
domain name in reverse order: com.javasoft.jag - the "com" or
"edu" (etc.) part used to be in upper case, but now lower case is
the recommendation.
2. Class and interface names are descriptive nouns, with the first
letter of each word capitalized: PolarCoords. Interfaces are often
(not always) called "something-able", e.g. "Runnable", "Sortable".
Caution: java.util.Observable is not an interface, though
java.util.Observer is. These two are poorly designed.
3. Object and data (field) names are nouns/noun phrases, with the
first letter lowercase, and the first letter of subsequent words
capitalized: currentLimit
4. Method names are verbs/verb phrases, with the first letter
lowercase, and the first letter of subsequent words capitalized:
calculateCurrentLimit
5. Constant (final) names are in caps: UPPER_LIMIT
6. Also in the FAQ:
Where can I find a Java style guide on naming conventions?
Other sites:
JLS 6.8 Naming Conventions
9. (Sect. 6) Should I prefer importing {package}.{class} over {package}.*?
Does it make a difference to the class file in any way, if I import a
package versus use the full name, i.e.
import java.rmi.server.*;
...
RemoteObject ro;
versus:
java.rmi.server.RemoteObject ro;
[*] No, it makes no difference to the class files or runtime speed.
Import is just a shorthand for quoting the full name package and class
name (as in the examples in the question). Importing a class does not
cause the class to be loaded at run time. There is no runtime penalty
for using the * form of import. The class file will contain the name of
the packages it uses, and the loader will look for those classes as
needed at runtime.
At compile time, the different forms of import may or may not make a
difference to compile time. Such a difference is likely to be
negligible, and should not be a factor in which form of import you use.
However, there are style advantages. Some say that stating which
classes you are importing can help program readability. In a program
with many * import statements, it may take a programmer time to find
which package an obscure class is imported from. If you explicitly list
each class you import at the top of the program, you document which
package each class you use comes from. These people suggest that you
use
import java.rmi.server.RemoteObject;
in preference to:
import java.rmi.server.*;
Other people say that it is clearer still to use the full package and
class name, at the point where you use classes in other packages.
These people suggest that you use:
java.rmi.server.RemoteObject ro;
But that gets a little lengthy when you instantiate:
java.rmi.server.RemoteObject ro
= new java.rmi.server.RemoteObject();
You always have the option of stating the full package and class name,
whether you use import or not.
Another good reason not to use the * form is when you are importing two
packages that have classes of the same name and you want to use only
one of those classes. E.g.
import com.sun.*;
import com.ms.*;
where there is a class called Modem in both those packages. If you use
the * form of import, you import both of the Modem classes and then
must fully qualify the class each time you use it, to say which of the
two you mean. In Java 1.2, the class java.util.List was introduced.
That had the same unqualified name as java.awt.List. If your code had
"import java.awt.*; import java.util.*;" it would no longer compile.
You'd get a message about ambiguous classname. If you import all of a
package indiscriminately you might get bitten when the package API
changes.
In Java 1.0, if you import a class that has the same name as a class
defined in that source file, you will get an error that the class names
clash. In Java 1.1, the local class will be used when the package name
is not given; to use the imported class, you have to use the full
package name.
The best advice is to write the program so that it is as readable as
possible. Where you have a group of well-known classes, as in java.awt,
there is no reason not to use "import java.awt.*;"
10. (Sect. 6) How can I use Math.cos() etc. without the prefix "Math."?
Is there some declaration that I can use to make "acos", "cos", "sin",
etc. (from java.lang.Math) recognizable in my own class, so I don't
have to prefix "Math." to them?
[*] No. There is no good alternative. There are several bad
alternatives:
1. Using "import" doesn't work.
The import stament only imports packages, subpackages, and
classes, not class members. This doesn't work:
import java.lang.Math.*;
2. Minimizing class name usage is unclear and bad style.
- You could wrap the functions in your own class.
double sin(double x) {
return Math.sin(x);
} // etc. for each function
But you'd have to use your class name everywhere but inside your
class, so it doesn't help.
- You can make a null reference to the Math class and use it to
refer to the static methods. Declare
java.lang.Math M = null;
angle = M.cos(i);
Besides not being clear, this invites abuse and errors.
- You could inherit the names
If java.lang.Math were not final and your class did not extend
another class, you could have your class extend Math, to bring the
namespace in. However, it is poor OOP style to use inheritance to
obtain a name abbreviation rather than to express a type
hierarchy.
11. (Sect. 6) Why is there a standard JNI?
[*] JNI is the Java Native Interface. It defines the way that a Java
program can call C programs. The industry has agreed on, and Sun has
codified, JNI as the standard. Microsoft shuns the standard and uses a
protocol of its own called Raw Native Interface, RNI.
You might think that once a Java program uses JNI, portability is lost,
and hence it doesn't matter if vendors diverge from the JNI standard.
Not so. Code that accesses a native library using JNI can run on any VM
that supports JNI, so it's portable across VMs on the same platform.
Further, you can port a native library to all platforms Java supports
(indeed, this is how Sun implements the Java Platform), so JNI
_enables_ cross-platform development where it's necessary to use
platform-specific idioms for certain functionality.
Conversely, code that uses RNI can only run on Microsoft's VMs on the
win32 platform. Microsoft's RNI has the effect of limiting RNI programs
to the Microsoft VM. Further, Microsoft's failure to support JNI locks
out JNI-based functionality on Windows. Microsoft's non-standard RNI is
the reason that programs using the Microsoft JVM cannot use the
standard Java jdbc-odbc library. That library has a piece written in C.
It works for all JVMs except Microsoft's.
The standard JNI thus has two purposes:
1. Source code compatibility between different platforms.
2. Binary code compatibility between different JVMs on the same
platform.
Microsoft's use of RNI locks in programmers who use it, and Microsoft's
failure to support JNI locks out programmers who don't use RNI. Users
can't run standard JNI applications on Microsoft VMs, or RNI
applications on non-Microsoft VM's. As a result, since most users will
support only one VM, they'll be locked in to complementary software -
in the case of Microsoft, a proprietary standard. A standard JNI means
that you can use any standard JVM to run your code on this platform.
12. (Sect. 6) How do I find out more about JNI? How do I find out more
about Java Anything?
[*] Taking the questions one at a time. Use of JNI detracts from
program portability. So you would only do it when you need some
critical single-platform effect. The documentation on JNI is at:
http://java.sun.com/docs/books/tutorial/native1.1/index.html
If your interest extends to reading a book on JNI, a good one is
"Essential JNI Java Native Interface" by Rob Gordon; ISBN
0-13-679895-0. See
http://www.amazon.com/exec/obidos/ASIN/0136798950/afuinc
In general, if you want to find out about topic "X" in Java, your first
stop should be to search the http://java.sun.com website for "X". For
example if you want to know about Internationalization in Java, a
search at the site quickly takes you to
http://java.sun.com/products/jdk/1.1/docs/guide/intl/intlTOC.doc.html.
13. (Sect. 6) How do I get unsigned ints in Java?
[*] Java doesn't have unsigned ints. The reason is that this is a
poorly designed area of C. The rules for what type you end up with when
you mix signed and unsigned in expressions are complicated, and they
changed between K&R and ANSI C (you might have heard this under the
name "unsigned preserving vs. value preserving"). Worse, they depended
on the underlying hardware, so they varied from platform to platform,
causing bugs in all kinds of unexpected places. The book "Expert C
Programming" goes into this in more depth (page 25). So, to avoid
bringing over the hidden complexities, Java does not bring over
unsigned types from C.
Use type char if you are OK with 16-bit unsigned quantities. Otherwise,
go to the next larger type and use masking. If you don't have to worry
about division by numbers with the high bit set, then when you use >>>
for right shift and remember to AND each byte with 0xFF where they are
being expanded into a larger type, then there is no difference in the
bit patterns generated.
Specifically, to convert an int to its unsigned representation, use:
((long)i) & 0x00000000FFFFFFFFL
This promotes the signed (and possibly negative) int to long (with sign
extension) then chops off the sign-extension, leaving it as the same
32-bit quantity held in a 64-bit type.
Also worth noting is that if you're going to work with unsigned bytes,
int is a more efficient larger type to use than short or char, since
smaller values have to be promoted to int to do any arithmetic or
testing on them.
14. (Sect. 6) What happened to "private protected"?
[*] It first appeared in JDK 1.0 FCS (it had not been in the betas).
Then it was removed in JDK 1.0.1. It was an ugly hack syntax-wise, and
it didn't fit consistently with the other access modifiers. It never
worked properly: in the versions of the JDK before it was removed,
calls to private protected methods were not dynamically bound, as they
should have been. It added very little capability to the language. It's
always a bad idea to reuse existing keywords with a different meaning.
Using two of them together only compounds the sin.
The official story is that it was a bug. That's not the full story.
Private protected was put in because it was championed by a strong
advocate. It was pulled out when he was overruled by popular
acclamation.
Inheritance
15. What are the differences between an interface and an abstract class?
[*] Some use a semantic distinction: an abstract superclass models the
"is" relationship, while an interface models the "has" relationship.
The rule would be, if it's a subtype, inherit; otherwise, implement.
But, in the absence of real-world characteristics to distinguish the
objects from their properties and parents, that becomes a circular
argument. In this case, you have to look at the practical differences
in Java (compared with C++).
Most differences between interfaces and abstract classes stem from
three characteristics:
1. Both define method signatures that a derived class will have.
2. An abstract class can also define a partial implementation.
3. A class can implement many interfaces, but inherit from only one
class.
In greater detail, these topics are:
1. Method signatures Both interfaces and abstract classes permit one
to treat the derived-type class as the derived-from-type class.
Both define a set of available methods in a way that can be
enforced by the type-checking mechanism. This is typically used to
permit different (derived) types to behave in the same way (as
what they are derived from - i.e., they all support particular
methods). For example, all java.* types can be printed as Strings
because Object, the superclass of all java.* types, has a
toString() method. Similarly, all types that implement the
Observable interface can be passed an Observer to signal when an
event has occurred. This permits an algorithm or service to
operate on different (derived) types as if they were the same
(derived-from) type.
This mechanism supports not only polymorphism (one object treated
as another), but differentiation. In either case, the (derived)
types can implement the method in the way appropriate to that
type. However, you're not likely to override inherited
functionality, but you must implement interface methods, so if you
expect significant differentiation, then an interface might be
warranted.
Finally, this mechanism supports a weak variant of access control.
Only inherited methods are available to callers who only have a
reference to the superclass or interface type. It's weak because
they can attempt a narrowing cast if they know their target type.
Nonetheless, it reduces some complexity.
2. Inheriting implementation Inheriting an implementation is useful
where the code should be shared. This happens where derived types
vary the functionality only a little bit, or where a complex set
of method interfaces can through mutual reference be implemented
with relatively few methods that can be implemented by derived
types. You can also reuse code by having your class use or keep an
object of another type that implements that code, but that doesn't
permit your callers to treat you in a particular way. To both
"get" functionality and to be treated "as" the superclass are the
essentials of the type/subtype relationship.
3. Java's rule of single inheritance Java differs from C++ in
permitting only single inheritance. This makes for some difficult
choices, if you would like to share combinations of inherited
functionality and polymorphism from more than one source. However,
it does reinforce the notion of inheritance as a subtyping (is)
relationship, and implicitly that type relationships form a tree
rather than a network.
Other differences to consider:
1. Abstract class implementations may include fields
2. Interfaces may include final data members
3. It is slightly slower to call an implemented method via an
interface reference. There is an even smaller penalty for calling
a superclass method via a subclass reference (i.e., where the
subclass does not override the method). There is almost no penalty
for calling a subclass method via a superclass reference. (All are
compared to a direct method call, i.e., calling the derived class
method via a derived class reference).
16. (Sect. 6) How do static methods interact with inheritance?
[*] Static (per-class, rather than per-object) methods do not
participate in overriding (choosing the right method at runtime based
on the class of the object). Probably the best and simplest way to
think about this (and to write your code) is to write every invocation
of a static method using the fully qualified class name:
class A {
public static method1() {
A.method2();
}
public static method2() {
}
}
class B extends A {
public static method3() {
A.method1();
}
public static method2() {
}
}
Now it is perfectly clear that the static method2() that is called is
A.method2(), not B.method2(). A.method2() will be called regardless of
whether you use the fully-qualified class name or not, but using "A."
makes it obvious to all.
17. (Sect. 6) Why is the String class final? I often want to override it.
[*] Being final guarantees that instances of String are read-only. (The
String class implements read-only objects, but if it were not final it
would be possible to write a subclass of String which permitted
instances to be changed.) Strings need to be read-only for security and
efficiency.
As for efficiency, Strings are very commonly used, even behind the
scenes by the Java compiler. Efficiency gains in the String class yield
big dividends. If no one can change a String, then you never have to
worry about who else has a reference to your String. It's easier to
optimize accesses to an object that is known to be unchanging.
Security is a more compelling reason. Before String was changed to be
final (while Java 1.0 was still in beta) there was a race condition
which could be used to subvert security restrictions. It had to do with
one thread changing a pathname to a file after another thread had
checked that the access was permitted and was about to open it.
There are other ways to solve these problems, but the designers
preferred making String final, particularly since the StringBuffer
class is available as an alternative.
18. (Sect. 6) If I extend/subclass a class, are the constructors inherited?
[*] "Constructor declarations are not members. They are never inherited
and therefore are not subject to hiding or overriding." The default
constructor is not inherited, but provided. (See JLS 8.6.7 Default
Constructors)
If you don't give your child class any constructors, a default no-arg
constructor that invokes the superclass' constructor is provided for
you. If the superclass doesn't have a no-arg constructor, you should
create a constructor and call the appropriate superclass constructor.
Also in the FAQ:
Compiler message No constructor {superclass}()
Other sites:
JLS 8.6.7 Default Constructors
19. (Sect. 6) How can I safely store particular types in general
containers?
I often want to store particular types of objects but don't want to
subclass my basic storage classes to enforce the particular type; that
would make for too many subclasses (e.g., IntegerLinkedList,
StringLinkedList, etc.).
[*] Generic programming in java (the rough equivalent of C++'s
templates) works reasonably well since all java classes are subclasses
of Object. There is, however one potential problem - there is always a
possibility that a generic container may contain different classes of
objects.
This naturally leads to the question of how to do this in a type-safe
way. If you've created a generic LinkedList class, how can you be type
safe without having to create a multitude of subclasses
(IntegerLinkedList, StringLinkedList, etc.)?
One way to handle this would be to offer up an additional constructor
in your generic class that takes a parameter of type "Class" and uses
that parameter along with Class's "isInstance" method to guarantee that
Objects added to the container are the expected type.
public class LinkedList {
Protected Class type = Object.class;
public LinkedList(Class type) { this.type = type; }
public void addElement(Object element) throws Exception
{
if(!type.isInstance( element ))
throw new Exception(
"Expected element of type (" + type + ")" +
" got element of type (" + element + ")" );
...
}
}
Note that the comments in the source for isInstance() refer to a
"specified Class parameter", suggesting that you are supposed to write
something like:
public void addElement(Object element) throws Exception
{
Class c = element.getClass();
if(!type.isInstance(c))
This works, but the documentation for isInstance is clear that the
parameter should be an Object rather than a Class. Also, note that
"Collections" are coming in JDK 1.2, and they provide a much safer and
more extensible mechanism. More information about this is available at
the Java Developer Connection at the Java website: http://java.sun.com/
Method interfaces
20. How do I send a variable number of arguments to a method?
[*]
1. (Easy) Use method overloading to support different parameters.
This makes things easy on the caller but can get out of hand if
you want to support a wide number and variety of parameter types.
You should ask yourself if your code design is well-organized if
you need to do this.
2. (More complicated) Use arrays. It's even possible to declare
arrays inline as shown below:
foo("A param",
new Object[] {"param3", "param4", new Integer(5)} );
// ...
void foo(String param1, Object param2[]) {
System.out.println(param1);
for (int i = 0; i < param2.length; i++) {
System.out.println(param2[i].toString());
}
}
You can even pass arrays of arrays using this method. Of course,
inside the method you need to be able to decode what the arguments
are and how you use them.
3. Alternatively you can invent a class that just contains all the
possible fields you might want to pass into a method (plus
booleans to say if each field is set or not), and make an object
of that class be a parameter to the method. You can return
multiple values from a method the same ways; either have the
method return an array or a wrapper object.
However, remember the wise words of Professor Alan Perlis, "if your
procedure has more than about half a dozen parameters, you probably
forgot a few." Passing large numbers of arguments into a function
suggests your function is badly organized.
21. (Sect. 6) How can I return a different object in a method parameter?
How can I pass an object to a method, and have the method change the
reference so it points to a different object back in the calling code?
indirection". Wrap the object in another class, whose purpose is simply
to be passed as a parameter, allowing the nested object reference to be
modified.
The second alternative is a clearer variant of this. Pass in a single
element array. Since arrays are objects, this works.
void jfoo(Object ref[]){
ref[0] = new Object();
}
...
Object kludge[] = new Object[1];
kludge[0]= myObj;
jfoo(kludge);
if (kludge[0] == myObj) ...
else ...
Note that changing a global variable/object inside a method is an
egregious programming practice; it usually violates basic OOP
constructs.
22. (Sect. 6) How do I get multiple return values back from a method?
[*] You can just have the function return a Vector. This is
particularly convenient when you're not sure how much you are going to
be returning, based on what occurs in the method. A Vector is
essentially a dynamically-growable array. Regular arrays can't grow
after you declare them - you have to declare a bigger array and move
the old stuff into it.
Arrays
23. (Sect. 6) How do I allocate a multidimensional array?
[*] There are several ways. If you want a rectangular array, you can
allocate the space for the array all at once. The following creates a
4x5 array:
int arr[][] = new int[4][5];
If you want each row to have a different number of columns, you can use
the fact that a two-dimensional array is actually an array of arrays.
The following code allocates a triangular array:
int arr[][] = new int[4][]; // allocate the four row arrays
for (int i = 0; i < 4; i++) // initialize each of the four rows
arr[i] = new int[i + 1]; // row i has i + 1 columns
Note that if you allocate an array of any kind of object (as opposed to
primitive type), all the references will be null by default. These null
references can result in NullPointerExceptions if you try to
dereference them.
In other words, after doing:
int arr[] = new int[4];
you can say
if (arr[2] == 0)
But after doing
Integer Iarr[] = new Integer[4];
you must fill in the object reference before using it. E.g.,
Iarr[2] = myInt;
or
arr[2] = new Int(27);
before you can say
if (Iarr[2].equals(myInt))
24. (Sect. 6) How do I copy an array?
[*] If the array only contains primitive types or if you want to copy
only the object references, not duplicate the objects, then use the
method
java.lang.System.arraycopy(Object src, int src_position,
Object dst, int dst_position, int length);
Otherwise, if you want to duplicate the objects, you have to initialize
your new array and write a loop that duplicates each object in the old
array into the new.
Note that the documentation for arraycopy() says that if src and dst
refer to the same object, then arraycopy behaves as if the source array
elements are copied into a temporary array (i.e., they are preserved).
Some interpret this as meaning a temporary array will be so allocated,
but that's not Sun's implementation.
Other sites:
JLS 20.18.16 {java.lang.System.arraycopy()}
25. (Sect. 6) How do I clear an array?
[*] There is no method to clear an array to 0.0, 0, null, false,
'\u0000' etc. When you allocate an array, the elements are set to their
default values, but that doesn't help when you want to reuse an array.
If you want to set the same array to the same set of values many times,
create a template array. Fill it with the reset value, then use
System.arraycopy() to copy it into the work array each time you need to
set the work array.
26. (Sect. 6) What is a fast way to set all elements of an array?
I don't want to use a template array. I would like to set all array
elements to a given value without duplicating the (possibly large)
array.
[*] Using a loop that does it one by one is probably 20 to 40 times
slower than good old memset() in C.
A fast way on many VM's is to set the first byte of the array, then use
System.arraycopy() repeatedly to fill the next byte, the next two
bytes, the next four bytes, the next eight bytes, etc., and when you
get past halfway, fill in the rest.
public static void bytefill(byte[] array, byte value) {
int len = array.length;
if (len > 0)
array[0] = value;
for (int i = 1; i < len; i += i)
System.arraycopy( array, 0, array, i,
((len - i) < i) ? (len - i) : i);
}
This is faster on Sun's VM than a simple loop, and probably even faster
under JITs because it only performs at most log2(array.length) bounds
checks. This is a clever code idiom applying the binary chop algorithm
to arrays even when their size is not a power of 2.
-------------------------------
7. I/O
1. (Sect. 7) How do I read a file containing ASCII numbers?
[*] There are several ways to do this. Here is one way. Let's assume
your file is called "C:\work\mydata.txt" and it contains lines like:
135 7512 3659814 328 1 54829
68522 19982810 38
i.e. lines contain several ASCII strings that are numbers separated by
spaces.
The code fragment is as follows:
// Open the file with
RandomAccessFile f = new RandomAccessFile("c:\\work\\datafile.txt", "r");
// read an entire line from it
String s= f.readLine();
// get some methods to break up a line into tokens
StringTokenizer st = new StringTokenizer(s);
// extract the next int from the line
i = Integer.parseInt(st.nextToken());
We use a RandomAccessFile because that supports the readLine() method
directly. An alternative would be to instantiate a FileReader, and wrap
a BufferedReader around it. Putting it all together, including the
exception handling in the event the file is missing, the code looks
like:
import java.io.*;
import java.util.*;
public class c {
public static void main(String args[]) {
try {
RandomAccessFile f = new RandomAccessFile
("datafile.txt", "r");
String s;
while ( (s=f.readLine()) != null ) {
System.out.println("read: "+s);
StringTokenizer st = new StringTokenizer(s);
int i=0;
while (st.hasMoreTokens()) {
i = Integer.parseInt(st.nextToken());
// i now holds the next int on the line
// could also use Double.parseDouble(), etc.
System.out.print(" "+ i);
}
System.out.println();
}
} catch (Exception e) {System.out.println("Excpn: "+e); }
// file I/O, from book "Just Java" by Peter van der Linden
}
}
See also the next question on how to read data interactively.
2. (Sect. 7) How do I read a String/int/boolean/etc from the keyboard?
[*] The easiest way is to pick up the source for the 100% pure Java
class EasyIn from http://www.afu.com/ (same place as this FAQ). Compile
it with your code and use it like this:
EasyIn easy = new EasyIn();
int i = easy.readInt(); // gets an int from System.in
boolean b = easy.readBoolean(); // gets a boolean from System.in
double d = easy.readDouble(); // gets a double from System.in
... etc.
EasyIn is free, comes with source, and you can do what you like with
it, including improve it, and send me back the results.
If, instead, you want to "roll your own" code (why?!), in JDK 1.0.2
java.io.DataInputStream in = new java.io.DataInputStream(System.in);
String s = in.readLine();
One way in JDK 1.1:
java.io.BufferedReader in =
new java.io.BufferedReader( new InputStreamReader(System.in));
String s = in.readLine();
Once you have the token in a String, it is easy to parse it into one of
the other types, as shown earlier in the FAQ. Yes, it is bone-headed,
as it makes the simplest case of keyboard I/O unnecessarily
complicated. A bug was filed with Javasoft to record this problem, but
don't count on this being fixed any time soon.
3. (Sect. 7) Why do I have trouble with System.out.println()?
[*] Check the spelling. The last two characters are the letters "ell
enn" not "one enn".
The name of the method stands for "print line", since it prints a
String and goes to the next line, rather than staying on the same line
as System.out.print() does. Yes, the name is yet another Java naming
inconsistency, since the input equivalent is readLine(), not readln().
4. (Sect. 7) How do I write to the serial port on my PC using Java?
[*] There is a platform-independent serial port API introduced in JDK
1.2. You can download the documentation by registering with the Java
Developer Connection (it's free, http://java.sun.com) and browsing
http://java.sun.com/jdc/earlyAccess/communications.html.
For systems prior to JDK 1.2, read on. At least two companies have
written a library to drive the port. See
o http://www.sc-systems.com has a library for Windows 95, WindowsNT,
OS/2, Macintosh PPC, Solaris Sparc, Linux x86, FreeBSD x86, HP/UX
PA-RISC, and possibly others too.
o http://www.cd.com/portio
o In addition, there is a Unix serial port utility available with
source at http://jarvi.ezlink.com/rxtx/ It's free under the GPL,
and works on Linux, Irix, Solaris, Windows 95, and NT.
While not helpful to typical home users, there is an alternative
portable COM port solution for Java 1.1 and even 1.0. Buy your COM
ports in the form of "terminal servers". Using a COM port is now as
easy as connecting to it with a Socket. Port parameters can be changed
programatically using SNMP for most terminal servers (but this is never
necessary when a modern modem or other fixed-rate equipment is
attached). Any networked box can serve as a terminal server - even
Win95 - with a simple native server application for that box, but
buying an actual firmware based hardware box is much easier.
Furthermore, your Win95 native applications can now share the COM ports
(and any attached modems) via a Win95 product called "Dial-out IP" at
http://www.tactical-sw.com/.
If the port exists as a pathname in the filesystem, you can open it as
a file and read/write. You can also print text this way by writing to
"prn" or "lpt1" on a pc, and "/dev/something" on Unix. Writing a
formfeed at the end of the file is essential on Windows 95. Here is
some sample code:
// class that opens the printer as a file
// and writes "Hello World" to it
import java.io.*;
public class lpt {
public static void main (String[] argv) {
try {
FileOutputStream os = new FileOutputStream("LPT1");
//wrap stream in "friendly" PrintStream
PrintStream ps = new PrintStream(os);
//print text here
ps.println("Hello world!");
//form feed -- this is important
//Without the form feed, the text will simply sit
// in print buffer until something else gets printed.
ps.print("\f");
//flush buffer and close
ps.close();
} catch (Exception e) {
System.out.println("Exception occurred: " + e);
}
}
}
If you wish to change the characteristics of the port (e.g. baud rate,
parity, etc.), not just read/write data, Java currently offers no
portable way to do this. You will need to use one of the packages
mentioned above or some native code or a system command.
5. (Sect. 7) How do I append to a file?
[*] There are two ways. JDK 1.1 introduced new constructors for two of
the output classes that allowed you to set a boolean flag:
public FileWriter(String fileName, boolean append) throws IOException
public FileOutputStream(String name, boolean append) throws IOException
Another way is to do this:
RandomAccessFile fd = new RandomAccessFile(file,"rw");
fd.seek(fd.length());
Then write using fd. Note that the latter method does not take
advantage of the "append" mode present in many operating systems (such
as all Unixes). Such a difference may make a difference with multiple
processes or threads appending to the same output file. This can happen
frequently, even if not intended by the programmer, e.g. with logfiles
in multitasking environments.
6. (Sect. 7) Is it possible to lock a file using Java ?
[*] JDK 1.2 introduces the ability to lock a file (indirectly) using
the File class. Use createTempFile() with delete on exit. Prior
releases of Java do not feature an API to lock a file or regions within
a file. Code that needs to do this must take one of four approaches:
1. Implement an advisory locking scheme using features that Java does
have (synchronized methods). This allows you to lock files against
use by other Java code running in the same JVM.
2. Use the atomic operation File.createNewFile(), and mark it as
deleteOnExit(). Have all processes (Java and non-Java) follow the
same protocol: if the create operation succeeded, you have the
lock. To give up the lock, you either delete the file or exit the
JVM.
Note that this may fail if the file is remotely mounted using NFS
version 2. (There's a window of opportunity bewteen the LOOKUP to
see if it's already there, and the CREATE if it's not). However,
NFS version 3 does guarantee exclusive create of remotely mounted
files, and is not subject to this race condition failure.
3. Make calls to native code to issue the locking ioctls. This
approach is not portable, but gives you a shot at having your
locks respected by legacy code in non-Java programs using standard
locking ioctls.
4. Push the work to a central server. Since socket connection
requests arrive in a single queue on the server, this can be used
to serialize lock requests. There might be some merit in copying
the NFS lockd protocol for a general approach. Rolling your own
simple version for a specific application is pretty easy. A
database would be better off locking records or fields, not byte
offsets. In theory, the server socket approach would make it
easier to perform automatic cleanup of a lock on abrupt VM process
failure, e.g. by asking "are you still alive?" to the lock holder
occasionally.
7. (Sect. 7) How do I make the keyboard beep in Java?
[*] In JDK 1.1, java.awt.Toolkit has the method beep(). The pre-1.1
alternative of
System.out.print("\07");
System.out.flush();
(the ASCII BEL character) doesn't work on Macs, but does on some other
platforms. Java doesn't support the C abstraction of '\a' for an alert
character.
8. (Sect. 7) How do you do file I/O from an applet?
[*] For security reasons, untrusted applets accessed across the network
are restricted from doing certain operations, including I/O. This
prevents rogue applets from sending out your private data, or deleting
it. A trusted (signed) applet can perform these operations (JDK 1.1
on).
The simplest approach for server-side I/O is to use the Linlyn class
available from http://www.afu.com. This is free software under the GNU
license, and uses FTP to move files between an applet and the server.
It is suitable for low-volume non-critical use like keeping a
high-score file. The Linlyn class has a very simple application
programmer interface.
o The following suggestion is for server-side input.
You can read a file on the server if you can create a URL
referencing the file. Then open a stream, then use any of the
stream-based methods to read.
This allows reading but not writing. It requires an http daemon
running on the server, which will usually be the case.
try{
URL url = new URL("http://somewhere.com/test.txt");
// or URL url = new URL( getDocumentBase(), filename);
BufferedReader in = new BufferedReader(
new InputStreamReader(
url.openStream() ) );
String s = in.readLine(); //read till you get a null line.
} catch(MalformedURLException e){
System.out.println("URLException:"+e);
} catch(IOException e){
System.out.println("IOException:"+e);
}
}
You cannot write a file on the server this way.
o The following suggestions are for server-side output.
It absolutely requires the cooperation of the server to allow an
applet to write a file to the server. This cooperation may take
any of several forms:
+ FTP server
+ File server (webnfs or custom written)
+ Listening on a socket for data from applets
+ CGI script
+ Java RMI (remote method invocation)
+ JDBC process
In particular:
+ FTP code. Use the Linlyn class mentioned above.
+ WebNFS. This is an evolution of the NFS (Network File System)
to make file resources visible in browsers. More information
at http://www.sun.com/webnfs
+ Open a socket back to the server and read/write the data.
Have a process on the server that listens for socket
connections from applets and does the requisite I/O. This
does I/O on the server.
+ Or use a CGI script or servlet on the server to write when
browsed.
o The following suggestions are for client-side I/O. Use a trusted
applet (see section on security). This will permit local I/O
without any of the restraints mentioned above. In this regard, the
appletviewer and many other browsers regard applets loaded from a
local filesystem (rather than across the net) as being more
trustworthy, and perhaps even allowed to do I/O.
o The simplest form of output is probably for the applet to connect
to the mailserver port on the server, and prompt the user to enter
his email address. Then email him the data to save locally if he
wishes. If a small amount of data he can later enter it by
cut-and-paste into the applet when he next invokes it.
o Or use a browser that has a security policy that is configured to
allow file I/O (such as Sun's appletviewer).
o Read this article
http://www.javareport.com/html/features/archive/9802/somers.shtml
for an introduction to the basics.
9. (Sect. 7) What are Resources files and how do they work?
[*] In Java 1.1 and above, a resource file is a file that your code
should be able to access no matter where it was loaded from: the local
file system, via http, or some other means. This is a different concept
than system resources. See question (TBD) for more information about
system resources. (Volunteer sought to write answer on system resource
files).
There are two ways to specify resources.
o using an absolute path name, such as
"/myPackage/resourcedir/myResource.txt"
With an absolute path name, the resource file is actually relative
to the classpath.
o using a relative path such as "dir/myResource.txt".
If you specify a relative path, then the file is found relative to
where the class loader found the package of the Class that is
loading the resource. The consequence of this is that relative
resource files can only be in the same directory as your class
file or in a directory below.
If you want to access resources from an unsigned applet, use relative
resources.
There are a couple of methods to get at resources, including methods in
the java.lang.Class and java.lang.Classloader. It uses the class loader
because that code knew where to find the class on the filesystem. If
the resource file is nearby, it can be found the same way. One simple
method to get a stream for a resource for a particular class, say
Mypackage.MyClass, is as follows:
String relativePath = "resourceDir/somefile.txt";
String absolutePath = "/somePackage/somefile.txt";
InputStream in=Mypackage.MyClass.class.getResourceAsStream(relativePath);
If the class cannot be loaded, "in" would be assigned a null value.
Otherwise, you can use the inputStream just as any other.
A brief note about the syntax used above. In java 1.1 and above, a
java.lang.Class object for a particular class can be acquired by
appending .class to the class's name. Though it looks like every object
has a static member variable, this is not actually the case.
When using resources with Netscape, be aware of the Netscape
restrictions that:
o Netscape does not implement the getResource() methods in
java.lang.Class and java.lang.ClassLoader, but only
getResourceAsStream() methods.
o All resources must be in a Jar/archive file
o Resource files must have a Netscape-approved extension, or you
must call certain functions before hand. See the following for
details:
http://developer.netscape.com/docs/technote/java/getresource/getresource.html
Internet Explorer does not seem to have these restrictions.
One final "gotcha" is that jpgStream.available() (wrongly) returns 1
when .gif files in the .jar file are compressed! Some people now create
a .jar file in two steps, essentially:
jar cvf x.jar *.class
jar uf0 x.jar *.gif
This ensures .gifs are not compressed, and hence that available()
doesn't lie to you. David Alex Lamb stumbled onto this after wondering
why his .gif files looked bad, then realized that compression of a .gif
might throw away detail. The conjecture is that available() returns 1
because it has to block to do uncompression.
10. (Sect. 7) How do I execute a command from Java? How do I do I/O
redirection in Java using exec()?
[*] See the answer to Question 18.7.
11. (Sect. 7) I used a C program to write a binary file. When I instantiate
a DataInputStream on the file in Java, and try to readInt, I do not get
the correct numbers. Why is this?
[*] Java does everything in network byte order (big-endian order), as
do many computers including Motorola, and SPARC. The Intel x86 uses
little-endian order in which the 4 bytes of an int are stored least
significant first. Rearranging the bytes on the way in will get you the
results you need. This is only necessary when the file was written by a
non-Java program on a little-endian machine such as a PC.
The following code will byte-swap little-endian integers into network
standard order:
public int swap(int i) {
int byte0 = i & 0xff;
int byte1 = (i>>8) & 0xff;
int byte2 = (i>>16) & 0xff;
int byte3 = (i>>24) & 0xff;
// swap the byte order
return (byte0<<24) | (byte1<<16) | (byte2<<8) | byte3;
}
Alternatively, the following code assembles bytes from a byte array
that is in big-endian order (as used by Java) into an int:
byte[] bytes = ... // whatever
int start_index = ... // wherever
int value = 0;
for ( int i = start_index; i < start_index+4; ++i ) {
value = ( value << 8 ) | ( bytes[i] & 0xFF );
}
If the bytes are in little-endian order, just change the "for"
for ( int i = start_index+3; i >= start_index; --i )
And this code will assemble a double that has been written in reverse
byte order:
byte[] gnol = new byte[8];
stream.read(gnol);
long l = (
( (gnol[7] & 0xff) << 56) |
( (gnol[6] & 0xff) << 48) |
( (gnol[5] & 0xff) << 40) |
( (gnol[4] & 0xff) << 32) |
( (gnol[3] & 0xff) << 24) |
( (gnol[2] & 0xff) << 16) |
( (gnol[1] & 0xff) << 8) |
(gnol[0] & 0xff)
);
double d = Double.longBitsToDouble(l);
12. (Sect. 7) How do I make I/O faster? My file copy program is slow.
[*] This is the purpose of BufferedInputStream. It is a flaw in Java
that buffered I/O is not the default, with a flag or different
constructor to turn it off. I/O is the second worst designed package in
Java, after the Date class. ,
13. (Sect. 7) How do I do formatted I/O of floating point numbers?
[*] Use the class java.text.NumberFormat.
Or use http://www.newbie.net/sharky/lava/. Or use Cay Horstmann's
http://www.horstmann.com/corejava/Format.java
Although many utilities claim to handle all varieties of C's printf, as
far as has been found, this is the only one to correctly handle the
equivalent of %e in printf.
See also the standard packages java.text.DecimalFormat and
java.text.NumberFormat
14. (Sect. 7) How do I read numbers in exponential format in Java?
[*] The program below (written by Steve Chapel) uses StreamTokenizer to
read data from the standard input and recognizes doubles in exponential
format (e.g. -1.23e-45).
import java.io.*;
public class ReadExponential {
public static void main(String argv[]) {
DataInputStream in = new DataInputStream(System.in);
StreamTokenizer st = new StreamTokenizer(in);
try {
while (st.nextToken() != StreamTokenizer.TT_EOF) {
switch (st.ttype) {
case StreamTokenizer.TT_NUMBER:
double num = st.nval;
int exp = 0;
st.ordinaryChars('\0', ' ');
st.nextToken();
st.whitespaceChars('\0', ' ');
if (st.ttype == StreamTokenizer.TT_WORD &&
Character.toUpperCase(st.sval.charAt(0)) == 'E') {
try {
exp = Integer.parseInt(st.sval.substring(1));
} catch (NumberFormatException e) {
st.pushBack();
}
} else if (st.ttype < 0 || st.ttype > ' ')
st.pushBack();
System.out.println("Num " + num * Math.pow(10, exp));
break;
case StreamTokenizer.TT_WORD:
System.out.println("Word " + st.sval);
break;
default:
System.out.println("Char '" + (char) st.ttype + "'");
break;
} // end switch
} // end while
} catch (IOException e) {
System.out.println("IOException: " + e);
}
} // end main
}
15. (Sect. 7) I'm trying to read in a character from a text file using the
DataInputStream's readChar() method. However, when I print it out, I
get ?'s.
[*] Remember that Java characters are 16-bit Unicode characters, while
many hosts systems store characters as 8-bit ASCII characters.
Therefore, to read individual chacters from a text file, you need to
ensure the proper conversion. The proper way to do this is to use an
InputStreamReader, which converts from 8 to 16 bit streams:
FileInputStream fis = new FileInputStream("myfile.txt");
InputStreamReader isr = new InputStreamReader(fis);
char c3 = (char) isr.read();
The less-favored way (because it is not so portable, as the encodings
translation is not done) is just to read a byte and cast it into a
character:
FileInputStream fis = new FileInputStream("myfile.txt");
DataInputStream dis = new DataInputStream(fis);
char c1 = (char) dis.readByte();
16. (Sect. 7) How do I delete a directory in Java?
[*] JDK 1.0 did not support directory removal. JDK 1.1 supports
directory removal with the method:
public boolean delete() in class java.io.File
Make sure you don't have any open streams in the directory you're
trying to remove. Do a close() on all streams, even if the underlying
file is gone.
17. (Sect. 7) How do I tell how much disk space is free in Java?
[*] There currently aren't any good Java APIs for system introspection.
There is no Java way to control processes, or look at system resources.
You can use Runtime.getRuntime().exec() to do "df" on unix or "dir" on
Windows right now.
Alternatively, check out JConfig:
http://www.tolstoy.com/samizdat/jconfig.html
JConfig is a cross-platform library that fills in many of the gaps in
the core Java API, and makes it possible to work with files, processes,
file types, video monitors, etc. in a much more Windows- and
Mac-friendly manner.
18. (Sect. 7) How do I get a directory listing of the root directory C:\ on
a PC?
[*] The obvious approach of calling File.list("C:\"); does not work.
There are two reasons why this fails. First, slash is an escape
character in Java, so if you want a literal slash, you have to repeat
it. Second, you need to give the name of the directory, i.e. dot.
Putting this together, either of the following calls will work
File.list("C:\\.");
or
File.list("C:/.");
Note: a file separator of "/" works just as well as "\" in most Windows
programs and library calls. It is an artifact of DOS's origin's as a
ripped-off port of CP/M. When Microsoft bought the rights to DOS from
Seattle Computer Products (what, you didn't know Microsoft didn't
develop DOS? They didn't even own the rights to DOS at the time the
contracted with IBM to supply DOS for the PC) they were buying software
which was an unauthorized port of the CP/M operating system.
CP/M didn't have directories, so it didn't use pathname separators. The
forward slash "/" was already used for giving options to CP/M commands,
so "\" was pressed into service as the DOS pathname separator, but the
DOS shell was taught to understood "/" for compatibility with other
OS's. See also JConfig in Q6.15.
19. (Sect. 7) What is the difference between the various ZIP formats: ZIP,
GZIP, and PKZIP?
[*] Zip is an archive file format, popularized on PCs, that contains
multiple compressed files.
GZIP comes from Gnu. It is essentially a one file subset of the Zip
format. You can't put a whole directory into a GZIP archive, just a
single file. It's intended for compressing a tarball of many files.
PKZIP is a set of commercially available programs that create Zip
files. All three use the deflate compression format, which is based on
the LZ77 algorithm. This compression is also used by the ZLIB library
and hence the PNG graphics file format (which uses ZLIB). PNG -
Portable Network Graphics - provides a patent-free replacement for GIF
and TIFF.
An alternative compression technology, LZW compression, is encumbered
by Unisys's patent. LZW is used in GIF files and by the Unix compress
command. Luckily, as well as being free from patent restrictions, LZ77
also gives better compression than LZW. LZW is the initial letters of
the last names of the three computer scientists who developed the
algorithm (Lempel, Ziv, Welch).
The basic classes (all in java.util.zip) that read LZ77 Zip format are
Deflater and Inflater. These are used by the stream classes
DeflaterOutputStream and InflaterInputStream. The java.util.zip classes
GZIPInputStream and ZipInputStream inherit from InflaterInputStream.
PKZIP is a commercial program for DOS, Windows, and OS/2, sold by
PKWARE Their FAQ, at http://www.pkware.com/zipgfaq.html, specifically
says
"Because PKWARE has dedicated the .ZIP file format to the public
domain, it is possible for other people to write programs which
can read .ZIP files. NOTE THAT THE PKZIP, PKUNZIP, PKSFX PROGRAMS
AND THEIR ASSOCIATED SOURCE CODE AND SUPPORT PROGRAMS ARE THE
EXCLUSIVE PROPERTY OF PKWARE INC. AND ARE NOT PUBLIC DOMAIN
SOFTWARE PROGRAMS.
The "other people" PKZIP's FAQ refers to is the InfoZIP project, a
group of public-minded programmers spread over the world producing free
software that works on most ANSI C compilers and platforms. See
http://www.cdrom.com/pub/infozip/.
Jar files are in ZIP format, but are not as complete as a full
filesystem archive format since file permissions are not saved. Some
versions of WinZip are known to be inadequate for processing the full
PKZIP format. Use InfoZIP instead.
20. (Sect. 7) How can I use characters other than ASCII in Java?
[*] Search for the article titled "Adding Fonts to the Java Runtime" or
visit:
http://java.sun.com/products/jdk/1.2/docs/guide/internat/fontprop.html
The article explains how to add fonts to Sun's JDK 1.2 using the
font.properties file. [If anyone has summarised the information, please
send it in].
21. (Sect. 7) I did a read from a Buffered stream, and I got fewer bytes
than I specified.
[*] This is the way that BufferedInputStream works up to and including
the current release. The behavior is so unintuitive that it really
represents a bug. Javasoft has "resolved" the bug by writing comments
in the program so that the broken behavior is in the range of legal
outcomes. Ugh.
When you instantiate a buffered input stream, you can specify the size
of buffer it should use. Let's call this the internal buffer. When you
call read() you can say how many bytes to read. Let's call this the
request. If the request is smaller than the internal buffer and not a
multiple of the internal buffer, then the last read returns only the
odd bytes left in the internal buffer! The more reasonable and
intuitive behavior would be for the internal buffer to be refilled, so
that the whole request can be granted.
For example, if you create a BufferedInputStream with an internal
buffer of 1000 bytes, and try to read 512 byte chunks, your first read
will return 512 bytes, but your second read will only return
(1000-512), or 488, bytes. (Assuming that the file has at least that
many bytes in it). The following code illustrates the problem.
// troubleshooting by Tov Are Jacobsen
import java.io.*;
class filebug {
public static void main(String args[])
throws FileNotFoundException, IOException {
BufferedInputStream bis =
new BufferedInputStream(
new FileInputStream("test.txt"), 1000 );
byte[] buf = new byte[2000];
int numread;
System.out.println( "Available: "+bis.available() );
while (true) {
numread = bis.read(buf,0,512);
if (numread<0) break;
System.out.println( "got "+numread
+", avail:"+ bis.available());
}
}
}
Of course, a valid reason for getting less than you asked for is that
you asked for more data than is actually available in the Stream, e.g.
you requested 512 bytes from a file that only contains 40 bytes. In
general, there are no guarantees about how much data is returned for a
given buffered input stream read request. To avoid this problem, push a
DataInputStream on top of your buffered stream. Then you can call
readFully(), which will do what you want.
A similar "got less than I asked for" occurs when reading a socket.
Network protocols frequently packetize data and send it across in
bursts. Nothing is lost of course, and you are always told how many
bytes you actually got. You will get the remaining bytes on a
subsequent read. This happens regardless of the language used. Be sure
to check the "amount of data returned" when using the read(byte[], int,
int) method of BufferedInputStream, or when reading from a socket.
Another problem with java.io.InputStream.read(byte[], int, int) is that
it catches and ignores IOExceptions. Instead, these exceptions should
be passed on to the caller. Ace programmer Jef Poskanzer, [email protected],
has a version to do this at
http://www.acme.com/java/software/Acme.Utils.html. See Jef's read() and
readFully() routines.
22. (Sect. 7) How do I redirect the System.err stream to a file?
[*] You cannot assign a new FileOutputStream to System.err, as it is
final. Instead use the System.setErr() library call, like this:
FileOutputStream err = new FileOutputStream("stderr.log");
PrintStream errPrintStream = new PrintStream(err);
System.setErr(errPrintStream);
This was introduced with JDK 1.1. There is also a corresponding setIn()
for redirecting standard in, and a setOut() for standard out.
Note that you will get a compiler warning about a deprecated construct
when you do this. PrintStreams are deprecated in favor of PrintWriters,
which handle Unicode properly. The PrintStream is marked as deprecated
by marking all its constructors as deprecated. There is no way to
create the PrintStream needed to redirect System.err or System.out
without triggering the deprecated warning.
23. (Sect. 7) What are the values for the Unicode encoding schemes?
[*] If you review the String constructor with this signature
String(byte[] bytes, String encoding)
you can see that one argument is a value for the encoding scheme that
the conversion of 8-bit bytes to 16-bit Unicode chars is to use.
There are three: "Unicode", "UnicodeBig" and "UnicodeLittle". The first
one expects the first two bytes of your data to be a Byte Order Mark,
FEFF or FFFE, which specifies whether the data is in little-endian or
big-endian order. If there isn't a BOM but you already know the
endianness, you can use "UnicodeBig" or "UnicodeLittle" directly.
There is also a Sun document at
http://java.sun.com/products/jdk/1.1/docs/guide/intl/encoding.doc.html
with some related information (not much).
There is another Sun document at
http://java.sun.com/products/jdk/1.1/intl/html/intlspec.doc7.html which
shows the table of encodings. There is a new system property called
"file.encoding" which translates between codes known to Java like
"Cp1252", and locale encoding names like "Windows Western Europe /
Latin-1"
24. (Sect. 7) Is there a way to read a char from the keyboard without
having to type carriage-return?
[*] You can do this in a GUI (e.g. in a text component). There is no
pure Java way to do character-by-character I/O without using a GUI.
-------------------------------
8. Core Libraries
1. (Sect. 8) I can't seem to change the value of an Integer object once
created.
[*] Correct. Integer (Float, Double, etc) are intended as an object
wrapper for a specific value of a number, not as a general-purpose way
of shipping a primitive variable around as an Object. If you need that
it's easy enough to create: class General { public int i; }
2. (Sect. 8) How do I print from a Java program?
[*] Use the Toolkit.getPrintJob() method
Component c = this.getParent();
while (c!=null && !(c instanceof Frame))
c=c.getParent();
// With a JComponent use c=getTopLevelAncestor();
PrintJob pj = getToolkit().getPrintJob((Frame) c, "test", null);
Graphics pg = pj.getGraphics();
printAll(pg);
pg.dispose();
pj.end();
This feature was introduced with JDK 1.1. A common place to put this is
in the code that handles a button press. Printing from an untrusted
applet is subject to a check from the SecurityManager.
The JDK 1.1 printing API is more a screen hardcopy facility than a full
blown publishing and illustration hardcopy API. JDK 1.2 offers a more
full-featured printing API.
If you simply want to print text, then write it to a file and print the
file. Or open a filename that corresponds to the printer. On Windows,
that is "LPT1" and the code looks like:
try {
FileOutputStream fos = new FileOutputStream("LPT1");
PrintStream ps = new PrintStream(fos);
ps.print("Your string goes here");
ps.print("\f");
ps.close();
} catch (Exception e) {
System.out.println("Exception occurred: " + e);
}
The final formfeed is needed by windows to start the printjob.
3. (Sect. 8) What are the properties that can be used in a PrintJob?
[*] The properties are
o awt.print.destination - can be "printer" or "file"
o awt.print.printer - printer name
o awt.print.fileName - name of the file to print
o awt.print.numCopies - obvious
o awt.print.options - options to pass to the print command
o awt.print.orientation - can be "portrait" or "landscape"
o awt.print.paperSize - can be "letter","legal","executive" or "a4"
paperSize=letter, and numCopies=1.
You can search for info like this by joining the Java Developer
Connection (it's free) at http://java.sun.com/jdc.
and doing a search for "PrintJob".
4. (Sect. 8) Is there any package in Java to handle HTML?
[*] See the answer to Question 13.14.
5. (Sect. 8) Why don't Dialogs work the way I want them to?
[*] Modal dialogs (dialog windows that stay up until you click on them)
are buggy in many browsers and in the 1.0.2 JDK. One bug is that the
dialog is not necessarily put on top when it is displayed. Most of the
modal dialog bugs are fixed in JDK 1.1.
6. (Sect. 8) Where can I find information about the sun.* classes in the
JDK?
[*] You're not supposed to. Those classes are only to support functions
in the java.* hierarchy. They are not part of the API, and won't be
present in Java systems from non-Sun vendors. Some people have
reverse-engineered the code and published an API for these classes but
you use it at your own risk, and it may change without warning.
Worst of all, those programs will not have the portability of true Java
but will only run on Sun JDKs. For the same reason you shouldn't use
classes outside the java.* packages when using JDKs from other vendors.
If you still insist on going ahead, check these URLs:
http://java.sun.com/products/api-overview/index.html
http://www.parmly.luc.edu/javaudio/
http://www.users.interport.net/~mash/javamidi.html
7. (Sect. 8) How do you read environment variables from with a Java
program?
[*] Environment variables are not used in Java, as they are not
platform-portable. The Mac doesn't have environment variables, for
example. A Windows 95 application not started from a DOS window does
not have environment variables. Use properties instead. It was a design
error in JDK 1.0 that programmers had to set the CLASSPATH environment
variable. This should have been set in a property file
Create your own properties file (see java.util.Properties) or specify
them with the -D option when you invoke the interpreter or JRE.
Additionally, on some systems you can set a property from the command
invocation line like this:
java -Dfoo=$foo MyClass (Unix)
or
java -Dfoo=%foo% MyClass (Win95/NT)
This sets the "foo" property to the value of the environment variable
foo, and makes it available in the System properties. Make sure you do
not leave any spaces after the -D or around the = sign. Inside the
program you get the value with:
String env = System.getProperty("foo");
More simply, just put the environment variable on the command line and
read it as arg[0].
java MyClass %FOO% ; Win32
java MyClass $FOO ; Unix
Finally, you could execute a Runtime process to get the environment
variables if you are on a platform that has them.
import java.io.*;
import java.util.Properties;
public class Main {
public static void main(String[] argv) {
Properties envVars = new Properties();
try {
envVars.load( // use "set" on Windows
Runtime.getRuntime().exec("/bin/env").getInputStream());
} catch (Throwable t) {t.printStackTrace();}
System.out.println("\n\n" + argv[0]
+ " = <" + envVars.get(argv[0]) + ">");
}
}
This is not a Pure Java approach as it builds platform-specific
knowledge into the program. See Question 10.6 for more details. On
Unix, the command that prints environment variables is "/usr/bin/env".
On Windows95, it is "set"
8. (Sect. 8) How do I get Java talking to a Microsoft Access database?
[*] Use the JDBC-ODBC bridge. It is not especially challenging to set
up, but it does require painstaking attention to detail. There is a
step-by-step example in the van der Linden text "Just Java" mentioned
in the sponsorship section of this document.
Note that the Microsoft version of the Java kit does not support
JDBC-ODBC access because it uses a non-standard native code interface.
The JDBC FAQ can be found at
http://java.sun.com/products/jdbc/jdbc-frequent.html
9. (Sect. 8) I can't seem to change the current working directory.
[*] Correct. This missing functionality is an oversight that we hope
will be corrected in the future. The bug id is 4156278, please feel
free to join the JDC, and vote to have this (or any other) fixed.
Changing the user.dir property merely changes the text property, not
the underlying reality that it is supposed to reflect.
There are several workarounds.
o Run your java app from a .bat or .sh file and do the "cd" in that
(before you run your java app), assuming that all the external
processes you need to exec can be run from the same directory.
o Do: exec("cd /home/wherever; externalApp.exe") on unix, (there
doesn't seem to be an equivalent on NT).
o Instead of running the .exe directly, run (or write on the fly) a
.bat or .sh file that does the cd and then runs the .exe for you
(this could well create trouble with getting back the correct
return status)
10. (Sect. 8) How do I create a Vector of ints?
[*] ints are primitive types and hence can't be stored by the Vector
class, which stores objects. You'll need to wrap the ints. Try this:
int i =7;
Vector holdsInts = new Vector(5,1);
holdsInts.addElement(new Integer(i));
int j = ((Integer)holdsInts.elementAt(0)).intValue();
11. (Sect. 8) I have several worker threads. I want my main thread to wait
for any of them to complete, and take action as soon as any of them
completes. I don't know which will complete soonest, so I can't just
call Thread.join on that one. How do I do it?
[*] You need to use the wait/notify mechanism to allow any of the
worker threads to wake up your main thread when the worker has
completed.
12. (Sect. 8) How do I get random numbers?
[*] If you just need a quick random double between 0.0 and just less
than 1.0
double myrandom = Math.random(); // [0,1)
The notation "[0,1)" is common math notation for "zero to .9999999 etc"
The Sun documents say this returns 0.0 to 1.0, but inspection of the
source shows they are wrong. However, due to the inherent inaccuracies
of floating point arithmetic, multiplying N by 0.999999 etc can result
in an answer of N, not N * .999999. So watch out when N is big.
JDK 1.2 adds another version of nextInt that accepts a parameter for
returning ranged random numbers.
Where things get trickier is when you use JDK 1.1 and want an int in a
certain range, say 1 to 6 to simulate the throw of a die or 1 to 52 to
represent a playing card. Class Random has a nextInt method that will
return any integer:
import java.util.Random;
Random r = new Random();
int i = r.nextInt();
However, that has an (almost) 50% chance of being negative, and it
doesn't come from the right range. So you just take the abs() value and
then mod it into the right range:
int dice_throw = 1 + Math.abs(i) % 6;
Except, the abs() method fails gracelessly in the presence of the
Integer.MIN_VALUE (it returns the same, negative, result!). So it is
better to AND to get the non-negative value: In general, to get a
random int between high and low limits (inclusive):
java.util.Random r = new java.util.Random();
int j = (r.nextInt() & Integer.MAX_VALUE) % (high-low+1) + low;
The sentence above states "(almost) 50% chance" because there is one
more value in the negative integers than in the positive integers in
two's complement arithmetic as used by Java. For most purposes, the
bias introduced will be insignificant, but we "and" the nextInt() to
convert it to zero. Sure, it's unlikely to occur, but you don't want
the reactor going critical just because you missed this case while
testing.
A worse problem is that with the algorithm used, the low order bits are
significantly less random than the higher order bits. And the low order
bits are precisely the ones you get when you do a (mod 2^n) operation.
You may consider using java.security.SecureRandom which provides much
better randomness by using a Cryptographic hash, although it is much
more expensive to compute.
13. (Sect. 8) What does "deprecated" mean? I got this in a compiler error
message.
[*] The compiler will flag a now-obsolete API as "deprecated". The word
means "officially disapproved of". Compile again with the
"-deprecation" option to see what is deprecated. In almost all cases,
the old API has been replaced by a new one. Update your code to use the
new one.
An example of using a deprecated API is calling component.size(). That
has been replaced by component.getSize().
14. (Sect. 8) Where/why should I use the Enumeration interface?
[*] It's a very convenient way to step through some of the library data
structures, such as HashTable, Vector, and ZipFile. It is thread safe.
If you're looking at an element in one thread while another thread is
trying to delete it, it won't half vanish.
Here's how you might look at every file in a ZIP file:
ZipFile z = new ZipFile("foo.zip");
Enumeration e = null;
for (e=z.entries(); e.hasMoreElements(); ) {
ZipEntry ze = (ZipEntry)e.nextElement();
System.out.println("got " + ze.getName() );
}
And here's how you might look at all the tokens in a String:
StringTokenizer tokens =
new StringTokenizer (SomeArbitraryString, "\n", false);
Enumeration enum = tokens.elements();
while enum.hasMoreElements()
System.out.println(enum.nextElement());
You should look for opportunities in your own data structures to
implement Enumeration anywhere where the structure has repeated
elements.
15. (Sect. 8) Which version of WinZip is compatible with java.util.zip?
[*] You need WinZip version 6.2 or later. Version 6.1 or earlier is not
good enough. WinZip can be downloaded from
http://www.winzip.com/download.cgi. The pkzip software works fine.
Infozip is better than WinZip because it lacks the winzip "feature" of
failing to recreate directories unless given a special option. Use
Infozip.
16. (Sect. 8) How can Java access other fonts on my system?
[*] You do it by editing the fontnames in the font.properties file in
the lib directory of your JDK release. Watch out for program
portability when you do this.
For more details, check the website
http://www.alumni.caltech.edu/~dank/javafont.htm and the JavaSoft site
at http://java.sun.com/products/jdk/1.1/docs/guide/intl/fontprop.html
They have lots of information on this.
17. (Sect. 8) How can I trap Control-C in Java?
[*] Control-C is used on some OS's to break into a running program
interactively and terminate it. On Unix Control-C is sent to the
process as a signal. If a C program declares a handler for that signal,
the program will be able to continue even if Control-C is sent to it.
Control-C is not a Java concept, and there is no way to do this
portably.
On Unix you can write the signal handler in C, and impose the handler
by calling the C routine through the Java Native Interface.
On Win32 in a GUI a KeyPressedListener will see a control-C as
java.awt.event.KeyEvent[KEY_PRESSED,keyCode=67,modifiers=Ctrl]
On Win32 in a command line app you can't read a control-C in Java
because it is sent to the process as a signal (as with Unix).
Instead of interposing a signal handler thru JNI, you could launch java
from perl or a shell script which is capable of doing this.
Alternatively, use javaw to launch the JVM so that it has no keyboard
which can ctrl-C it.
18. (Sect. 8) How do you parse commandline arguments to a Java program?
[*] Perhaps surprisingly, commandline arguments are not encouraged, as
they make a program not 100% pure Java. The reason is that some systems
like MacOS don't normally have a command line or command-line
arguments. Consider using properties instead so that your programs fit
more naturally into the environment.
If you must use command-line arguments, have them comply with the POSIX
conventions, i.e. they should
o use a "-" instead of a "/"
o be case sensitive, and
o be concatenatable (-pst == -p-s-t).
See
http://java.sun.com/docs/books/tutorial/essential/attributes/_posix.html
If such arguments are used, it should NOT be up to the invocating
application (ie java.exe) to propperly parse them. According to the
getopts routine from gnu, getopts is required to know exactly what the
usable parameters are before it can properly parse them and create
argc/argv.
Because of that, it becomes unduly hard for calling programs such as
java.exe to properly parse class arguments by themselves. Instead, they
should create an argc/argv pair (actually a java specific array
dependant on the way in which the method is called) where the number of
elements equals the number of text words in the options. Text word
means a group of ascii characters outside of quotation marks, not
separated by white space. Once these text words are passed to the
class, it is up to the class to actually parse them correctly.
The text words passed to the class can be parsed however the class
author wishes. It is up to the author to correctly parse the options.
There are several implementations of getopts which perform such
parsing, in C, Perl, and Java. You will find one at
http://www.urbanophile.com/arenn/hacking/download.html#getopt
19. (Sect. 8) How many threads can I create?
[*] By default, on a Win32 machine, each ("native") thread gets one MB
(1024*1024 bytes) of stack space reserved for it. The memory is not
actually allocated. If it were, you'd run out of memory with dozens of
threads instead of hundreds. The space needs to be reserved as a
contiguous block of memory so the stack can grow as needed.
Under Win32, the max amount of memory is 2GB, which corresponds to
2,000 threads each reserving 1MB of memory. That's an upper bound; in
practice you'd run out of other OS resources before running out of
memory. Each thread has some system resources that will be used to
manage the thread -- register storage area, thread-local storage, state
information, exception condition info, etc.
You should design your programs so that you use threads only where
necessary. It's almost always a mistake to have one thread per object.
Successful programs are much more likely to have 5 threads (or fewer)
than 500.
-------------------------------
9. Dates and Times
Credit to Paul Hill who completely rewrote this section, and to
the programmers at IBM who implemented much of the Java Date code,
and reviewed this section of the FAQ for accuracy.
java.util.Date
1. (Sect. 9) Is Java "Year 2000"-compliant?
[*] Java is Y2K compliant in release JDK 1.1.6 and later. See
http://www.sun.com/y2000/cpl.html. Prior to this release there were
certain corner case bugs that had to be fixed.
The Date class, as you can see from the discussion, contains more than
enough resolution to represent dates in this century and the next and
the last. The SimpleDateFormat when parsing a 2 digit year could cause
problems; see discussion below.
2. (Sect. 9) What happen to java.util.Date between JDK 1.0 and JDK 1.1?
[*] In JDK 1.1 the java.util.Date class was split to provide better
support for timezones, and internationalization.
The classes specifially related to dates are summarized below:
1. The class Date represents a specific instant in time,
with millisecond precision.
2. The class TimeZone is an abstract class that represents
a time zone offset, and also figures out daylight
savings time adjustment.
3. The class SimpleTimeZone is the only concrete subclass
of TimeZone in the JDK. It is what defines an ordinary
timezone with a simple daylight savings and daylight
savings time period.
4. The class Calendar is an abstract class for converting
between a Date object and a set of integer fields such
as year, month, day, and hour.
5. The class GregorianCalendar is the only concrete
subclass of Calendar in the JDK. It does the
Date-to-fields conversions for the calendar system in
common use.
6. The class DateFormat is an abstract class that lets you
convert a Date to a printable string with fields in the
way you want (e.g. dd/mm/yy or dd.MMM.yyyy).
7. The class SimpleDateFormat is the only concrete subclass
of DateFormat in the JDK. It takes a format string and
either parses a string to produce a date or takes a
date and produces a string.
At least one critic has used the term "baroque" when describing the
complexities of the Java date related classes, but other critics would
spell that "broke". The good news is that as of JDK 1.2 all of the
common problems have been corrected and many of the bugs were corrected
in 1.1.4 and 1.1.6. Even in 1.1.1, you can avoid most of the most
annoying bugs by always keeping in mind which timezone each class is
using.
3. (Sect. 9) Exactly what is a java.util.Date?
[*] A java.util.Date stores a moment in time as a long integer
representing the number of milliseconds since 00:00:00 Jan 1, 1970 UTC
(Coordinated Universal Time). This zero moment is known as the "Epoch".
This is the same Epoch as is used on Unix systems. Dates earlier than
the Epoch are represented as negative numbers, counting away from
1/1/1970.
The scheme is sufficient to represent dates from 292,269,053 B.C. to
292,272,993 A.D. (64 bits covers -9,223,372,036,854,775,808 to
+9,223,372,036,854,775,807 milliseconds. But note that prior to JDK
1.2, a GregorianCalendar will not accept values earlier than 4713 B.C.
A java.util.Date is the light-weight object intended to just hold a
millisecond value. It is used to hold, communicate and store a moment
in time. Other tasks like creating a formated string, or calculating
dates are best done using other classes.
4. (Sect. 9) Does a java.util.Date really represent the true UTC?
[*] No, but it is close enough for most human time-keeping purposes. On
most computers, it only represents the time since the epoch as
converted from the value on the underlying hardware. If you have
hardware that is synchronized with an atomic clock your time is UTC;
most hardware assumes a day is 24 hours long, but there have been more
than 20 leap seconds added to UTC, since the first one was added in
1972.
5. (Sect. 9) How do I create a Date object that represents the current
time?
[*] The default value of a date object is the current time, so the
following code creates a date object that contains the current time.
Date now = new Date();
6. (Sect. 9) I want to create a string that represents a date in a format
other than what is returned by java.util.Date.toString() do I have to
use a Calendar?
[*] No. Instead of creating a Calendar and pulling out all of the
appropriate fields and making a string, you could use
SimpleDateFormat.format() to create a string.
7. (Sect. 9) Why are all the methods in java.util.Date deprecated?
[*] Mostly because the original java.util.Date was not completely aware
of the timezone and "not amenable to internationalization". To make it
timezone aware and internationalizable would have required adding some
of the functionality which can now be seen in java.util.Calendar and
some of the functionality in java.util.DateFormat. If you find the
combination all of the date related classes complex, just be glad they
were separated into different classes.
8. (Sect. 9) I really don't need a internationalizable, timezone aware,
extra flexible formating set of date classes. Is there anything else I
can use that stores only a date, and allows me to do some date
calculations?
[*]You could consider using the BigDate class written by Roedy Green,
and available in his very informative glossary (search for BigDate). If
you want to store the result in a database as a Date or TimeStamp, you
should read the section below on java.sql.Date.
9. (Sect. 9) Since the Date( String ) constructor is deprecated what do I
use instead?
[*] The best choice is to use SimpleDateFormat.parse() to create a
java.util.Date object.
The Date constructor that accepts a string calls Date.parse( String ).
The Date.parse() function had its own rules for converting 2 digit
years (it used a 1980 pivot date) and other limitiations which makes it
of limited value. Other "features" of Date.parse() that are not
supported in SimpleDate have not been missed by many developers.
10. (Sect. 9) Since Date(int year, int month, int date) and related
constructors are deprecated what do I use instead?
[*] The constructor GregorianCalendar(int year, int month, int date) is
the newer replacement. Other choices are the Calendar.set( year, month,
day ) method. Note that the year in the GregorianCalendar starts at 1
A.D., not at 1901 like the old Date constructor.
java.util.TimeZone
11. (Sect. 9) How can I see if my JVM is using the right timezone?
[*] The following codes displays the ID of the current default
timezone.
System.out.println( TimeZone.getDefault().getID() );
12. (Sect. 9) The value of TimeZone.getDefault is not what I expected. What
is the problem?
[*] The value of the default timezone is based on the value of the
system property "user.timezone". The JVM is supposed to set this
value. In releases such as JDK 1.1 the value of user.timezone was often
not set to anything, so TimeZone.getDefault() used its own built in
"fallback" value (the default when there is no default value). In later
JDK 1.1 releases and in JDK 1.2 the setting of the value of
user.timezone is much better and the "fallback" value is now GMT
(Greenwich Mean Time). Up until JDK 1.1.3, the fallback value was "PST"
(North American Pacific Timezone).
13. (Sect. 9) Do all the standard objects use the same default timezone?
[*] Not until JDK 1.2. In JDK 1.1, Date.toString() and Calendar used
the value of TimeZone.getDefault() which could often be undefined (see
the previous question). In JDK 1.1, The Calendar in a SimpleDateFormat
was set to the 1st timezone resource for the locale (for the US this is
PST).
System.out.println( "Date format TZ = " + TimeZone.getDefault().getID() );
sdf = DateFormat.getDateTimeInstance( DateFormat.LONG, DateFormat.LONG );
System.out.println( "Date format TZ = " + sdf.getTimeZone().getID() );
Calendar cal = Calendar.getInstance();
System.out.println( "Calendar TZ = " + cal.getTimeZone().getID() );
When run on a system running JDK 1.1.6, NOT in the North American
Pacific Time nor in the GMT timezone results in:
Timezone default = GMT
Date format TZ = PST
Calendar TZ = GMT
This example shows two bugs, the value of user.timezone is undefined,
so its defaulting to GMT (see discussion of TimeZone.getDefault()) and
it shows that the DateFormat depends on the 1st locale entry which in
this case is PST.
If you don't want the DateFormat to use the Locale timezone, see the
code provided below.
14. (Sect. 9) If I explicitly set the default timezone, don't I need code
to choose between (a) the daylight savings version or (b) the standard
version of a timezone?
[*] No. The ID that you use to select a timezone with
TimeZone.getTimeZone refers to a predefined timezone that contains
daylight savings information (when applicable). For example, the
following code selects the timezone used in New York, USA.
// Get the North American Eastern Time definition.
TimeZone theTz = TimeZone.getTimeZone( "EST" );
// get a really detailed date format and set it to the right timezone
DateFormat df = DateFormat.getDateTimeInstance( DateFormat.LONG,
DateFormat.LONG );
df.setTimeZone( theTz );
// create a date in the locale's calendar, set its timezone and hour.
Calendar day = Calendar.getInstance();
day.setTimeZone( theTz );
day.set( 1998, Calendar.FEBRUARY, 1 );
day.set( Calendar.HOUR, 12 );
// print that date/time and
// that date/time 150 full days of milliseconds later.
System.out.println( df.format( day.getTime() ) );
System.out.println( df.format(
new Date( day.getTime().getTime() + // get the millis
150L*24*60*60*1000L ) ) ); // add exactly 150 days of millis
Results in:
February 1, 1998 12:00:00 PM EST
July 1, 1998 1:00:00 PM EDT
Notice that this example selected something refered to as "EST", but
that this TimeZone was aware of the daylight savings time change and
either printed as "EST" or "EDT".
The confusion is reduced in JDK 1.2, you can use longer TimeZone IDs
each maps to its own set of text resources. For example the following
IDs are 5 hour West of GMT and have various DST rules:
"America/Nassau", "America/Montreal", "America/Havana",
"America/Port-au-Prince", "America/Grand_Turk", "America/New_York" and
"EST".
You can look at a list of other timezone names and offsets in the file
$JAVAHOME/src/java/util/TimeZone.java
15. (Sect. 9) How do I create my own Time Zone to apply to dates?
[*] You can create a TimeZone object with the GMT offset of your
choice. The following code creates British Time, a timezone that was
not defined in 1.1.
britTime = new SimpleTimeZone(0*ONE_HOUR, "Europe/London" /*GMT/BST*/,
Calendar.MARCH, -1, Calendar.SUNDAY /*DOW_IN_DOM*/, 1*ONE_HOUR,
Calendar.OCTOBER,-1, Calendar.SUNDAY /*DOW_IN_DOM*/, 1*ONE_HOUR,
1*ONE_HOUR),
TimeZone.setDefault( britTime );
Or you can then apply that TimeZone to a particular Calendar object
like so:
Calendar myCal = Calendar.getInstance();
myCal.setTimeZone( britTime );
If you are running 1.2, You can choose this existing timezone as the
default with the code:
TimeZone.setDefault( TimeZone.getTimeZone( "Europe/London" ) );
Note that BST is defined from JDK 1.1 and later, but it is Bangladesh
Standard Time. For a longer example of creating and testing the British
timezone, Tony Dahlman provides a nice example in his BSumTime.java
code.
16. (Sect. 9) How do I create the BST timezone specifically?
[*] You can create an arbitrary TimeZone object with the code below. In
most or all other timezones, daylight savings time is handled
automatically and internally. But GMT is the reference for all other
timezones, and so does not have the summertime update applied
automatically. The rules for BST can be found at
http://www.rog.nmm.ac.uk/leaflets/summer/summer.html
Here is the code to create a British Summer Time timezone that is
offset one hour from GMT between two dates:
import java.util.*;
import java.text.*;
// create a BST timezone (code courtesy of Tony Dahlman).
public static GregorianCalendar setBritSummTime(String zoneName){
// Set up the default GMT0BST time zone
SimpleTimeZone bst_tz =
new SimpleTimeZone( 0, // no offset from GMT
zoneName, // individualized tz id
// last Sun Mar 1AM
Calendar.MARCH,-1,Calendar.SUNDAY,1*60*60*1000,
// last Sun Oct 2AM
Calendar.OCTOBER,-1,Calendar.SUNDAY,2*60*60*1000
);
SimpleTimeZone.setDefault(bst_tz);
// Apply TimeZone to create a Calendar object for UK locale
return (new GregorianCalendar(bst_tz,Locale.UK) );
}
and here is how you would print out values using BST:
// create a template for printing the date
DateFormat df = DateFormat.getTimeInstance(
DateFormat.LONG,Locale.UK);
// tell the template to use BST tz.
df.setTimeZone(TimeZone.getDefault());
System.out.println("Using British Summer Time "
+"the time is: "
+ df.format( BritishSummerTime.getTime() ) );
// Now get and compare with current time in GMT
df.setTimeZone(TimeZone.getTimeZone("GMT") );
System.out.println("\nCurrent time in GMT is: "
+ df.format(BritishSummerTime.getTime() ) );
In the winter, this BST zone is aligned with GMT; in the summer it is
one hour later (4 a.m. GMT is 5 a.m. BST).
You can look at a list of timezone names and offsets in the file
$JAVAHOME/src/java/util/TimeZone.java
java.util.Calendar and java.util.GregorianCalendar
17. (Sect. 9)How do I a create a specific date in the Gregorian Calendar?
[*]If you have a Date use:
myCal.setTime( myDate );
If you have a set of integers representing the year, month and day of
month use:
Calendar myCal = Calendar.getInstance();
myCal.set( 1998, Calendar.MARCH, 15 );
Note: Months start with January = 0!
18. (Sect. 9) How do I use a GregorianCalendar to extract a few fields from
a Date?
[*]The following code shows how to get some fields from a Date.
Calendar g = Calendar.getInstance();
g.setTime( aDate );
int year = g.get( Calendar.YEAR );
int mon = g.get( Calendar.MONTH );
int date = g.get( Calendar.DATE );
mon++; // in class Calendar & GregCal, months run 0-11 ;-(
System.out.println( mon + "/" + date + "/" + year);
If you want to build a string that has a formated date consider using
SimpleDateFormat.format().
19. (Sect. 9) Some people use Calendar.getInstance() while others use new
GregorianCalendar(). Which one is the correct way to get a Calendar?
[*] Either way is correct, it depends on what you want to be able to
do. You should use Calendar.getInstance(), if you want your code to be
ready when the loading of other Calendars are added to the JDK and some
other calendar is the default for the locale. A particular locale might
have configured a Hebrew or Islamic Calendar as the default calendar
and you might want a user to enter a date in his own Calendar, i.e.
1-Jan-2000 (Gregorian) = 23-Tevet-576 (Hebrew) = 24-Ramadan-1420
(Islamic). If you really are trying to place a particular Gregorian
date, i.e. 4-July-1776, into a Date object, you might as well create a
GregorianCalendar directly.
20. (Sect. 9) I changed a field using Calendar.set() and then I checked
another field. Its value is inconsistent with the value I just set.
What is going on?
[*]In JDK 1.1.0 the Calendar class did not update all of its fields
until you called getTime to retrieve the Date that corresponds to the
fields in the Calendar. To get the earlier version of the Calendar to
"turn the crank" and calculate all fields you can use the trick:
myCal.setTime( myCal.getTime() ) // pull the date out and put it back
in.
21. (Sect. 9) When I create the date July 4th, 1776 using new
GregorianCalendar( 1776, 7, 4 ) the month is off by one. What is the
problem?
[*]You need to be be aware that months start with January equal to 0. A
better way to create that date would be:
independanceDayUSA = new GregorianCalendar( 1776, Calendar.JULY, 4 );
22. (Sect. 9)Why aren't there constants for milliseconds per day, week or
year?
[*]The short answer is: these values are not constants. While some date
calculations would find these useful, it is important to remember that
in areas with daylight savings time rules, there are two days per year
that are not 24 hours long, therefore not all weeks are the same length
(2 out of 52). Also, because of leap years, not all years are the same
length.
If you adding values to a calendar consider using either add or roll;
for example:
myCal.add(Calendar.YEAR, 1 ); // get a value 1 year later.
23. (Sect. 9) By my count the week of the Year is off by one. What is the
problem?
[*]The GregorianCalendar class uses the value set by
setMinimalDaysInFirstWeek() to determine if the fractional week at the
beginning of the year should be week 1 or week 0. If you don't change
it, any fractional week could be week 1, depending on the value defined
for the locale.
24. (Sect. 9) What timezone does a calendar use when I don't explicitly set
one?
[*]The Calendar uses the TimeZone.getDefault() (see discussion under
TimeZone).
25. (Sect. 9) Should I stop using Date all together and just use Calendar?
[*] Probably not. The Calendar class is a much larger than a Date
object. Many other interfaces in standard APIs are defined using a Date
object. Use Date objects to hold, store or communicate a date-time
value. Use a Calendar object to manipulate a date-time value.
26. (Sect. 9) The GregorianCalendar will not accept a date prior to 4713
B.C. Why?
[*]January 1, 4713 B.C. is the "epoch" date for the Julian Day calendar
which was invented in the 16th century by the Joseph Justus Scaliger.
"[T]he Julian day calendar, ... does not use individual years at all,
but a cycle of 7980 astronomical years that counts a day at a time,
with no fractional days, no mean year, and no leap years. He came up
with his number by mulitplying three chronological cycles: an 18-year
solar cycle, a 19-year lunar cycle, and the 15-year indication period
used by Romans. All three cycles began together at the same moment at
the start of the "Julian cycle. ... [This] Calendar lives on among
astronomers."
-- David Ewing Duncan, "Calendar", Avon Books, 1998; p 207
Note that the Julian Day calendar is not the same as the Julian
calendar. The Julian Calendar is named for Julius Caesar. The Julian
Calendar was used in the Europe from what we now call January 1, 45
B.C. until at least October 4, 1582 and is still used today by the
Eastern Orthodox Church to date holidays.
The limitation on dates prior to 4713 BC has been dropped in JDK 1.2.
27. (Sect. 9) The Calendar class is said not to handle certain historical
changes. Can you explain some of the limitations?
[*]The date of change from the Julian to the Gregorian calendar depends
on where you lived at the time. The date can vary from 1582 (most
Catholic countries, which of course followed the Papal edict) to 1949
(China). The date of the cutover from using the Julian Calendar (leap
years ever 4 years) to using the Gregorian Calendar (every 4 years,
except every 100 unless divisable by 400) is controlled by the method
GregorianCalendar.setGregorianChange(Date).
It is also the case that January 1 was not always the beginning of the
year. January 1 was standardized by Julius Caesar in 45 B.C. and Pope
Gregory XIII in 1582, but others who used the Julian Calendar between
those dates used other dates for New Years Day. (Anyone who has ever
been involved in a standardization effort may find it interesting that
neither an emperor nor a pope could actually complete the
standardization effort).
The Calendar class uses a TimeZone which does not handle historical
changes, i.e. the SimpleTimeZone contains only two dates the "spring
forward" and "fall back" dates and a date that the DST starts (see
SimpleTimeZone.setStartYear() ). If the local definitions have changed
then a date/time may not accurately reflect the historical local time.
As noted above, the Date object does not usually include leap seconds,
unless your hardware includes leap seconds.
While the Calendar class is more than useful for international
business, it may not be what you want for doing UTC timebased
calculations or historical dates and times without a more careful
analysis of its design limits.
java.text.DateFormat and java.text.SimpleDateFormat
28. (Sect. 9) How do I convert a date in one format to another?
[*] The following code illustrates the technique:
import java.text.*;
public class DateTest {
public static void main( String[] args ) {
SimpleDateFormat df1 =
new SimpleDateFormat("yyyy-MM-dd hh:mm:ss.S");
SimpleDateFormat df2 =
new SimpleDateFormat("dd-MMM-yy");
String startdatetime = "1998-09-09 06:51:27.0";
try {
System.out.println("Date is " +
df2.format( df1.parse(startdatetime) ));
} catch (ParseException pe) {
System.out.println("ParseException " + pe );
}
}
}
When run, the program outputs "Date is 09-Sep-98"
29. (Sect. 9) How do I use DateFormat to parse a string containing a date?
[*] The easiest way to parse a date that is in a known format is to use
SimpleDateFormat.parse().
DateFormat df = new SimpleDateFormat( "HH:mm" );
df.setTimeZone( TimeZone.getDefault() ); // if using JDK 1.1 libraries.
df.setLenient( false ); // to not allow 26:65 etc.
Date lateLunchOnDayZero = df.parse( "12:30" );
System.out.println( lateLunchOnDayZero );
The above code would result in (when in the MST timezone).
Thu Jan 01 12:30:00 MST 1970
To parse other date and time fields, refer to the SimpleDateFormat
documentation.
30. (Sect. 9) How do I use a DateFormat to create a text string from a
Date?
[*] The easiest way to create a string from a date is to use a
SimpleDateFormat.format(). The following code illustrates how this can
be done.
DateFormat df = new SimpleDateFormat( "yyyy.MMM.dd HH:mm:ss.SSS z" );
df.setTimeZone( TimeZone.getDefault() ); // JDK 1.1
System.out.println( df.format( d ) ); // where d is a Date
For other possible fields from the calendar, see the document for
SimpleDateFormat.
31. (Sect. 9) What timezone does a SimpleDateFormat use when I don't
specify one?
[*]In JDK 1.1, the SimpleDateFormat uses the first timezone defined for
the locale. In JDK 1.2, it uses the default timezone. See the
discussion above on how this differs from the Calendar class).
32. (Sect. 9) I'm not yet using JDK 1.2 and I don't want the DateFormat to
use the 1st timezone for the locale. How do I change the timezone in a
SimpleDateFormat to use a different timezone?
[*] The following code sets the timezone of a DateFormat to the current
default.
DateFormat df = DateFormat.getDateInstance();
df.setTimeZone(TimeZone.getDefault());
or to set it to a timezone of your chioce.
df.setTimeZone(TimeZone.getTimeZone( "MST" ) ) // Mtn Time, Denver USA
33. (Sect. 9) What century is assumed when I use a two digit year in a
SimpleDateFormat string?
[*]In JDK 1.1, the default start for the century used by
SimpleDateFormat for 2 digit years is 80 years before the current date.
This means that in 1998: 1 = 2001, 2 = 2002, ... 17 = 2017, 18 = 2018,
19 = 1919, 20 = 1920, ... 98 = 1998, 99 = 1999,
In JDK 1.2 you can change this "default century start date" with the
method set2DigitYearStart( Date) and get its current value
with the method get2DigitYearStart(). One thing to note is that since
set2DigitYearStart takes a date not a year, you can have your default
century begin at any day or hour.
When running under JDK 1.1, it is probably best to avoid two-digit year
fields, when the dates entered could possibly fall outside of the range
-- now less 80 years and now plus 20 years. If you want to allow
two-digit year fields in JDK 1.2 and beyond, consider setting the
2DigitYearStart property to something appropriate, For example, set it
to today, when all dates to be entered are in the future (i.e. an
expiration date), or set it to today less 100 years, when the value is
always in the past (i.e. birthdate, death date).
34. (Sect. 9) Does the above mentioned limitation of 2 digit years in JDK
1.1 mean that java.text.SimpleDateFormat is not Y2K compliant?
[*] No. It means that any code you write that (1) allows the entry of 2
digit years and (2) does not make sure they are in an appropriate
century, would not pass a careful Y2K analysis. This code was put here
so you could sensibly read old files with non-Y2K compliant dates, not
so you could create new ones. Once you are using JDK 1.2 it is better
to set the 2DigitYearStart property to something appropriate for any
two-digit year field which you are parsing.
java.sql.Date and java.sql.TimeStamp
35. (Sect. 9) What timezone does a java.sql.date use when converting to an
SQL DATE?
[*]This is another hidden use of the default java.util.TimeZone. If you
have carefully set every timezone in every Calendar and DateFormat you
are using, but you don't set the default in java.util.TimeZone when a
java.util.Date is converted to a java.sql.Date you may not end up with
the value you expected in your database.
36. (Sect. 9) When I print a jave.sql.Timestamp it doesn't include any
milliseconds. What is the problem?
[*] If you print the java.sql.Timestamp directly you will see this
problem. The following code demonstrates this surprising behavior.
// incorrect use of java.sql.Timestamp
DateFormat df = new SimpleDateFormat( "MM/dd/yy hh:mm:ss.SSS a" );
df.setTimeZone( TimeZone.getDefault() ); // needed in JDK 1.1
java.sql.Timestamp t = new java.sql.Timestamp( 94, Calendar.JANUARY, 1,
13, 45, 59, 987654321 );
System.out.println( df.format( t ) ) ; // Wrong! no fractions of a second.
The results of the above code are:
01/01/94 01:45:59.000 PM
The above code is using whatever is in the super class (java.util.Date)
and assumes all of those parts are filled in. java.sql.Timestamp could
have stored the whole milliseconds in the millisecond part of a
java.util.Date, and stored the nanoseconds that are not whole
milliseconds in an additional field. They chose to ignore the fractions
of a second in the java.util.Date and put all fractional parts in an
additional nanosecond field.
The following code shows how to convert a java.sql.timestamp to a
java.util.Date.
Date d = new Date(t.getTime() + (t.getNanos() / 1000000 ));
// 1 Milli = 1x10^6 Nanos
System.out.println( df.format( d ) ) ; // Right! At least we have the millis
The result of the above code is a better approximation of the timestamp
value:
01/01/94 01:45:59.987 PM
37. (Sect. 9) How do I calculate the number of days between two dates?
[*] There is no API for this (there should be), but you can calculate
static final long ONE_HOUR = 60 * 60 * 1000L;
Calendar earlierDate = new GregorianCalendar();
Calendar laterDate = new GregorianCalendar();
earlierDate.set(1997, 1, 5, 0, 0, 0); // FEB!! 05, 1997
laterDate.set(1998, 1, 5, 0, 0, 0); // Feb 05, 1998
// the first getTime() returns a Date, the second takes
// that Date object and returns millisecs since 1/1/70.
// The API has misleading and horrible naming here, sorry.
long duration = laterDate.getTime().getTime() -
earlierDate.getTime().getTime();
// Add one hour in case the duration includes a
// 23 hour Daylight Savings spring forward day.
long nDays = ( duration + ONE_HOUR ) / (24 * ONE_HOUR);
System.out.println("difference in days: " + nDays);
Note: this method works only if the two times are both created to be
exactly midnight. Otherwise you might end up finding that for example,
11 PM tonight is 0 days away from 6 AM tomorrow, instead of 1 day as
many applications expect. This is because 24 hours haven't elapsed
between 11pm on one day and 6am on the next.
Or, use int julian = myCalendar.get(Calendar.DAY_OF_YEAR); and
subtract, making sure to subtract the years too. Alternatively, use
BigDate at http://mindprod.com.
For the ultimate in generality, you could get ACM's Collected Algorithm
199, recode it in Java (takes about 30 minutes), compute the Julian
date for each end point, and subtract the two numbers. Here is the code
in Java to get the GMT Julian day number and a "local Julian Day" that
leverages the existing algorithms in the GregorianCalendar.
import java.util.*;
import java.text.*;
/**
* This Calendar provides Julian Day and "Calendar Day" methods,
* The Calendar Day starts at _local_ midnight.
*
* @author Paul A. Hill
* @version 1.1
* @since JDK 1.1.7 3/99
* @see java.util.Calendar
*/
public class FAQCalendar extends GregorianCalendar implements Cloneable {
/**
** this constant and the ones that follow are actually private
** in the GregorianCalendar, so we'll redefine them here.
**/
public static final long
ONE_SECOND = 1000,
ONE_MINUTE = ONE_SECOND * 60,
ONE_HOUR = ONE_MINUTE * 60;
/**
* this next constant and the others with it have their uses,
* but watch out for DST days and the weeks that contain them.
* Note also that 1/1/1970 doesn't start on the first day of the week.
*/
protected static final long
ONE_DAY = ONE_HOUR * 24,
ONE_WEEK = ONE_DAY * 7;
/** The number of days from Jan 1, 4713 BC (Proleptic Julian)
** to 1/1/1970 AD (Gregorian). 1/1/1970 is time 0 for a java.util.Date.
**/
public static final long JULIAN_DAY_OFFSET = 2440588L;
/**
* Same as GregorianCalendar(). Creates a calendar with the time set to
* the moment it was created.
*
Note: for Brevety I have not provided all of the other
* constructors that you can find in GregorianCalendar.
*
* @see java.util.GregorianCalendar#GregorianCalendar
*/
public FAQCalendar() {
super();
}
/**
* A calendar day as defined here, is like the Julian Day but it starts
* and ends at _local_ 12 midnight, as defined by the timezone which is
* attached to this calendar. This value is useful for comparing
* any date in the calendar to any other date to determine how many days
* between them, i.e. tomorrow - today = 1
*
* @return the calendar day (see above).
* @see #getCalendarDay
*/
public long getCalendarDay() {
TimeZone tz = getTimeZone();
// Figure the exact offset for the exact time of day.
int offset = tz.getOffset( get( ERA ), get( YEAR ),
get( MONTH ), get( DAY_OF_MONTH ), get( DAY_OF_WEEK ),
(int)((long)get( HOUR_OF_DAY ) * ONE_HOUR +
get( MINUTE ) * ONE_MINUTE +
get( SECOND ) * ONE_SECOND ) );
return round( ONE_DAY, getTime().getTime() + offset ) + JULIAN_DAY_OFFSET;
}
/**
* Sets the date in the calendar to 00:00 (midnight) on the local calendar day.
* See getCalendarDay for the definition of calendar day as used in this class.
*
* @param calendarDay the day to set the calendar to.
* @see #setCalendarDay
* @see java.util.TimeZone#getRawOffset
*/
public void setCalendarDay( long calendarDay ) {
// Set to the beginning of the Julian day.
// Then add in the difference to make it 00:00 local time.
setJulianDay( calendarDay );
setTimeInMillis( getTime().getTime() - getTimeZone().getRawOffset() );
// we may have gone slightly too far, because we used the
// raw offset (diff between Standard time to GMT/UT, instead of the
// actual value for this day, so during DLS we may be at 1 AM or whatever
// the local DLS offset is), so we'll just drop back to midnight.
set( HOUR_OF_DAY, 0 );
}
/**
* Finds the number of days after 12/31/4312 BC 24:00 GMT on a proleptic
* Julian Calendar (i.e. extending the Julian Calendar into pre-history)
* to the current time.
*
The Astronomers Julian Day begins at noon. The Julian Day used here
* sometimes called the Chronologists or Historians Julian Day
* starts at midnight. For more information see
* http://www.magnet.ch/serendipity/hermetic/cal_stud/jdn.htm#astronomical
*
Note: This routine does NOT take into consideration
* leap seconds.
*
* @return the day number of the current time from 1/
* @see #getCalendarDay
*/
public long getJulianDay() {
return round( ONE_DAY, getTime().getTime() ) + JULIAN_DAY_OFFSET;
}
/**
* Sets the current date contained in this calendar to exactly
* 00:00 GMT on the date defined by the Julian Day provided.
*
* @param julianDay the Julian Day to set the calendar to
* @see #setCalendarDay
*/
public void setJulianDay( long julianDay ) {
setTimeInMillis( ( julianDay - JULIAN_DAY_OFFSET ) * ONE_DAY );
}
/**
* This is a utility routine for rounding (toward negative) to the nearest
* multiple of the conversion factor.
*
BUG? Why is this different than the formula given in
* java.util.GregorianCalendar private millisToJulianDay?
*
* @param conversion typically one of the constants defined
* above ONE_MINUTE, ONE_DAY etc.
* @param fractions the value to convert expressed in the same units
* as the conversion factor (i.e milliseconds).
*
* @return the value divided by the conversion factor, rounded to the negative.
* @see java.util.Calendar
*/
protected static final long round( long conversion, long fractions ) {
long wholeUnits;
// round toward negative:
// For secs rounded to minutes -61/60=-1, -60/60=-1, -59/60=0,
// but we want -2, -1, -1 not -1,-1,0
// or month 0..11 => year 1; -12..-1 => 0; -24..-13 => -1
if ( fractions >= 0 ) {
wholeUnits = fractions / conversion;
}
else {
wholeUnits = ( fractions + 1 )/ conversion - 1;
}
return wholeUnits;
}
} // FAQCalendar
-------------------------------
10. AWT
Text, Textfield, and TextArea
1. (Sect. 10) How can I write text at an angle?
[*] Check out http://www.nyx.net/~jbuzbee/font.html. Jim has some code
to do exactly this. A good way to do it is to draw the text to an
offscreen image and write an ImageFilter to rotate the image.
Also, from JDK 1.2 on, the Java 2D API handles arbitrary shapes, text,
and images and allows all of these to be rotated, scaled, skewed, and
otherwise transformed in a uniform manner. A code example would be:
import java.awt.*;
import java.awt.geom.*;
public class r extends Frame {
public static void main(String args[]) { new r(); }
r() { setSize(200,200); setVisible(true); }
public void paint(Graphics g) {
Graphics2D g2D = (Graphics2D) g;
AffineTransform aft = new AffineTransform();
aft.setToTranslation(100.0, 100.0);
g2D.transform(aft);
aft.setToRotation(Math.PI / 8.0);
String s = "Rotated Hello World";
for (int i = 0; i < 16; i++) {
g2D.drawString(s, 0.0f, 0.0f);
g2D.transform(aft);
}
}
}
There is more info about the 2D API at
http://java.sun.com/products/java-media/2D/index.html and
http://developer.javasoft.com/developer/technicalArticles/
2. (Sect. 10) How do you change the font type and size of text in a
TextArea?
[*] Like this.
myTextArea.setFont(new Font("NAME", <STYLE>, <SIZE>));
where:
o NAMEis the name of the font (e.g. Dialog or TimesRoman).
o <STYLE> is Font.PLAIN, Font.ITALIC, Font.BOLD or any additive
combination (e.g. Font.ITALIC+Font.BOLD).
o <SIZE> is the size of the font, e.g. 12.
Example: new Font("TimesRoman", Font.PLAIN, 18);
3. (Sect. 10) Can you have different fonts for individual words in a
TextArea?
[*] No. If you're trying to write a word processor, use the Canvas
class to render on. Note that this can be done using the Swing JText
classes.
4. (Sect. 10) How much text can be put in a TextArea?
[*] TextArea just uses the corresponding widget of the underlying
window system. It will be bounded by the limit imposed in the native
window system. In Windows 95 TextAreas can hold about 28Kb. The native
widget allows 32Kb, but there is some overhead which reduces the amount
available to the programmer. The limit is removed in JTextComponent in
Swing (JDK 1.2) which dispenses with peer controls.
5. (Sect. 10) How do I clear the contents of a TextArea?
[*] Set it to an empty String with this:
area.setText("");
6. How do I get back to a normal echo after I have used
TextField.setEchoChar('*')?
[*] TextField.setEchoChar('\0') works on most Windows-based
browsers...but for most other platforms (i.e. Netscape under UNIX), it
just locks up the textfield.
There is only one good solution, and that is to make two TextFields on
top of each other, one normal, and one with .setEchoChar('*'), and
switch between them.
7. (Sect. 10) How do I get word wrap in a TextArea?
[*] It's a little obscure. Creating a TextArea with no horizontal
scrollbar causes wrapping to occur automatically. The idea is that if
you ask for a scroll to scroll viewing over to the right, there is no
reason for the widget to do word wrap. So take away the scrollbar, and
word wrap will be done instead.
Supply TextArea.SCROLLBARS_NONE or TextArea.SCROLLBARS_VERTICAL_ONLY to
the TextArea constructor to get word wrap. By default, a TextArea is
created with both horizontal and vertical scrollbars.
8. (Sect. 10) How can I limit a TextField to no more than N characters, or
to only allow numeric input?
[*] The approach is to look at keystrokes as they happen, and disallow
input that does not meet your criteria.
A neat variation is to extend the basic AWT component, and in your
subclass also include the handler that will look at the keystrokes.
This bundles everything neatly in one place. The code may look like:
import java.awt.*;
import java.awt.event.*;
public class XCTextField extends java.awt.TextField implements
java.awt.event.TextListener {
public XCTextField(int columns) {
super(columns);
enableEvents(AWTEvent.FOCUS_EVENT_MASK);
addTextListener(this);
}
// other constructors may be useful, too
public void textValueChanged(java.awt.event.TextEvent event) {
int col = this.getColumns();
int len = getText().length();
// int caret = getCaretPosition();
if (col > 0 && len > col) {
// or if the char just entered is not numeric etc.
String s = this.getText();
Toolkit.getDefaultToolkit().beep();
this.setText(s.substring(0,col));
this.setCaretPosition(col-1); // caret at end
}
}
public void processFocusEvent(java.awt.event.FocusEvent e) {
// this routine highlights according to focus gain/loss.
super.processFocusEvent(e);
int id = e.getID();
if (id==java.awt.event.FocusEvent.FOCUS_GAINED)
this.selectAll();
else if (id==java.awt.event.FocusEvent.FOCUS_LOST)
this.select(0,0);
}
}
Here is a much briefer example, which very cleverly does the work in
the Listener. Oracle really dislikes the "apostrophe" character in a
data text fields, as it is interpreted as part of an SQL statement.
Here is the code that James Cloughley wrote to suppress apostrophes
("ticks") in a TextField.
import java.awt.*;
import java.awt.event.*;
public class NoTick extends KeyAdapter {
final char tick = '\'';
public void keyPressed( KeyEvent event ) {
TextComponent tc = ( TextComponent )event.getSource();
char c = event.getKeyChar();
if ( c == tick ) { event.consume(); }
}
}
Use it like this:
TextField sometextfield = new TextField();
sometextfield.addKeyListener( new NoTick() );
Brief and clever - make the event handler consume unwanted characters.
However, it doesn't filter out text that arrives in the component via
cut & paste! If you use ctrl-v to paste, you get key events for the
ctrl and v, but not for the characters that are pasted.
Finally, check out iDate, iTime, and iNumeric from IBM's alphaworks
javabeans, available free at http://www.alphaworks.ibm.com/alphaBeans.
These beans do the kind of validation you want.
Size and Position
9. (Sect. 10) I use add(Component) to add Components to the Container. Is
there any way to explicitly set the z-order of these Components?
[*] JDK 1.0 has no way to explicitly set the z-order of components. You
can try it heuristically, based on the browser you're using, or you can
use CardLayoutManager to ensure the panel you want is at the front.
In JDK 1.1, the z-order of components ("z-order" means "front-to-back"
order, i.e. which window is in front of which) can be controlled by
using the the method add(Component comp, int index). By default,
components are added 0 to N. The method paint of class Container paints
its visible components from N to 0.
10. (Sect. 10) How can I get the dimensions and resolution of the screen?
[*] Use
java.awt.Toolkit.getDefaultToolkit().getScreenSize()
or
java.awt.Toolkit.getDefaultToolkit().getScreenResolution()
Screen resolution is in dots-per-inch.
Take a look in the Toolkit class for other useful methods.
Toolkit.getDefaultToolkit().getColorModel().getPixelSize()
gets you the color model in terms of bits per pixel.
Math.pow(2, Toolkit.getDefaultToolkit().
getColorModel().getPixelSize())
gets you the color model in terms of number of colors. Or use this:
1 << Toolkit.getDefaultToolkit().
getColorModel().getPixelSize()
That does a shift left to calculate the power of two.
11. (Sect. 10) How do I allow for the size of the title bar and border when
I draw a Frame?
[*] Use MyFrame.getInsets(). This returns a java.awt.Insets object
which has four ints: top, left, bottom, right, giving the number of
pixels each of those margins are inset from the top. You can use these
value to adjust the Dimension object returned by component.getSize().
If you are doing this in the constructor you need to ensure that the
Frame's peer object is created first. Otherwise the Insets object
returned by getInsets() will have all zero values. Make a call to
Frame.addNotify() to have the peer created.
12. (Sect. 10) How do I resize a List? I had a List defined as
List tlist = new List(10);
but the Strings in the list were 80 characters long and only the first
15 were being shown. I was not able to resize the List to display the
contents without using the scroll bar.
[*] A List cannot be resized in a constructor, so add the following to
the Applet (or wherever):
public void paint (Graphics g) {
tlist.setSize(200,200);
}
Then before showing panel/frame with the List:
tlist.resize(400,400);
13. (Sect. 10) How can my program tell when a window is resized?
[*] Override the setBounds(int,int,int,int) method of Component to do
what you want. Of course, have it call super.setBounds() as well. Note
that setBounds() replaces reshape() which is deprecated.
Note the new APIs call the deprecated APIs instead of the other way
round. For example, Component.setBounds calls Component.reshape,
instead of reshape calling setBounds. This is because the AWT sometimes
needs to call these for its own purposes. If it called the old one
which then called the new one, and you overrode the new one, the AWT
would (wrongly) not call your routine. By having the AWT call the new
one (and then the new one call the old one), any overrides of the new
one will correctly be called by the AWT as needed. If that didn't make
sense, forget I mentioned it.
14. (Sect. 10) How do I center a dialog box?
[*] You cannot currently get the applet's absolute screen coordinates.
Its location (0,0) is relative to the browser, not the screen itself.
But you can center something that it pops up or displays centered on
the screen with code like this:
Dimension screen = Toolkit.getDefaultToolkit().getScreenSize();
my_window.move(
( screen.width - my_window.size().width ) / 2,
( screen.height - my_window.size().height ) / 2 );
my_window.show().
In a related fashion, you can center something on its parent like this.
Note the intelligent use of APIs like translate() to do the work for
you.
void center(Component parent) {
pack();
Point p = parent.getLocation();
Dimension d = parent.getSize();
Dimension s = getSize();
p.translate((d.width - s.width) / 2,
(d.height - s.height) / 2);
setLocation(p);
}
15. (Sect. 10) How can I get the absolute mouse coordinates?
[*] You mean, if the browser is about 640x480, you want a y-coord
between 0 and 480. If the browser window is about 800x600 you want a
y-coord between 0 and 600. This might be needed for a pop-up menu,
where you want to pop up at the absolute mouse position.
The approach is to sum up the event's (x,y) and the locations of the
target and its parents until there is no parent. Though on some
browsers, it seems this is not reliable. [Better suggestions are
solicited.]
16. (Sect. 10) How do I detect a resize of a Frame or other Component?
[*] If you are using JDK 1.0.2, you can override the reshape(int, int,
int, int) method of Component to do what you want; of course, have it
call super.reshape() as well.
In JDK 1.1.x, setBounds() replaces reshape(), which is deprecated -
however, there is a better way of detecting the resize using the new
event model, than overriding setBounds(). Note the new APIs call the
depecated one.
The proper way to detect the resize in 1.1.x is to register a
ComponentListener on the Frame, like this:
import java.awt.*;
import java.awt.event.*;
class MyFrame extends Frame {
public MyFrame() {
addComponentListener(new CmpAdapter());
}
class CmpAdapter extends ComponentAdapter {
public void componentResized(ComponentEvent evt) {
//doSomething();
}
}
}
Alternatively, the same effect can be achieved like this:
class MyFrame extends Frame implements ComponentListener {
public MyFrame() {
addComponentListener(this);
}
public componentHidden(ComponentEvent evt) { }
public componentMoved(ComponentEvent evt) { }
public componentShown(ComponentEvent evt) { }
public componentResized(ComponentEvent evt) {
//doSomething
}
}
Or even with an anonymous inner class
public MyFrame() {
addComponentListener(new ComponentAdapter() {
public void componentResized(ComponentEvent evt) {
// doSomething;
}
} );
}
17. (Sect. 10) What are those preferredSize() and minimumSize() methods in
Component?
[*] Those methods allow a LayoutManager to calculate the preferred and
minimum sizes of the Components it is arranging. You can control the
values that the LayoutManager gets by creating subclasses of the
Components you are using and overriding these methods. You don't call
them; you override them and they are called on your behalf.
18. (Sect. 10) Why isn't my component showing up at all? Or in some cases
not until I resize the main window?
[*] The initial sizes of components are not always exactly what the
programmer would expect. When a component doesn't show up, often it is
getting added to its parent, but with a size of 0x0. Even when
getPreferredSize gives a non-zero value. If this seems to be what's
happening, try calling setSize(getPreferredSize()). If that doesn't
seem to be the problem, look into your layout manager.
Drawing and Pixels
19. (Sect. 10) How do I plot a single pixel to the screen?
[*] Use g.drawLine(x1,y1,x1,y1) to draw a line one pixel in length. If
you are drawing a very large number of individual pixels, consider
using a java.awt.MemoryImageSource object and measure whether this
offers better performance.
20. (Sect. 10) Is it possible to draw a polygon or a line more than 1 pixel
wide?
[*] JDK 1.1 doesn't have support for this. The standard workaround for
drawing a thick line is to draw a filled polygon. The standard
workaround for drawing a thick polygon is to draw several polygons.
There is a useful class at
http://www.apl.jhu.edu/~hall/java/GraphicsUtil.html which extends the
drawxxx and fillxxx methods of java.awt.Graphics. It adds a Line Width
argument to most of the drawxxx methods, a Color argument to most of
the drawxxx and fillxxx methods, and a Font argument to drawString and
drawChars.
JDK 1.2 introduces the java.awt.BasicStroke class. You set the stroke
on a Graphics object, and line rendering is done using that info.
BasicStroke bs = new BasicStroke(width, BasicStroke.CAP_BUTT,
BasicStroke.JOIN_MITER, 1.0f, null, 0.0f);
g.setStroke(bs);
g.drawLine( ... );
21. (Sect. 10) How can I make an offscreen image with transparent pixels?
How can I grab the pixel values from an offscreen image?
How can I use AWT drawing primitives (e.g. drawString() or drawOval())
on an image I created from an ImageProducer?
[*] None of these things can be done.
Despite the fact that there is only one class called Image in the AWT
libraries, it suffers from a (currently undocumented) severe case of
schizophrenia: The code behaves as though there are two unrelated types
of Image. The first type are those created by the
Component.createImage(int, int) call, known as "offscreen" images, and
the second are those created by the
Component.createImage(ImageProducer) call, or by the
Toolkit/Applet.getImage() calls, which I will call "produced" images.
The only common ground between these kinds of Image is the following:
o You may find their width and height by the methods of the Image
class.
o You may use them as the argument to the various
Graphics.drawImage() calls.
The differences between these objects are the following:
o You may not put transparent pixels into an offscreen - note that
all Java primitives accept Color objects, which all represent
completely opaque colours as if Produced from an int with the
upper 8 bits equal to 0xff. (See also Question 8.3.)
o You may not call Image.getGraphics() on a produced image, and
hence may not use any AWT primitives.
o You may not grab pixels from an offscreen image using
PixelGrabber.
o Anyone know of any other limitations?
In these cases, "you may not" generally means "you may not
successfully". Symptoms on attempting these range from Exceptions to
garblings of the Image. Any or all of these restrictions may be removed
in Java 1.2, which features a new 2D API. Wait and see.
Workaround: cause a peer to be created for the Image, and then do the
operation. It will work. You can add it to a Frame, for example. You do
not have to show() the Frame. Causing the peer to be created is enough.
There are some relevant bugs shown in the Java Developer Connection:
Bug ID 4098505. Apparently, from the report from the Sun engineer,
PixelGrabber is specified to work with offscreen images, just it is
currently buggy, and invariably gets the wrong color model. No fix has
been scheduled yet.
Bug ID 4077718 reports that setting transparent Colors in offscreen
images has been available since Java 1.2b1. I am personally unable to
verify this.
There is an incorrect answer from Sun to the third matter, of
getGraphics() on produced images, in article 1501 in Questions&Answers.
22. (Sect. 10) How can I grab a pixel from an Image object?
[*] This is the purpose of the java.awt.image.PixelGrabber class. A
fragment of code showing its use is:
import java.awt.image.PixelGrabber;
import java.awt.Image;
...
public static int pixelValue(Image image, int x, int y) {
// precondition: buffer must not be created from ImageProducer!
// x,y should be inside the image,
// Returns an integer representing color value of the x,y pixel.
int[] pixel=new int[1];
pixel[0]=0;
// pixel grabber fills the array with zeros if image you are
// trying to grab from is non existent (or throws an exception)
PixelGrabber grabber = new PixelGrabber(image,
x, y, 1, 1, pixel, 0, 0);
try {
grabber.grabPixels();
} catch (Exception e) {System.err.println(e.getMessage());}
return pixel[0];
}
By the way, one issue on working with images is that the Java VM will
consume virtual memory pretty fast if you are loading lots of images
without calling the Image.flush() method when done. The getImage()
method probably caches old images so they aren't garbage collected.
Other AWT FAQs
23. (Sect. 10) How do I change the icon on my Frame or JFrame from the Java
coffee cup to my own icon?
[*] Just use
f.setIconImage( Toolkit.getDefaultToolkit().getImage(iconfilename) );
On Windows, the icon must be 16-by-16 pixels.
24. (Sect. 10) What's all this about subclassing Canvas and overriding
paint() ? Can't I just do a getGraphics() for a component, and draw
directly on that?
[*] You can do that, and it might work up to a point (or it might not).
A problem arises when the window system wants to refresh that component
e.g. because it has been partially obscured and is now revealed. It
calls paint(), and paint() has no knowledge of the other g.drawing()
you have just done.
25. (Sect. 10) But couldn't the AWT just remember what has been drawn to a
Graphics context, and replicate that instead of calling paint()?
[*] Possibly it could, but how do you unremember something that has
been drawn? How do you start drawing over again with different
contents? You could solve these by creating extra methods, but that is
not how it works. In practice it is a lot simpler to be able to look at
the paint method, and see explicitly all the things that will be done
to draw that component. Bottom line: Use paint(), not g=getGraphics();
g.drawString( ...
26. (Sect. 10) When I call repaint() repeatedly, half my requests get lost
and don't appear on the screen. Why is this?
[*] repaint() just tells the AWT that you'd like a paint to happen. AWT
will fold several adjacent repaint requests into one, so that only the
most current paint is done. One possible workaround might be to use a
clip rectangle and only paint the different areas that have changed.
27. (Sect. 10) Why do I get this when using JDK 1.1 under X Windows?
java.lang.NullPointerException
at sun.awt.motif.MFramePeer.<init>(MFramePeer.java:59)
at sun.awt.motif.MToolkit.createFrame(MToolkit.java:153)
at java.awt.Frame.addNotify(Frame.java)
at java.awt.Window.pack(Window.java)
[*] There's a missing font on your system. Move font.properties from
the "lib" subdirectory aside to font.properties.bak. Then it won't look
for the font and fail to find it.
The problem occurs because the Motif AWT libraries use the Font "plain
Dialog 12 point" as a fall-back default font. Unfortunately, when using
a remote X server sometimes this font isn't available.
On an X terminal, the diagnostic may be slightly different, a segv
% appletviewer HelloWorldApplet.html
SIGSEGV 11* segmentation violation
si_signo [11]: SIGSEGV 11* segmentation violation
si_errno [0]: Error 0
si_code [1]: SEGV_ACCERR [addr: 0x14]
To determine which fonts you have, issue a command such as
xlsfonts > ~/fonts.txt
Then pick through the long list of fonts to determine which ones you
want to use. The xfd program will let you look at a font:
xfd -fn "your font name here" &
28. (Sect. 10) Why is GridBagLayout so hard to use?
[*] There are two reasons. First, while simple layouts are easy.
detailed GUI layout is difficult. Second, GridBagLayout wasn't designed
with human factors and ease of use in mind. If that bothers you (it
bothers me) then don't use it. Create your GUI on several panels and
use the other layout managers as appropriate to get the exact effect
you want. The official story from the project leader of the AWT
project, as explained to the Mountain View Java Users Group on December
4 1996, is:
"The case has been made and is now accepted that GridBagLayout is
too hard to use for what it offers. GBL will continue to be
supported, and something better and simpler will eventually be
provided as well. This 'better GBL' can be used instead of GBL."
Bottom line: nobody has to spend any effort on GBL, there are simpler
alternatives available now. In addition GBL is the source of a memory
leak. GBL puts "added" components into a hashtable, but
removeLayoutComponent() never removes them. See bug 4195295.
GBL documentation is hard to come by. Based on the obvious similarity,
it could have been derived from the Tk (of Tcl/Tk) grid layout manager.
If you're not a fan of nesting panels, and none of the other layout
managers do what you want (or you're working with legacy code that
already uses it), you might find some Tk documentation worth a look.
29. (Sect. 10) MyClass works fine except when I try to set a particular
font. I just can't seem to get it to work in Win95, but I can get it to
work on a MacOS and Unix.
[*] You probably specified a font name that isn't available under your
Win95 installation; this is one of those cross-platform differences
that can bite you if you over-specify for one platform, like specifying
"Arial" as a font and expecting it to work on something other than
Windows.
On both Windows 95 and Solaris 2.6, these fonts
o Dialog
o SansSerif
o Serif
o Monospaced
o Helvetica
o TimesRoman
o Courier
o DialogInput
o ZapfDingbats
are revealed by this code:
import java.awt.*;
class foonly {
static public void main(String s[])
{
String n[]= new Frame().getToolkit().getFontList();
for (int i=0;i<n.length; i++)
System.out.println(n[i]);
System.exit(0);
}
}
In other words, You can get a String array of the names of the fonts by
String[] fonts = Toolkit.getDefaultToolkit().getFontList()
The names of actual fonts like Helvetica, TimesRoman, and Courier are
deprecated in JDK 1.1 in favor of font styles like SansSerif, Serif,
and Monospaced (respectively). The font style will be mapped into the
nearest actual font on a platform.
The font styles are now mapped into a system font name using the
entries in one of the font.properties files in $JAVAHOME/lib. There are
multiple font.properties files corresponding to different locales. If
you wanted a quick hack for testing, you could modify the file or add
to it so a different mapping is done to a new font you want to try.
30. (Sect. 10) I've made a Lightweight Component (a Component directly
extending Component), and it keeps flickering/doesn't repaint itself
properly. Why is this?
[*] Lightweight Components, since they are notionally meant to be
"transparent", aren't painted directly in response to repaint(), but in
fact, Component.repaint() goes up the stack of Components until it
finds an "Opaque" Heavyweight Component (necessarily a Container), and
then calls repaint() on *that*.
At this point, a call is eventually scheduled to Container.update().
His first action is to call super.update, plunging us into
Component.update(), which clears the component to the background color,
since it has been called on a heavyweight, and returns. Then
Container.update() proceeds merrily to call update on all contained
Lightweight Components, recursively.
The bottom line: "transparency" of lightweight components will only
work correctly (without flickering) if the first upwardly accessible
heavyweight component in the containment hierarchy is
o a double-buffered heavyweight Component (necessarily a Container),
or
o a heavyweight that never updates, but always paints (i.e. one that
has overriden the default update() mechanism to not clear the
background).
If this is not done, the default Component update() will always clear
the background before any repainting is done, leading to annoying
flickering.
Another important point is that if your Container has its own paint()
method, that paint method of the container must call
super.update/paint(), otherwise the contained lightweight components
will never be painted. Putting this all together, the minimal
alteration to code to cause it to work in this case is to place the
method
public void update(Graphics g) {
super.paint(g);
}
in the most closely containing heavyweight Container, in a Component
hierarchy where you want to smoothly render lightweights that do not
paint areas extending past that painted by their parents, i.e. ones
that are not "transparent". This is dirty, but quick.
If you want smooth transparency, the call above should read
public void update(Graphics g) {
// standard offscreen generation here.
offg.fillRect(required background colour, full size);
super.paint(offg);
g.drawImage(myimage, 0, 0, null);
}
public void paint(Graphics g) {
// can generally expect resizes will hit update() first.
super.paint(offg);
g.drawImage(myimage, 0, 0, null);
}
It's possible to intertwine these, by having this.update() calling
this.paint(), with various replacings of the argument, but it is
clearest to override them separately, as in the example.
31. (Sect. 10) What is the difference between Component's
setForeground(Color c) and Graphics's setColor(Color c) ?
[*] First of all, these methods do the same thing: Set the foreground
color to the value of the parameter. The difference lies in where you
use them. There is also a Component.setBackground that will set the
background color.
If you are in a constructor or an event handler (e.g. "click here to
turn the canvas blue") you have a Component and should use the
setForeground() method. If you are in a paint() method, that takes a
Graphics context as its argument so you will typically use
g.setColor(c).
Unlike a Component, a Graphics object doesn't have a background color
and a foreground color that you can change independently. A Graphics
object arrives in the color(s) inherited from the drawing surface. From
then on, any rendering (drawLine(), drawRect(), fillOval(), etc.) will
be done in the setColor() color. Because they do different things, the
Component and Graphics methods have different names.
32. (Sect. 10) When I start a mouse drag inside a Component, and go outside
the Component, still dragging, the mouse events still get sent to the
Component, even though I am outside it. Is this a bug?
[*] No, it is the specified behavior. The Java API documentation says:
"... Mouse drag events continue to get sent to this component even
when the mouse has left the bounds of the component. The drag
events continue until a mouse up event occurs...."
It is done for the convenience and ease of the application programmer.
It allows you to handle all drags from the place of origin. If you
don't want this, simply look at the coordinates of the mouseDrag Event,
and if they are outside the Component, ignore them.
33. (Sect. 10) Why doesn't my window close when I click on the X in the
title bar?
[*] Here's how to make your program do that.
o JDK 1.0.2: Handle Event.WINDOW_DESTROY to do a hide() and
dispose() on the Frame.
o JDK 1.1:
+ Listen for WindowEvent and do hide(); dispose(); in
windowClosing() - this really ought to be the "default"
behaviour, so was made so for a Swing JFrame.
+ Enable AWTEvent.WINDOW_CLOSING and do the hide() and
dispose() in processWindowEvent().
o JDK 1.2: The Component JFrame does a close by default (see section
10).
34. (Sect. 10) How can I force a synchronization of the graphics state,
e.g. of a cursor change, or an animation frame to be rendered?
[*] This is done by the sync() method in Toolkit. So just use:
AnyComponent.getToolkit().sync();
35. (Sect. 10) How can I tab between components?
[*] In JDK 1.0, you have to read the key press, and program it
explicitly. JDK 1.1 supports tab and shift-tab (previous field)
automatically. The tab order is the order that the components were
added to the container.
36. (Sect. 10) What is the difference between "low level" and "semantic"
events?
[*] Low-level events are tied to a specific component (resizing a
window, dragging the mouse, striking a key, adding a Component to a
Container, etc.). Semantic events are those generated when you frob a
control (move a scrollbar, click on a button, select from a menu,
etc.), and the same kind of event can be generated by several different
components. A Button and a List both fire an Action event when they are
clicked on.
To the programmer, the important difference is that you can change a
low-level event such as the key value in a keypress, and it will
display the new value. You can also consume low level events so that
they do not appear in the widget. You can't do these things with
semantic events - they have already "occurred" to the widget.
Semantic events: Use the method addXListener() to add a listener object
which implements the XListener interface to get XEvent objects
delivered (usually via the AWTEventMulticaster). Low level events: Use
the method enableEvents() and override performX() to grab those events
in the object itself.
37. (Sect. 10) Is it possible to have a Java window float above all other
windows. For example, a tool palette floats in a super-layer always
above all the regular document windows on which you use the palette's
tools?
[*] On MS Windows, a Window object floats above all other windows,
unlike a Frame, which is layered in with ordinary windows. This
behavior yields a "floating" effect. Whether a Window object is really
supposed to float is another question entirely.
On Mac, a Window object is either layered in with other windows, just
like a Frame is, or else it is entirely modal - depending on which VM
you use. In Java - there appears to be no easy way to get floating
behavior. If anyone knows otherwise, please send in your comments.
38. (Sect. 10) How can iconify/deiconify a window in Java?
[*] JDK 1.1 had no way to write code to force a window to iconify or
deiconify. Support was added in JDK 1.2.
MyFrame.setState( Frame.ICONIFIED );
MyFrame.setState( Frame.NORMAL );
will do the trick. There is a corresponding getState();
39. (Sect. 10) How do I know which mouse button was pressed, and how often?
[*] To handle mouse events you have to implement the MouseListener
interface, or derive from the MouseAdapter class in order to use one of
the mouse-handling methods. The MouseEvent argument passed to the
handling methods contains fields that say which button was pressed, and
the click count. Use code like this.
public void mouseClicked(MouseEvent m) {
boolean leftButtonPush =
(m.getModifiers() & java.awt.event.InputEvent.BUTTON1_MASK) != 0;
boolean centerButtonPush =
(m.getModifiers() & java.awt.event.InputEvent.BUTTON2_MASK) != 0;
boolean rightButtonPush =
(m.getModifiers() & java.awt.event.InputEvent.BUTTON3_MASK) != 0;
int click = m.getClickCount(); // might be 1,2,3 clicks or more
You can also call at m.isPopupTrigger(). If it returns a true value,
the user has asked for a pop-up menu. On a lot of window systems, the
right mouse button is the trigger for pop-up menus.
You can overload processMouseEvent for your component.
public void processMouseEvent(MouseEvent e) {
if (e.isPopupTrigger()) {
// do what you want
}
else
super.processMouseEvent(e);
}
The code above applies to JDK 1.1. You can also call
java.awt.swing.SwingUtilities.isRightMouseButton(MouseEvent me).
See also question 15.10.
-------------------------------
11. Swing
1. (Sect. 11) What is Swing?
[*] Swing is a new GUI toolkit bundled with JDK 1.2, and available as
an add-on extension library for JDK 1.1. Swing is part of the Java
Foundation Classes and supports a GUI toolkit that lets developers
create components that have a pluggable look-and-feel. From an
architectural standpoint, the Swing component set extends - but does
not completely replace - the Abstract Windowing Toolkit (AWT).
Swing has many components that can be used in place of components in
the AWT (e.g. JFrame instead of Frame, JButton instead of Button,
JApplet instead of Applet, JPanel instead of Panel). It also has many
components that don't exist in the AWT (e.g. tool tips, toolbars, and
progress bars). However Swing relies on the underlying AWT being there.
The Swing toolkit allows the creation of GUI's that are every bit as
sophisticated as native code toolkits like MFC -- with the Java
advantage that they run on every platform. The pluggable look and feel
means that they can have the same appearance on every platform, or you
can choose to have it look like Windows on a PC, like Motif on a Unix
box, etc, just as the user chooses.
Swing also supports the Accessiblity API. That API allows the adaptive
software used by disabled computer users to directly query the Java VM
and extract information about the running program, the usual purpose
for this being to determine the UI components. Then the existing
adaptive software can interpret it to the user (e.g. read the contents
of a window, etc). Swing doesn't use native components, and the
adaptive software taps into native components, so without the
accessibility API has a real role in bringing the two together. The
beauty of the Accessibility API is that developers need to do little
work to support it, but they do need to be made aware of it if they add
new components.
With Swing, native window behavior is confined to external window
frames (and their borders) and a few other things such as fonts and the
buffers used to hold window contents. The composition, layout, and
drawing of controls is now all handled by Java code. So identical code
is executed to create and manage your user interface on every platform.
Swing provides a much greater consistency of behavior across different
platforms.
Swing works with JDK 1.1 if you download the swing.jar file and add it
to your path. Swing is built in to JDK 1.2, and Javasoft has just
changed its 1.2 Swing package-naming strategy. It is now called
javax.swing.
2. (Sect. 11) Should I use Swing or AWT to build my GUIs?
[*] Use Swing to build your apps now instead of AWT components,
wherever you have a choice. Swing is a GUI toolkit that is at least as
good as other commercial GUI toolkits, and better in several respects.
With Swing, it is easier to build an application that is portable
between Mac, Solaris, Windows 95 and NT, than it is to use Win32 and
build an application that just runs on Windows 95 and NT.
3. (Sect. 11) Where can I find a Swing tutorial?
[*] There is a Swing tutorial at
http://java.sun.com/docs/books/tutorial/ui/swing/index.html which is
part of this tutorial:
http://java.sun.com/docs/books/tutorial/ui/TOC.html
There is also a Swing FAQ at http://www.drye.com/java/faq.html
See also http://www.codeguru.com/java/Swing/index.shtml
There is a Swing Developer Connection website with white papers and
examples at http://www.theswingconnection.com/.
Please let this FAQ maintainer know about other good Swing tutorials
and online resources.
4. (Sect. 11) What is the Model/View/Controller paradigm?
[*] Model/View/Controller is a design pattern or framework originally
developed by Prof. Trygve Reenskaug at Xerox PARC in 1978/9. MVC was
developed to allow Smalltalk to conveniently support GUIs.
Model/View/Controller is a design pattern used extensively in Swing.
Basically, the "model" contains your data, the "view" is the graphical
representation, and the "controller" is responsible for the interaction
between the other two. As an example, think of visually editing the
Tree widget that represents a directory. The display is the view.
Selecting a file, and dragging it to the trash can will delete the
file. In order for the delete to happen, the controller must tell the
model what just happened in the view.
In practice, inter-communication between the view and the controller is
complex, so the two are bundled together in one category in Swing. The
model (data) is separate though.
There's a reasonable white paper on MVC in Swing at
http://java.sun.com:81/products/jfc/swingdoc-static/swing_arch.html .
There is information on other OO design patterns at
http://www.parallax.co.uk/cetus/oo_patterns.html.
5. (Sect. 11) When I run the Swing demo on Windows 95 I get an error "Out
of environment space."
[*] That's because you don't have enough space for your DOS
environment. You can fix this with:
o Right click your MS-DOS Prompt icon or window and choose
Properties.
o Choose "Memory" and on "Initial Environment", choose 4096 instead
of "auto".
o Run Swing again, you'll be OK.
6. (Sect. 11) How can I run Swing code in a browser?
[*] Most current browsers have to be specifically set up to run Swing
applets. Read the article at
http://www.javasoft.com/products/jfc/swingdoc-current/applets.html for
information about this. The article also contains a simple Swing
example applet, so you can confirm that that's your problem.
Another approach is to use the Java plug-in, which automatically gives
a Swing-compatible Java in the browser. See
http://java.sun.com/products/jfc/tsc/swingdoc-current/java_plug_in.html
7. (Sect. 11) Why is my menu showing up behind other components when I use
Swing?
[*] The answer relates to lightweight and heavyweight (peer-based)
components. There is a good article about it at
http://www.javasoft.com/products/jfc/swingdoc-current/mixing.html
For those who want the quick fix, and will read the article later,
adding the line:
javax.swing.JPopupMenu.setDefaultLightWeightPopupEnabled(false);
before you create any menus will probably fix it (even if you're using
menus other than JPopupMenu).
The summary answer is that a Lightweight component will not appear over
a heavyweight component by default.
8. (Sect. 11) Why is there no JCanvas? How do I get a lightweight Canvas?
[*] Use a JPanel as a Swing replacement for Canvas. All Swing
components have a paint(Graphics) routine that you can override, just
as you would have with Canvas, (but you probably want to override
paintComponent(Graphics) instead, see next question).
9. (Sect. 11) Why don't the borders of my Swing components look right when
I override paint(Graphics)?
[*] Swing splits painting into several different routines:
o paintComponent(Graphics),
o paintBorder(Graphics),
o paintChildren(Graphics)
all of which are called from paint(Graphics). If you override paint(),
unless you remember to do it, the paintBorder() and paintChildren()
won't get done.
In most cases, what you really want to do is override paintComponent()
instead of paint().
10. (Sect. 11) Why does my JFrame go away after I choose system close on
the window?
[*] Assume that you have a Swing JFrame component, and you handle the
windowClosing event, but do nothing in the handler. You will see that
the JFrame disappears anyway.
The reason is that JFrame's have default handling of the system close
operation, separate from the windowClosing event. You have to override
that by calling:
setDefaultCloseOperation(DO_NOTHING_ON_CLOSE);
on your JFrame.
11. (Sect. 11) Why can I run the Mac Look and Feel only on Mac OS?
[*] (This answer comes from the Swing Connection, see
http://java.sun.com/products/jfc/tsc/swingdoc-current/911.html).
Sun has not determined that it has the right to deliver the Mac look
and feel on platforms other than MacOS. If Apple were to confirm Sun's
right to deliver this look and feel on other operating systems, Sun
would be delighted to remove the lock. To date, Apple has declined to
do this.
Although you cannot use the new Mac L&F on non-Macintosh platforms,
there is a way to examine the source code so developers can use it as
an example of how to create a custom L&F. The Mac L&F is distributed in
"stuffed-binhexed" format, which is standard for the Macintosh. If you
develop on a MS-Windows platform and would like to examine the source
code for the Mac L&F then you can do that by downloading and using a
program called Aladdin Expander for Windows. You can download Aladdin
Expander from this URL: http://www.aladdinsys.com/expander/index.html
When you have downloaded Aladdin Expander, you can use it to decode the
Mac L&F file posted on the JDC.
A recent posting on comp.lang.java.gui suggested the following user
workaround:
import com.sun.java.swing.plaf.windows.WindowsLookAndFeel;
class MyOwnWindowsLookAndFeel extends WindowsLookAndFeel {
public isSupportedLookAndFeel() { return true; }
}
The desire on Sun's part to avoid infringing the Windows Look and Feel
is also the reason why the JTree uses colored circles (and soon, little
circles with a short line coming out of them) for the nodes to indicate
whether they are open or not. The Swing team could have used the '+'
and '-' as Windows does, or even the triangles that MacOS uses, but
decided against it.
12. (Sect. 11) When I set the cursor to WAIT_CURSOR why does it only change
when my cursor is over specific components?
or
How can I change the cursor to a WAIT_CURSOR over my entire window and
all of its components, preventing any user action, while some other
process is happening. (i.e. database access, opening another window,
downloading an image, sorting some data, etc.)
[*] In JDK 1.0.2 only the awt Frame could change its Cursor. With the
advent of JDK 1.1 the Cursor manipulation has been move to the
Component clss. Now all Components have access to the Cursor class.
You could change your Cursor to a WAIT_CURSOR for each Component. This
could be time-consuming. You could have a potentially large number of
Components. With the advent of the JFC Swing, there is a mechanism to
change the Cursor over the entire Window regardless of the number of
components. The Swing component JFrame has a method:
public void setGlassPane(Component glassPane)
which sets an awt Component as the 'glassPane' for the JFrame.
This Component will cover the entire JFrame's visible user accessible
(when visible), area excluding the border set by the underlying
windowing system. With the 'glassPane' Component you can set the
'WAIT_CURSOR' over an entire JFrame, prohibiting user input (the
'glassPane' Component gets all user input) and blocking the user until
your 'other' processing is complete.
NOTE: You must spawn a Thread to accomplish your 'other' work if you
want to see the WAIT_CURSOR while the 'other' processing is happening.
When the 'other' work is being accomplished, the 'glassPane' is visible
with a WAIT_CURSOR and gobbling up all user input. When the 'other'
work is finished, the Thread uses your waitCursor() method to hide your
'glassPane' until its needed again.
13. (Sect. 11) Why does the compiler complain that the
javax.swing.ProgressMonitor method "isCancelled()" isn't found?
[*] In American English there are two acceptable spellings: "canceled"
and "cancelled". Note that the first one has one "el" and the second
two "el's". Sun spells this "canceled" for ProgressMonitors but many of
the secondary sources of documentation spell it "cancelled". To make
matters worse, Sun spells it "cancelled" at other times, such as the
method "isCancelled()" for PrinterJob.
14. (Sect. 11) Why doesn't pressing the Enter key activate the default
button on a Swing dialog?
[*] The default keymap for Swing text components (derived from
JTextComponent) binds the Enter key (VK_ENTER) to the ActionEvent for
text fields. This was done to be compatible with the behavior of
java.awt.Textfield. To enable use of the Enter key to activate the
default button, remove the Enter key binding from the default text
component keymap as follows:
static {
KeyStroke enter = KeyStroke.getKeyStroke(KeyEvent.VK_ENTER, 0);
Keymap map =
JTextComponent.getKeymap(JTextComponent.DEFAULT_KEYMAP);
map.removeKeyStrokeBinding(enter);
}
15. (Sect. 11) How do I create non-rectangular Components?
[*] Take a look at the following example that comes with the JDK:
$JAVAHOME\demo\awt-1.1\lightweight\RoundButtons\example.html
-------------------------------
12. Browsers
1. (Sect. 12) When will my favorite browser support Java 1.1?
[*] All popular browsers now have JDK 1.1 support. Netscape
Communicator 4.04 plus patch "J" fully supports the features of Java
1.1. It was released in December 1997, and is only missing the JavaSoft
support for applet signing (Netscape has gone its own way on this). See
http://developer.netscape.com/support/faqs/champions/java.html#21
If you have Netscape 4.05, and the console says anything other than
Java 1.1.5 then you do not have a fully 1.1 compliant Netscape. There
is a special preview version available here:
http://developer.netscape.com/software/jdk/download.html
Netscape badly fumbled its Java support in 1997 as its market was
seized by Microsoft. Microsoft is using IE as a strategic tool to
deploy what Microsoft employees call "polluted Java". For both of these
browsers, the Java Plug-in is currently a good approach.
Sun's HotJava browser fully supports the JDK 1.1 features. People who
are obliged to use a browser without standard Java support should use
the Java Plug-In. The Java Plug-In substitutes a standard Java virtual
machine for the one that shipped with the browser. It allows you to use
RMI, JavaBeans components, and Java Foundation Classes in Internet
Explorer 3.02, 4.0, and 4.01. The Java Plug-In also works flawlessly
with Netscape browsers. You can download the Java Plug-In from
http://java.sun.com/products/.
Note that you need to change the HTML a little, to ensure that the
plug-in JVM is invoked, not the browser JVM. A tool is included to do
the changes automatically.
2. (Sect. 12) What applet routines get called in various browsers and the
plug-in on different browsing actions (back, forward, load, etc)?
[*] Java supporter Dave Postill did the work to get this answer.
The life cycle of an applet is illustrated by logging calls to init(),
start(), stop() and destroy(). Use caution when your applets have
threads since in most sample applet code, the stop() method calls stop
on any separate threads within the applet, and then sets them to null.
This reckless threadicide is because most people think of the stop()
method as something called only when the user leaves the page and wants
to forget about it. But since Netscape calls stop() when you resize the
window, your users would lose the applet's state when they thought they
were only making a minor adjustment.
See "Java Tip 8: Threads, Netscape, and the resize problem - How to
deal with applet resizing in Netscape Navigator", JavaWorld
http://www.javaworld.com/javatips/jw-javatip8.html. Sadly the JavaWorld
workaround does not completely fix the problem, since it relies on
start() being called soon after stop() to identify a resize. However if
you minimise the browser it may send a stop() to the Applet and then
may not send a start() until the Browser is either restored or
maximised. In this case, using the workaround results in the Applet
being destroyed following minimising of the Browser - unless the
Browser gets un-minimised within the killThreads timeout.
Netscape Netscape Applet- Internet Internet
[4.04/JDK with Viewer Explorer 4 Explorer
1.1.4] Plug-In [JDK SP1 with Plug-In
[4.05/JDK [4.05/ JDK 1.1.5] 4.72.3110.8 [5.00.0518.10
1.1.5] 1.1.5/ [JDK on NT 4.0 / Plugin 1.1]
Plugin 1.1] 1.1.6] SP3 on NT 4.0 SP3
1. Clear
browser nothing nothing nothing nothing nothing
cache
init(),
2. start() or
Initial init(), init(), init(), init(), init(),
load of start(), start() start() start() start()
.html stop(),
start() [1]
3. Back stop() stop(), [4] stop(), stop(),
destroy() destroy() destroy()
4. init(), init(), init(),
Forward start() start() [4] start() start()
stop(), stop(), stop(), stop(),
5. Reloaddestroy(), destroy(), [4] destroy(), destroy(),
init(), init(), init(), init(),
start() start() start() start()
6.
<shift>
reload stop(), stop(), stop(), stop(),
[NS], destroy(), destroy(), [4] destroy(), destroy(),
<ctrl> init(), init(), init(), init(),
reload start() start() start() start()
[IE]
7. Resizestop(), [3] [3] [3] [3]
start()
8.
Minimize [2] [3] stop() [2] [3]
9.
Restore [2] [3] start() [2] [3]
10. Exit stop(), stop(), stop(), stop(), stop(),
destroy() destroy() destroy() destroy() destroy()
Notes:
[1] Results not repeatable.
[2] Not tested.
[3] Tested, and found that no logged methods are called.
[4] Test not applicable.
3. (Sect. 12) Is it possible to set and retrieve cookies from Java, in a
manner that is compatible with all browsers supporting cookies?
[*] Short answer: no.
Longer answer: probably no.
Ultimate answer:
A cookie is a morsel of tasty data that a server sends back to the
client, and can retrieve on demand. It allows the server to retain some
state information for each of its clients. The information is typically
something like "what pages has the user seen?" or "is this a privileged
user?".
The DevEdge site on Netscape's home page has a Javascript-Java example
on getting cookies. Also
http://www.geocities.com/SiliconValley/Vista/1337 has info on
connecting an applet with JavaScript functions. It's quite involved.
Stick to just Java if you can.
4. (Sect. 12) I am developing an applet and testing it in Netscape
Navigator. I find that after I recompile, I press reload, clear the
caches, retype the URL of the HTML wrapper, and I still have the old
version. Why is this?
[*] Note: a reader reports that as of Netscape Communicator 4.05 it is
possible to force the browser to reload the applet by holding down
"control"+"shift" while clicking "Reload"
In the past Netscape has completely failed to improve the defective
code that does this monstrously wrong thing. It has been like this for
many successive releases.
Flushing the network cache will make no difference; that isn't where
the caching is taking place. Although applets are sometimes "pruned"
and their ClassLoaders garbage-collected, this doesn't happen
predictably, so restarting Netscape is the only reliable work-around at
the moment.
A related question is "how do I make the browser reload from a
URLConnection instead of just getting the content from the local
cache?" The answer is to use
java.net.URLConnection.setUseCaches(false)
Browsers seem to vary in their conformance to this programmatic
request. Netscape caching varies depending on whether a proxy server is
in use, and which thread in the applet made the get request.
Another approach is adding "?" to the URL, e.g.
http://www.somesite.com/webcam/image.jpg?100
and increasing this number each time the Applet fetches the image.
5. (Sect. 12) Why didn't Netscape reload the applet when you pressed the
Reload button?
[*] For the applet to be reloaded, the new version would have to be
loaded in a different ClassLoader. Navigator/Communicator's policy for
assigning ClassLoaders to applets doesn't take into account whether a
reload has been done (although there is no technical reason why it
couldn't).
Some versions of Netscape reload the Applet if you use
Edit/Preferences/Advanced/Cache to Clear Memory Cache and Clear Disk
Cache, then <Shift> while you click on reload.
In Explorer, use View/Options/General/Delete Files, then <Control>
'Reload' button to reload the page containing the applet.
Until they fix it, use the appletviewer to test applets. And send them
mail - developers can only fix the bugs they know about.
6. (Sect. 12) Should I use Microsoft CAB files or Java JAR files?
[*] The question contains its own answer.
CAB format is a Microsoft-only format. So do not use it as it destroys
software portability.
JAR format is the Java standard format, based on PKZIP format including
data compression. JARs were introduced with JDK 1.1.
See http://www.ibm.com/java/community/viewarchive4.html for more
information.
You should use the Java standard format JAR (Java Archive) files, not a
vendor-specific format. JAR files are not just a Java standard, they
are in industry-standard PKZIP format. One reader comments that both
formats can be used with this tag:
<APPLET NAME=myapplet
ARCHIVE="myzip.zip"
CODE="com/nnnnn/nnnn/cccccccc.class"
WIDTH=n
HEIGHT=n>
<PARAM NAME="cabbase" VALUE="mycab.cab">
</APPLET>
IE3 does not support JAR
IE4 supports compressed and uncompressed JAR, but not signed JAR
7. (Sect. 12) How can I tell the version of Java that my browser supports?
[*] See http://java.rrzn.uni-hannover.de/insel/beispiele/vertest.html.
This page tells you whether your browser supports JDK 1.1.
See http://www.uni-kassel.de/~pfuetz/Properties.html This page tells
you which classes you may expect to be present in the browser's
runtime.
8. (Sect. 12) How can I tell the options/commands that Netscape's JVM
takes?
[*] You can open the Java Console and type a "?" to get a list of all
the commands/options available to you in Netscape's built-in JVM.
-------------------------------
13. Applets
1. (Sect. 13) What is the difference between an application, an applet,
and a servlet?
[*] An applicationis a standalone program.
An applet is a downloadable program that runs in a web browser.
Typically an applet has restricted access to the client system for
reasons of security. Other than that it is virtually no different from
a regular Java program.
A servlet is a Java program whose input comes from a server and whose
output goes to a server. Other than that it is virtually no different
from a regular Java program. Think of a servlet as an application, but
one that (like an applet) requires a context in which to run, namely
web server software. Servlets are used like CGI scripts, but take much
less processor resources, and they allow the server end to be written
in Java as well as the client. There is a page with much servlet
information at: http://www.frontiernet.net/~imaging/servlets_intro.html
There is a servlet tutorial at
http://java.sun.com/docs/books/tutorial/servlets/index.html
and another servlet tutorial at
http://www.novocode.com/doc/servlet-essentials/
There is a servlet FAQ at http://www.saint-elie.com
The web server starts up a servlet when the URL is referenced, and now
your applets have something that they can talk to (via sockets) on the
server that can write files, open connections to other servers, or
whatever.
There is also a software technology from IBM called an "Aglet".
An aglet is a mobile agent that can go from machine to machine,
performing tasks, serializing data collected, and "shipping itself"
(code and data) to the next machine. It's too early to say if aglets
are a flash in the pan or a dawning technology. Read about aglets at
http://www.trl.ibm.co.jp/aglets/
Finally, there is the ticklet (Tcl/Tk) plugin for your browser
(Netscape or Explorer) available at http://sunscript.sun.com/plugin/
Don't confuse Sun's JWS "Java Web Server" with JWS "Java Workshop".
Java Web Server supports servlets, as does the lightweight and free
server at Acme.com:
http://www.acme.com/java/software/Acme.Serve.Serve.html
2. (Sect. 13) My applet works on my machine, but fails when I put it on
our web server. Why?
[*] It could be one of several reasons, and unfortunately the messages
that you get in this situation aren't much help. In general, you can
assume that either your applet's class files are corrupted somehow, or
the web server can't find one or more of them when the browser needs
them.
Be careful of the following things:
o Make sure you transfer the class files in binary mode, rather than
text or ASCII mode. An error from the browser saying "cannot start
applet ... bad magic number" usually means that one of the class
files on the server is corrupted. Replace your class binary files
on the web server, clean up the cache of your browser, and reload
your applet.
o Make sure you transfer all of the class files that are a part of
your applet. Sometimes people are surprised by how many there are.
There will be a class file for every class and interface you
define, even if you define more than one in a single source file.
If you use the Java 1.1 "inner classes" feature, there will be
class files for each inner class as well.
o Make sure you maintain the appropriate case distinctions in your
filenames. If a class is called StUdLy, it must be found in a file
called StUdLy.class.
o Make sure you maintain the directory structure that matches your
package structure. If you declare a class in package com.foo.util,
the class either needs to be in a Jar file, or the class file
needs to be in directory com/foo/util under the applet's codebase
directory. Again, case distinctions are important for
package/directory names, just as they are for class/file names.
o Make sure that the web server process will have read access to the
class files, and search access to the directories that the files
are in. For example, if the web server runs on a Unix machine, use
the command "chmod o+r filename" for the files, and "chmod o+x
dirname" for the directories.
3. (Sect. 13) How do I load a webpage using an applet?
[*] Use code like this,
getAppletContext().showDocument(
new URL("http://www.here.com") );
Or, to show the page in another window or frame,
getAppletContext().showDocument(
new URL("http://www.here.com"), "windowname" );
4. (Sect. 13) How do I use an image as the background to my applet? How do
I set the background color of my applet the same as the browser?
[*] You can simply do a g.drawImage(yourImage, x, y, this) in the
paint() routine of your applet. If the image isn't big enough to fill
the entire background, tile it or scale it. Here is some code to tile
it
// The background image is named "bg".
int w = 0, h = 0;
while (w < size().width) {
g.drawImage(bg, w, h, this);
while ((h + bg.getHeight(this)) < size().height) {
h += bg.getHeight(this);
g.drawImage(bg, w, h, this);
}
h = 0;
w += bg.getWidth(this);
}
Alternatively, the AWT can scale your background image to the size of
the applet. The result quality will depend on the kind of image. Inside
an applet class, you can use:
drawImage(img, 0, 0, size().width, size().height, this);
You can set the background color to match the background color of the
browser by passing the value in as a parameter, like this:
In the HTML applet tag:
<param name=BrowserColor value=F1F1F1>
(value should be the same hex as the HTML COLOR value).
In the Applet init() method:
String colparam = getParameter("BrowserColor");
int col = Integer.valueOf(colparam,16).intValue();
setBackground( new Color(col) );
An applet cannot override the size imposed by the HTML. If you make the
applet larger, the browser will still clip to the original size. If you
need more room, open up a new Frame, Window or Dialog to show your
output.
5. (Sect. 13) How do you make the applet's background transparent?
[*] There is no way to give an applet a transparent background that
lets the web browser background show through. You can simulate it by
giving the applet a background that matches the underlying browser
background. (For a straight color, it will be the value of <BODY
BGCOLOR=nnnnnn> in the HTML file). It doesn't produce satisfactory
results with a patterned background because of problems aligning the
edges.
Lightweight components (new in JDK 1.1) have a transparent background,
but that merely allows other components to show through. A lightweight
component is always ultimately positioned in a heavyweight component.
6. (Sect. 13) How do you do file I/O from an applet?
[*] Unsigned applets are simply not allowed to read or write files on
the local file system (see answer to question 7.8).
Unsigned applets can, however, read (but not write) non-class files
bundled with your applet on the server, called resource files.
See answer to question 7.8 and question 7.9.
7. (Sect. 13) How do I pull a non-class file, such as a .gif, out of a jar
file?
[*] [Question has been retired, in favor of question Q7.9]
8. (Sect. 13) How do I read a text file stored in a Jar?
[*] [Question has been retired in favor of Q7.9]
9. (Sect. 13) How do you get a Menubar/Menu in an applet?
[*] In your applet's init() method, create a Frame instance and then
attach the Menus, Menubar etc to that frame. You cannot attach the Menu
or a Menubar to an applet directly.
Or get the parent Frame like this (doesn't work in all execution
environments):
Container parent = getParent();
while (! (parent instanceof Frame) )
parent = parent.getParent();
Frame theFrame = (Frame) parent;
This second suggestion definitely doesn't work in the appletviewer, and
probably won't work on Macs (where would the Menubar go?) or in some
browsers. In JDK 1.1, just use a popup menu, which isn't attached to a
Frame.
10. (Sect. 13) Can I get rid of the message "Warning:Applet Window" along
the bottom of my popup windows in my Applet?
[*] This is a security feature that prevents the applet programmer from
popping up a window that looks like it came from the native OS and
asking for passwords or credit card info (etc.). Users must always be
aware of when they are talking to an unsigned applet. You can get rid
of it by signing the applet, if the user accepts signed applets from
you. In Netscape (only), using the Capabilities API to make the call
PrivilegeManager.enablePrivilege("UniversalTopLevelWindow");
before creating the Frame eliminates the message, if the security
manager passes it.
11. (Sect. 13) When I subclass Applet, why should I put setup code in the
init() method? Why not just a constructor for my class?
[*] The browser invokes your constructor, then setStub, then init().
Hence when your constructor is invoked, the AppletStub (and through it
the AppletContext) is not yet available. Although in principle you can
do things in the constructor that don't rely (even indirectly) on the
AppletStub or AppletContext, it is less error-prone to simply defer all
setup to the init() method. That way you know that anything that needs
the stub/context will have it available.
12. (Sect. 13) I want to know about {applets,applications} but the lousy
book I got just talks about {applications,applets}. What can I do?
[*] The truth is that 95% of the material is the same, whichever your
book chooses to focus on. Some people write their apps to work
completely in a Panel, then depending on whether they're running
stand-alone or in a browser the Panel is either added to a Frame or an
Applet. The trick is that you need to add a listener to the
application's Frame to handle the WINDOW_CLOSING (previously
WINDOW_DESTROY) event yourself.
If you fail to do this, when running as an application, the window
won't close. See Question 15.7 for a sample of the right handler.
In this scenario the following code will tell you which environment
you're running in:
public boolean isRunningInBrowser() {
Component p = getParent();
while(p != null && !(p instanceof Frame)) {
p = p.getParent();
}
return (p == null);
}
13. (Sect. 13) How do I print a page with an applet?
[*] Browsers are starting to introduce support for this. Until they all
have it, your best bet is to print a screendump. Using the browser to
print the page may leave a blank where the applet is. Putting print
support in the applet will print the applet only, not the rest of the
browser page.
Also in the FAQ: Q5.2.
14. (Sect. 13) How can I position my dialogs centered (not top left)?
[*] Use some code like this:
void center(Component parent) {
pack();
Point p = parent.getLocation();
Dimension d = parent.getSize();
Dimension s = getSize();
p.translate((d.width - s.width) / 2, (d.height - s.height) / 2);
setLocation(p);
}
15. (Sect. 13) How can I get two applets on the same page to communicate
with each other?
[*] This is the purpose of the InfoBus protocol. See
http://java.sun.com/beans/infobus/index.html
The older way to do it was as follows. In your HTML page, give a NAME
in the APPLET tag for the applet receiving the message, say <APPLET ...
NAME=someName ...>. In the Java code of the other applet do
Applet anotherApplet = getAppletContext.getApplet("someName");
Cast anotherApplet to the correct applet subclass, and you can call any
methods in the applet subclass. Don't forget to use appropriate
synchronization when two threads tweak variables. This only works when
the applets are truly on the same page. If they are in different
frames, it doesn't work.
You can walk through all the applets on an HTML page using code like
that below. However this appears to be broken in Communicator 4.04 on
Win95.
Applet otherApplet;
AppletContext ac =getAppletContext;
Enumeration applets = null;
for (applets=ac.getApplets(); applets.hasMoreElements(); ) {
otherApplet=(Applet)applets.nextElement();
if (otherApplet!=this) break;
// do something with otherApplet, e.g.
// if (otherApplet instanceof FooApplet) ...
}
Some people suggest using the static members of a common class to
communicate information between the applets. This is not recommended as
it relies on class-loading behavior that may change in future. Netscape
changed it in one Beta so it didn't work, then changed it back again so
it did. It doesn't work if you use the "mayscript" tag though.
Inter-applet communication sometimes arises when you have a
multi-screen type program and you don't want to force the user into
downloading everything at once. One alternative is to make them into
one applet with two GUIs. Try to avoid the need for applets to talk to
each other. Also check the URL
http://java.sun.com:81/products/hotjava/1.1/applet_environment.html
which explains how it can be done in HotJava 1.1. Recommendation: avoid
code which is browser-specific.
16. (Sect. 13) How can I resize an applet?
[*] If you want resizing behavior from an applet, you should launch an
external Frame that can be resized independently.
One programmer suggests using percentages for the height/width
parameters in an applet tag, like this:
<APPLET CODE="lewinsky.class" WIDTH="100%" HEIGHT="100%">
You can't resize the applet directly, but it does get resized when you
resize the browser window (tested with Netscape 3.04 and 4.04, but does
not work with appletviewer). If you have nothing else on your HTML page
and use 100% for your width and height, the browser window looks almost
like a real application.
For the extremely tricky: have the browser reload the page with the
applet when the browser resizes using new values for width and height
(probably not what you want most of the time). You would need
Javascript to generate a page dynamically using document.write("...")
when the browser resizes. Not recommended. Another possibility is to
use the new SplitPane class in JFC.
17. (Sect. 13) How do I sign an applet?
[*] The browser vendors have produced independent and different
solutions for applet signature (alas). Here are some URLs on this
topic.
See the Java Signing FAQ at
http://www.fastlane.net/~tlandry/javafaq.txt
Read the basics of signing at:
http://www.javareport.com/html/features/archive/9802/somers.shtml
Be aware that the mechanism of signing and administering signed code
changed significantly between Java 1.1 and Java 2.
Netscape signing:
http://developer.netscape.com/docs/manuals/signedobj/javadoc/netscape_security_Target.html
https://certs.netscape.com/client.html
http://developer.netscape.com/docs/manuals/signedobj/capabilities/contents.htm
http://developer.netscape.com/docs/manuals/signedobj/targets/contents.htm
Microsoft signing:
http://www.microsoft.com/java/security/secfaq.htm
http://www.microsoft.com/java/sdk/20/tools/signcode.htm
-------------------------------
14. Multi-Media
1. (Sect. 14) Are there any good Java Image libraries?
[*] Yes. Try the Java Image Management Interface (JIMI), which offers a
free trial period. JIMI is a toolkit that lets your Java programs read
and write many graphics file formats (PNG, JPG, BMP, GIF etc). JIMI is
written in 100% Java, and best of all, it's a breeze to get started
with. See http://www.activated.com/jimi.html
2. (Sect. 14) Why won't my audio file play?
[*] Java 1.1 and earlier releases use one audio format exclusively. The
audio file must be in .au format, recorded at 8 KHz, mono, in mu-law
encoding. If your audio clip is in a different format (e.g., .wav) or a
different frequency it must be converted to the exact specifications
above before Java can play it. Support for .wav and other formats is
part of the Java Media Framework coming in JDK 1.2.
Search at http://www.yahoo.com for GoldWave for Win 95, sox for Unix
and similar conversion utilities for other systems.
Other sites:
o One conversion utility in Java is at
http://saturn.math.uaa.alaska.edu/~hursha
o The source of a Java class to play linear PCM .WAV files is at:
http://www.shef.ac.uk/~cs1mjp/Java/WhiteBoard/WavePlayer.html. It
can be used in any Java application or applet.
3. (Sect. 14) How can I do video streaming using Java?
[*] That is the purpose of StreamBean. See
http://www.streambean.com/streambean/
4. (Sect. 14) Does Java support animated GIFs?
[*] Java 1.0.2 and earlier releases use GIF and JPEG formats, and do
not use the GIF89 animated GIF format. (An animated GIF is one that
contains successive frames of an image, so when they are displayed in
quick sequence the image appears to contain movement). When you display
an animated GIF in Java 1.0.2, you will just get the first frame. There
doesn't appear to be any easy way to get other frames from the image.
The advantage of an animated GIF file is that there is only one file to
download, and it is simple to do simple animations. The advantage of
programmatic control over individual frames is that you control the
rate and order of displaying them.
Here's a surprise: JDK 1.1 supports the animated display of animated
GIFs. For simple animations animated GIFs are a lot easier and
lighter-weight than coding an animation explicitly.
5. (Sect. 14) How do I create animated GIFs?
[*] Use GIFanimator from ULead (said to be the best)
http://www.ulead.com, or GIF Construction Set from Alchemy Mindworks
6. (Sect. 14) How do I prevent animated GIFs from flashing while
displaying?
[*] The problem is most likely that in your paint method you have
g.drawImage(img, ix, iy, this);
You should change this to
g.drawImage(img, ix, iy, getBackground(), this);
This will change all the transparent regions of the image to the
background color before painting to the screen. If you paint
transparent images directly to the screen they flicker.
If that does not solve it then check that imageUpdate is
public boolean imageUpdate(Image img, int flags, int x, int y,
int width, int height) {
if ((flags & (FRAMEBITS|ALLBITS))!= 0) repaint();
return (flags & (ALLBITS|ABORT)) == 0;
}
update is
public void update(Graphics g) { paint(g); }
If you have a background Image behind the partly transparent animated
GIF you will have to double buffer. You can crop the backgound image so
you won't have to double buffer the full app and waste too much memory.
7. (Sect. 14) Does Java support transparent GIFs?
GIF89a images with a transparent background show up as transparent
without further filtering. This has been supported from 1.0 on. Java
correctly displays both animated GIFs and transparent GIFs.
Even better, you can fill the transparent pixels with a color (so they
appear non-transparent in Java). Just pass the fill color explicitly:
drawImage(img, x, y, w, h, fillcolor, this);
Further, you can filter the pixels of an Image to turn any bits you
wish transparent. However, the most you can do is reveal what is
underneath the image. You cannot reveal what is underneath the applet
(i.e. on the browser itself). By default applets have a plain grey
background.
8. (Sect. 14) How do I play video in Java?
[*] Use the Java Media Framework Player API.
Other sites:
o The Java Media Framework Player API spec can be found at
http://java.sun.com/products/java-media/jmf/
o Intel has released a SDK for the Java Media Framework Player API.
The SDK is for Windows 95 and Windows NT. For more information,
see http://developer.intel.com/ial/jmedia
o SGI has released an implementation of JMF for IRIX: See
http://www.sgi.com/Products/motion/
9. (Sect. 14) How can I play *.au files from an application?
[*] A new static method was introduced in JDK1.2 in the class Applet:
public final static AudioClip newAudioClip(URL url) {
The method turns a URL (pointing to an audio file) into an AudioClip.
Since the method is static it can be used without any objects of class
Applet needing to exist, e.g. in an application. Once you have an
AudioClip, you can play it with:
MyAudioClip.play();
The Java Media Framework provides a richer API for playing sounds in an
application.
For code prior to JDK 1.2, you can use the AudioClip or AudioPlayer
class in sun.audio
http://www.javaworld.com/javaworld/javatips/jw-javatip24.html. If you
do this your code is no longer 100% pure Java, as it relies on a vendor
library.
import sun.audio.*;
URL url; ...
AudioStream audiostream = new AudioStream(url.openStream());
AudioPlayer.player.start(audiostream);
...
AudioPlayer.player.stop(audiostream);
Also in the FAQ:
Use the new Java Media Framework API, allowing a wide range of video
and audio formats to be played back. See previous question.
10. (Sect. 14) How do I read in an image file, in an application?
Use
Image img = Toolkit.getDefaultToolkit().getImage(fname);
11. (Sect. 14) When I initialize a component,I call MyComponent.getImage()
to get its image. createImage() returns null! I know the image works
elsewhere. What's wrong?
[*] A peer component needs to exist for your component before you can
get its image. This is done by the method addNotify() (surely one of
the most poorly named methods in all Java -- it doesn't mean "add a
Notify". It means "Notify that the Component has been added to a
Container". It tells the system, "you need to create the peer for this
Component now"). addNotify will be called for you when you add your
component to a container.
Javasoft notes that most applications do not call addNotify() directly.
It is called for you when you add the component to a container. If you
have any code that requires peer resources, you can move it into a
thread that is started from a conditional branch of the paint() or
update() method. That way the peer will definitely exist when the code
is executed.
A common reason for seeming to require peer resources in a constructor,
or alternatively in the getPreferredSize() method, (which is also
usually before the peer is created) is to measure the area required for
your window, in terms of font and image sizes. Font sizes may be
obtained by calling
Toolkit.getDefaultToolkit().getFontMetrics(somefont). This does not
require a peer. Image sizes may be obtained by waiting for the relevant
Image to load from the ImageProducer by using an ImageObserver, or a
MediaTracker (see 8.15), also without requiring a peer. See 15.4 for
more discussion of this problem.
If you override addNotify(), don't forget to call super.addNotify() in
your overriding version.
12. (Sect. 14) How can I force a reload a fresh version of an image into my
applet? My image file is changed periodically, and I want the applet to
go and retrieve it, not cache it.
[*] You need to turn off caching for the URL.
URL url = null;
URLConnection con;
try {
url = new URL(getDocumentBase(),"image.jpg");
con = url.openConnection();
con.setUseCaches(false);
} catch (MalformedURLException e1) {
System.err.println(e1.getMessage());}
catch (IOException e2) {
System.err.println(e2.getMessage());}
Note: Some programmers have reported that it caches anyway, even if
they do this. That is a browser bug.
One programmer reported that even after turning off caching and calling
image.flush() before getImage(..), he was still seeing the same picture
even though it had been changed on the server.
He worked out a solution: access the image via a cgi script that
returned a URL. This redirects the browser, and he put in an Expires:
header as well to force the reload. Painful and laborious, but it got
the result.
13. (Sect. 14) How can I save an Image file to disk in JPG or GIF format?
[*]
o If you have an Image and you want a JPG in a file, download James
Weeks's code from http://www.obrador.com/essentialjpeg/
o Sean Breslin also wrote a program that compresses a Java Image
into a JPEG file. http://www.afu.com/jpeg.txt
o If you just want to convert some file, a non-Java solution is to
use the standard IJG 'cjpeg' utility. It supports GIF, PPM, BMP,
PNG and Targa input files.
o If you have an Image and you want a GIF or PPM in a file, download
Jef Poskanzer's abstract ImageEncoder class at
http://www.acme.com/java/software/
Also try the Java Image Management Interface (JIMI), which is free for
non-commercial use. JIMI is a toolkit that lets your Java programs read
and write many graphics file formats (PNG, JPG, BMP, GIF etc). JIMI is
written in 100% Java, and best of all, it's a breeze to get started
with. See http://www.activated.com/jimi.html
14. (Sect. 14) What causes this problem:
$ appletviewer m.html
Premature end of JPEG file
sun.awt.image.Im...Exception: JPEG datastream contains no image
at sun.awt.image. ... .produceImage(JPEGImageDecoder.java:133)
at sun.awt.image.Inpu...mageSource.doFetch(
InputStreamImageSource.java:215)
at sun.awt.image.ImageFetcher.run(ImageFetcher.java:98)
[*] There's a known bug in early releases of the JDK which can cause
the above failure when reading a JPEG across a slow connection. The
failure only occurs if the JPEG contains a large application data block
(APPn marker) - the problem is that the JPEG decoder is trying to skip
over the APPn and failing if not all of the APPn has been received yet.
The quoted error message is only one of several possible complaints,
but they all stem from the same root.
Photoshop is the most common source of JPEGs containing oversize APPn
blocks. In particular, if you allow Photoshop 4 to save a thumbnail
(preview) in a JPEG, the thumbnail plumps up Photoshop's private APPn
marker to several K, which is usually enough to cause this problem.
There are several possible workarounds:
o Get a newer JDK - this problem is said to be fixed in 1.1. (If you
are putting images up on the Web, this isn't much of a solution,
because you can't assume visitors to your site have an up-to-date
Java installation.)
o When making JPEGs for Web use from Photoshop, make sure you have
turned off the "save thumbnails" preference. (This is a good idea
quite aside from bug workarounds, because the thumbnail is just a
waste of download time as far as a Web browser is concerned.) You
might still have a problem if you've got verbose comments or lots
of paths being saved into the file, but 99% of the time, getting
rid of the thumbnail will make Photoshop's APPn small enough to
not trigger the Java bug.
o Use a tool such as 'jpegtran' (from the Independent JPEG Group) to
strip out the Photoshop APPn entirely without any loss of image
quality. Recommended answer for the compulsive byte-trimmer.
o (Last resort) Load and resave the image in a different image
editor that won't insert any APPn or other overhead data. This
implies a JPEG generational loss, so I don't recommend it if you
are picky about image quality.
Any large overhead marker will cause the same problem; 4K of comment
text, say, in a COM marker. So Photoshop is not the only source of
tickling this bug.
15. (Sect. 14) How can I convert between GIF and JPEG formats?
[*] In a word: don't.
There's hardly any overlap between the set of images that JPEG works
well on and the set that GIF works well on. Sometimes, with enough
care, you can get an acceptable conversion...but most of the time
GIF<->JPEG conversion will just turn your image to mush. It's better to
pick the right format in the first place.
Other sites:
o If you're determined to convert formats anyway, try the GBM
(Generalized Bitmap Module). The package is GNU licensed, in C and
is very good. Find it at
http://www.interalpha.net/customer/nyangau/
GBM does a good job converting to JPEG, and 'lossiness' is
adjustable to 0%. It also converts to/from about 20 other formats,
does cropping, sizing, color mapping, gamma correction,
halftoning, everything you could want. GBM source doesn't support
JPEG directly, but utilizes JPEG source from IJG called jpeg-6a
and found at
ftp://sun2.urz.uni-heidelberg.de/pub/simtel/graphics/jpegsr6a.zip
o For more info see the JPEG FAQ at
http://www.faqs.org/faqs/jpeg-faq/
16. (Sect. 14) If you have an InputStream (rather than a file) that
contains an Image, how can you display it?
[*] Use this method, and some adroit shuffling.
Toolkit.getImage(URL url)
Create a thread that pretends to be an http server. Make it listen to
some port (8765 for example) for incoming requests. When the thread
gets a request, it should simply whisk up the appropriate http headers
and follow it by the InputStream. Thus the component that has the input
stream and wants to do the getImage(url) can now invoke:
Toolkit.getImage("localhost:8765/")
The thread will act as a stream-to-url adapter, and send back the data
It saves you from having to read 200K of JPEG data before you can begin
drawing anything.
17. (Sect. 14) How can I record sounds in Java?
[*] The Java Media Framework will eventually support this, but it does
not yet. JMF 1.0.1 only supports playback.
JMF 1.0.1 is bundled with JDK 1.2, and available as a separate download
for JDK 1.1 and Netscape Communicator 4 with Java 1.1.
Other sites:
In the meantime, there is a package for Win95/NT available at
http://www.scrawl.com/store/. It supports 8, 16-bit, stereo, mono,
11025, 22050, 44100 Hz record/play, load/save .WAV files. You
could also interface to native code for your platform.
18. (Sect. 14) Does Java have any built-in support for displaying HTML?
[*] JDK 1.1 supports rendering HTML using the unbundled JFC 1.1 package
known as Swing. The Swing package is bundled in JDK 1.2. It has an
elementary (graphics, tables, text) HTML bean that is good enough for
simple rendering (help files, email, etc).
Other sites:
o JavaBrowser http://www.ii.uib.no/~alexey/jb/index.html Free
source, free for use under GNU LGPL licence, HTML 2.0 (sort of).
o ICE Browser - Java Bean Component
http://www.icesoft.no/ICEBrowser/ Free binaries for use in free
applications. Commercial licensing available including source -
flat fee licence. Thin HTML client! Lightweight! HTML 3.2
o HotJava HTML Component - Java Bean Component
http://www.javasoft.com/products/hotjava/bean/index.html $195 for
private use binary licence. HTML 3.2
o HTML browser (free source)
http://barium.tn.tudelft.nl/people/gool/java/html/Html.html
o Web Window Browser http://www.opencube.com/example_wwb.htm $139 -
no sources.
o jHelp ($20-650) http://w3.nai.net/~rvdi/jhelp/jhelp2/jhelp.html
jHelp is a HTML browser component written in Java, HTML 2.0
19. (Sect. 14) I loaded an Image file from a JPEG/GIF file using the
Toolkit/Applet.createImage(URL/String) method, and (the height and
width are -1 / it will not draw to the screen). What is wrong?
[*] The behaviour of the AWT on creating images in this way is to do
nothing at all.
When the image is first drawn using Component.drawImage(), or its size
is requested, the image begins to load in a different Thread.
As the image loads, the ImageObserver specified in the
drawImage()/getHeight() call is periodically notified of the loading
status of the image, by calls to its imageUpdate() method.
In the case of Component.drawImage() call, the default behavior of
Component.imageUpdate() is to schedule *another* repaint() call when
the image has fully loaded. This means that, in particular the
following code will not work:
class MyComponent extends Component {
...
public void paint(Graphics g) {
ImageFilter cropper=new CropImageFilter(0,0,16,16);
Image cropped_image=createImage(new
FilteredImageSource(image.getSource(),cropper));
g.drawImage(image,10,400,this); // this line works
// this line doesn't -
g.drawImage(cropped_image,400,15,this);
}
}
The cropped_image will not be created in time to be painted, and when
it is finally created, another call will be scheduled to paint, which
will try to draw another one, etc.
(Note also that creating objects like this in paint() methods is
generally a very poor idea in Java, since they are called very
frequently, and you will strongly offend the garbage collector.
In order to get round this problem, you may i) add all such Images to a
MediaTracker, and call the waitForAll() method. ii) implement your own
ImageObserver interface, and wait for the imageUpdate() method to be
called with the ALLBITS/FRAMEBITS value. i) is easier, but ii) is
recommended, since there are reports of MediaTracker not working in
some environments.
Also in the FAQ:
o See also Q13.12
o See Q6.4 for examples of how to reuse objects.
20. (Sect. 14) How can I record sound in an applet?
[*] If you are using win95/nt, you could use SoundBite - Audio
Recording in Applets. See http://www.scrawl.com/store/
It provides easy access to audio data in arrays:
short[] left, right;
21. (Sect. 14) Does Java support PNG? Yes. PNG - Portable Network Graphics
- provides a patent-free replacement for GIF and TIFF. If you save a
GIF, don't forget to pay the royalty to Unisys - see Unisys's web page
at http://www.unisys.com/LeadStory/lzwfaq.html. That patent is why GIFs
are a poor choice for internet images.
The PNG format is specified in RFCs 1950, 1951, and 2083, and is
unencumbered by licenses or patents. See also the PNG-1.1 specification
at ftp://swrinde.nde.swri.edu/pub/png/documents.
The PNG format is supported by the Java Advanced Imaging API which is
part of the 1.2 media APIs.
-------------------------------
15. Networking and Distributed Objects
RMI Issues
Note that there is an RMI FAQ at
http://java.sun.com/products/jdk/rmi/faq.html
1. (Sect. 15) Should I use CORBA in preference to RMI? Or DCOM? Or what?
[*] If your distributed programs are all in Java, then RMI provides a
simpler mechanism that allows the transfer of code, pass-by-value of
real Java objects, and automatic garbage collection of remote objects.
If you need to connect to C++ (or other language) systems or you need
CORBA-specific services, then CORBA is your choice.
In July 1997, Sun announced that it was aligning RMI to work more
closely with CORBA. Sun is simply adding an IIOP transport layer to RMI
to support interoperability with CORBA. Java programs can then use RMI
to access CORBA-based objects through IIOP, the OMG's CORBA-based
protocol. This is very good news for those building heterogenous
Enterprise systems, although it will take some additions to IIOP to
support the pieces that RMI uses.
In 1998 Microsoft spokespeople tried to promote DCOM by spreading
misinformation that RMI is changing or being dropped. That is totally
wrong. The RMI API continues unchanged in its current form. Using DCOM
restricts your code to only ever run on Microsoft platforms using Intel
hardware, and negates the "write once, run anywhere" Java philosophy.
You would have to recompile your DCOM code to run it on other Microsoft
platforms like Compaq's (formerly DEC's) alpha computer. Non-portable,
single vendor code should be avoided. DCOM/DNA has limitations for use
in the enterprise.
Other sites:
http://www.javaworld.com/javaworld/jw-10-1997/jw-10-corbajava.html
has a good intro to CORBA in the Java world.
http://www.objenv.com/cetus has a CORBA/RMI comparison.
2. (Sect. 15) How do I do RMI into a different domain?"
[*] Similar to the proxy answer in a section below; you must tell the
program where to find the server. In this case start up the client with
this commandline option: -Djava.rmi.server.hostname=hostname.domainname
3. (Sect. 15) RMI seems to have stopped working for me in JDK 1.1. Why is
this?
[*] The rules for where the client looks for a stub class seem to have
changed making it necessary to reset your class path on the client
after starting the RMI registry. In particular, it looks like rmic was
not updated to the new "don't need $CLASSPATH for current dir"
convention as the compiler was. You are best off setting classpath
explicitly.
Other sites:
There are several very good sources available from Sun which cover many
simple and advanced RMI problems.
o The documentation, of course:
http://java.sun.com/products/jdk/1.1/docs/guide/rmi/index.html
o Dedicated FAQs on RMI and Object Serialization
http://java.sun.com/products/jdk/rmi/faq.html
o Mailing list [email protected] with archive at
http://chatsubo.javasoft.com/email/rmi-users/ Visit the archive!
4. (Sect. 15) After a number of RMI client to server connections (55 on my
system), subsequent RMI clients trying to connect fail. Why?
[*] You are hitting the default limit of 64 open file descriptors. Try
increasing the limit in your OS.
In addition there is currently a practical RMI connection limit imposed
by the scalability of the VM and the performance of object
serialization. This is addressed in JDK 1.2. The actual number of
active clients you will be able to support will depend on the workload
mix you have (i.e. the number of clients, how often they talk to the
server, and how much work must be done per call).
5. (Sect. 15) I'm using RMI on Win95, and the Naming.lookup() call is
taking a long time, even for localhost. How do I fix it?
[*] (See also the first answer in next section below, and note that
this Windows workaround has never worked for some people). Try adding a
definition for the machine in your "hosts" file. Typically, this file
will be named c:\windows\hosts (if it doesn't exist, there should be a
file called c:\windows\hosts.sam). The hosts file is searched by your
TCP/IP stack before it resorts to DNS, so adding an entry in this file
can speed up your lookups considerably. The hosts file is used to map
IP addresses to symbolic addresses. To enter the name "localhost" with
address 127.0.0.1 (the IP loopback address), enter the following line
in your hosts file. 127.0.0.1 localhost
Windows Networking
6. (Sect. 15) Why does < Windows RMI/my java debugger/IDE/other> hang for
a couple of minutes if my Windows PC is not dialed up to the Internet?
[*] Java has networking support built in. When the Java program starts
the Winsock DLL automatically gets loaded. The first thing this does is
to try to resolve the fully qualified domain name for your machine
under the name "localhost". If your system doesn't have this name
mapped, it will try to query a nameserver on the internet, which is
typically (on a PC) your dialup ISP. So it either prompts you to
connect to the ISP, or waits till the attempt times out.
Some people say you can avoid the Win95 problem by giving your system
another way to resolve DNS names. This tip has never worked for me.
Edit the hosts file for your system so that localhost and the full
domain name are both mentioned. On Windows 95 systems the hosts file
is: %windir%\HOSTS (for example, C:\WINDOWS\HOSTS). On Windows NT
systems the hosts file is: %windir%\System32\DRIVERS\ETC\HOSTS (for
example, C:\WINNT\System32\DRIVERS\ETC\HOSTS).
One gotcha under Win95 is that if the last entry in the hosts file is
not concluded with a carriage-return/line-feed then the hosts file will
not be read at all. So if my system is called goober.best.com change
the hosts file from
127.0.0.1 localhost
to
127.0.0.1 goober.best.com localhost
Showing more of the file:
# Hosts file
127.0.0.1 localhost
129.146.77.177 goober
Another alternative is to dial up with a PPP connection to your ISP
whenever you want to run networking programs.
Fundamentally the experience of some people has been that networking is
not completely satisfactory on Windows95 using Winsock 1.1, and is
subject to sporadic unexplained failures. You could try downloading
Winsock 2.0. To get Winsock 2.0, you need to drag in all the other junk
from Microsoft Windows Sockets 2.0 Software Development Kit. This free
software can be downloaded from the following addresses:
http://www.microsoft.com/win32dev/netwrk/winsock2/ws295sdk.html or
ftp://ftp.microsoft.com/bussys/WinSock/winsock2/
The patches needed to improve Win95 networking are already in Win98.
Other Networking Issues
7. (Sect. 15) If I call the InetAddress.getByName() method with an
IP-address-string argument, like "192.168.0.1", get an
UnknownHostException on some platforms, but not others. Code like
Socket sock = new Socket("155.152.5.1", 23);
triggers the exception. Why?
[*] This is a platform difference that arises out of different
semantics in the underlying network libraries, and is [said to be, but
subject to confirmation] fixed in JDK 1.1. On Solaris and Windows NT,
the IP address string only works for IP addresses that have an
associated hostname. On Linux and Windows 95, the IP address string
works in all cases.
When InetAddress is instantiated with an IP address, a reverse DNS
lookup is done. If the IP address is not associated with a valid
hostname, the instantiation will fail. This is part of anti
DNS-spoofing, and in JDK 1.1 works because the reverse lookup will not
occur until the hostname is asked for. So in JDK 1.1,
InetAddress in = InetAddress.getByName("155.152.5.1");
[Note: this info is still to be confirmed. Net gurus?]
Other sites:
Microsoft has several network-related patches at its site
http://www.microsoft.com/
8. (Sect. 15) I want to pass a class file to willing recipients who are
using my applet. Any ideas how?
[*] You could use a trick: put your .class file(s) in a .zip archive
and use showDocument() on the URL. A person accessing this will get a
dialog box put up asking them about saving the file to their local hard
disk.
Other sites:
You can see this in action and try it out yourself at:
http://www.best.com/~rmlynch/saveit.html
9. (Sect. 15) How do I get a URLConnection to work through proxy
firewalls? I.e. How do you get your Java application to do its web
accesses through a proxy?
[*] This is typically needed for any net access to another domain. Tell
the run time system what you are trying to do, by using these
commandline arguments when you start the program.
java -DproxySet=true -DproxyHost=SOMEHOST -DproxyPort=SOMENUM code.java
Note proxyPort is optional and it defaults to 80. Without this, you
will see an exception like java.net.UnknownHostException or
java.net.NoRouteToHostException
The proxy settings work for both java.net.URLConnection, and for
java.net.Sockets.
Netscape's and IE's JVMs (at least in versions 4.x+) take the proxy
settings for applets from the browser's proxy configuration. You can
also do URL proxies in applications (not applets) with the following
code
// set up to use proxy
System.getProperties().put("proxySet", "true");
System.getProperties().put("proxyHost", "myproxy.server.name");
System.getProperties().put("proxyPort", "80");
But how do I know the name of the proxy server?
This code just tells you how you can get a URL connection to the
outside. Since it is your proxy server, you are expected to know the
name of it. There isn't any code that you can write that will allow
arbitrary URL connections to be initiated from outside the firewall.
Think about it! If there were, the firewall would not be doing its job.
Also note there are corresponding socksProxyPort and socksProxyHost for
when socks is used instead of proxy. The default socks port is 1080.
10. (Sect. 15) What is "swizzle", as in "Swizzle this object?"
[*] It means serialize. To swizzle an object is to recursively
serialize or flatten composed objects.
11. (Sect. 15) I have been using the Serializing capabilities in 1.1 to
save some objects to disk. I added a new field to one of my objects
that get serialized and now deserializing my old data no longer works.
I get this exception:
java.io.InvalidClassException: MacroData; Local class not compatible
[*] You need to add a declaration such as
static final long serialVersionUID = 4021215565287364875L;
in the modified class. The actual value of this long is supplied by the
"serialver" utilitity suppied with the JDK. Any versions of a class
other than the first version require this static to be defined in the
class. This is how versioning is achieved.
12. (Sect. 15) My socket code looks good, but is broken!
[*] When using sockets you typically open both inward and outward
streams. A TCP connection is full duplex, but either the send or
receive side may be closed independently. By default, the remote end
will take the close as indicating that the connection has simply been
closed, and will close its end as well. Check whether this is happening
for you, by adding the matched pair. Use tcpdump to check this.
13. (Sect. 15) How do I map between IP address and hostname?
[*] In Java 1.1 (earlier releases were buggy) use:
String host = InetAddress.getByName("211.10.2.119").getHostName();
14. (Sect. 15) How do I embed an anchor in a URL? Just putting it as part
of the string in the constructor doesn't work.
[*] Like this:
URL url = new URL("http://www.my_domain.com/my_page.html");
URL anchor = new URL(url, "#section2");
this.getAppletContext().showDocument(anchor);
15. (Sect. 15) How do I POST to a CGI script from an applet?
[*] Let's start by noting that this is more troublesome than it might
seem at first, and that GET is preferred. For an untrusted applet, the
CGI script can only be on the server that served the applet. POSTing to
the server involves sending key/value pairs, not just a message. Also
values must be encoded. To send "words to the server" you might try:
StringBuffer sb = new StringBuffer();
String str="words to the server";
sb.append("message=");
sb.append(java.net.URLEncoder.encode(str));
URLConnection cn = url.openConnection();
// set properties
cn.setDoOutput(true);
cn.setUseCaches(false);
cn.setAllowUserInteraction(false);
cn.setRequestProperty("content-type","application/x-www-form-urlencoded");
// send parameters
PrintWriter out = new PrintWriter(cn.getOutputStream());
out.print(sb.toString());
out.close();
// read stuff
BufferedReader in = new BufferedReader(
new InputStreamReader(
cn.getInputStream()));
sb = new StringBuffer();
String inputLine;
while ((inputLine = in.readLine()) != null){
sb.append(inputLine);
sb.append("\n");
}
in.close();
uresp = new URL(getDocumentBase(),"respond.html");
getAppletContext().showDocument(uresp); }
The CGI has to write its output to respond.html so that it can be
displayed by the browser. But it may still fail, because respond.html
could be overwritten by a subsequent request to the same CGI before the
results of the first POST are read back.
To get an acceptable solution takes quite a lot of effort. In general
you should prefer GET to POST for CGI access from Java. As it says on
the Javaworld page, the answers to the question are really: you can't,
don't POST (use GET), use a bean, or cheat.
Note, if you request a URL via the URLConnection/HttpURLConnection, the
server sets the content type, and your applet can use
URLConnection.getContentType() to get the type. Alternatively, use
setRequestProperty to set it, like this:
url = new URL(cgiUrl);
urlc = url.openConnection();
urlc.setRequestProperty(
"Content-type",
"application/x-www-form-urlencoded");
Other sites:
There's a pretty good explanation at
http://www.javaworld.com/javaworld/javatips/jw-javatip41.html
See also the Marty Hall book Core Web Programming (Prentice Hall ISBN:
0-13-625666-X). It has comprehensive coverage of writing Java programs
that interface to CGI scripts. Better still, use servlets, write
everything in Java, and discard your CGI code.
16. (Sect. 15) How can I write CGI programs in Java?
[*] CGI (the Common Gateway Interface for web servers) is an API for
writing programs that use the web as its user interface. By far, the
most popular language for this task is Perl, because of its powerful
text handling capabilities, and excellent resources available for
making the jobs of CGI programmers easier. CGI programs can be written
in any language, including Java.
Unfortunately, the interface between the web server and the CGI program
uses environment variables extensively. Use of environment variables
has always been deprecated in Java, because of portability issues (not
all systems have environment variables). The way to get around this is
to write a "wrapper" for the Java program in a language that supports
environment variables, and can then invoke the Java program with the
appropriate environment data passed in as Java properties.
Because the Java runtime environment is not a lightweight process, it
might take a moment for the CGI program to get started before anything
happens. This is particularly true on operating systems, like NT, that
have a large overhead to spawning new processes.
In preference to using Java for CGI on the server, you might consider
using the Java servlet API in Netscape's Enterprise Server. This allows
you to develop server-side programs in Java without suffering the same
performance restrictions and other limitations of the CGI API.
Other sites:
See http://search.netscape.com/comprod/server_central/
query/eval_guide/enterprise/advantage.html for more details.
17. (Sect. 15) How can I write the "ping" program in Java?
[*] You can't do it directly. Quoting from the Java Networking FAQ,
Ping requires ICMP packets. These packets can only be created
via a socket of the SOCK_RAW type. Currently, Java only
allows SOCK_STREAM (TCP) and SOCK_DGRAM (UDP) sockets. It
seems unlikely that this will be added very soon, since many
Unix versions only allow SOCK_RAW sockets to be created by
root, and winsock does not address ICMP packets (win32
includes an unsupported and undocumented ICMP.DLL).
You could write a program that handshakes with a remote system, to
simulate ping, or you could use native code for the raw socket. Other
sites:
You can find the Java Networking FAQ at
http://www.io.com/~maus/JavaNetworkingFAQ.html
There is an additional Java Networking FAQ (which is being
modified and reposted regularly) at
http://www.davidreilly.com/java/java_network_programming/
18. (Sect. 15) Why, when I make a call to DatagramPacket.getlength() does
it returns the wrong size?
[*] You are probably reusing a Datagram packet, and it has no way to
grow its buffer if a longer packet comes in. Take care of it by calling
setLength(max) before reuse.
Failing to do this is a very common mistake.
19. (Sect. 15) Why do my networking programs freeze on the Mac?
[*] You shouldn't println to a socket. This is the subject of Apple
Technical Note 1157 at
http://developer.apple.com/technotes/tn/tn1157.html
The problem is that many socket protocols expect CR+LF to terminate a
string, but println delivers a platform-specific EOL mark. On the Mac,
it is just CR. Therefore the server hangs waiting for the Mac to send a
LF. It never does. But the program works fine on platforms that send
CR+LF.
20. (Sect. 15) How do I "timeout" a socket read, as the "select" function
in Unix does?
[*] There are two timeouts at issue here.
Use the ServerSocket.setSoTimeout(int millis) for timing out your
'accept()' calls.
For a select() like mechanism, you can poll InputStreams of your
connections for available() > 0
URLConnection doesn't have a method for this, but you can do it by
using multiple threads. Create a timer thread that would just track
time and close the connection after your specified timeout period.
Check the URLConnection to see if it's got any data flowing through it
(calling URLConnection.connected() is probably a sufficient test). The
URLConnection should be in a separate thread.
-------------------------------
16. Security
1. (Sect. 16) What is a "trusted applet"?
[*] JDK 1.1 introduced the notion of a "trusted applet" which is one
that has been cryptographically signed to guarantee its origin and make
it tamper-resistant. Trusted applets can be granted more system access
privileges than untrusted applets.
You preconfigure your browser with a list of whose X.509 certificate
you trust, and then applets arrive with X.509's attesting to their
keys. It's easier than it sounds.
2. (Sect. 16) What is the story with Java and viruses? What is the
blackwidow virus?
[*] Java was designed with security in mind. The security features make
it very difficult, probably impossible, to attach a virus (self-copying
code) to a Java applet. There has never been a Java virus carried from
system to system by applets.
There has been mention of a "Java virus" called "BlackWidow" in the
media (it was mentioned in Unigram in late 1996, and obliquely on the
RISKS newsletter in February 1997). A request to the editor of Unigram
for more information brought the answer that there was no more
information, it was just a report of a rumor. As far as is known, this
story exists only as rumors reported on by the press. There is no
actual Java virus or blackwidow virus (there was a legitimate
commercial product of that name, since renamed).
In spring 1998 there were press reports of a "Java applet" called White
Ghost. It turns out that it relies on security flaws in ActiveX, and
the only susceptible systems are Microsoft's Active Desktop. If anyone
has a URL for a copy of this code, and an analysis of it, please
contact the FAQ author.
In August 1998, Symantec had some information about a Java program
(application) that could append itself to some other Java program. They
termed it "Strange Brew". That is just normal file processing, no
different to a program written in C, C++, Fortran, COBOL, etc. What
makes PC viruses dangerous is when they have a hidden means of
travelling between places, such as being attached to code that is
automatically executed, like boot sector code, or Word macro
initialization files. No one has yet produced a virus that does this in
Java.
In March 1999, reports surfaced of a virus called BeanHive. Again this
was not Java-related, but relied on the browser user explicitly
accepting an unknown certificate as trusted when prompted. If the user
allowed that, then any code from that source will be uploaded and run
without further input from the user. Don't remove security for
untrusted code simply because it asks for it, is the lesson here.
If anyone has more concrete information about a virus that can attack a
Java applet (again, this is thought to be impossible), please contact
the FAQ author.
3. (Sect. 16) Why do I get the warning string "Unsigned Java Applet
Window" at the bottom of popup windows in my applets?
[*] This is a security feature, to make certain that users can always
tell that a window asking for their password and credit card details
(or whatever) is from an applet. There should be no way for an
untrusted applet to work around this message.
Also in the FAQ:
See also the answer to Q12.7.
4. (Sect. 16) Where can I find information on signing applets?
[*] Please take a look at the "Code Signing for Java Applets" page at
http://www.suitable.com/Doc_CodeSigning.shtml. The page explains how to
sign your Java applet so that it can be used in both
Navigator/Communicator and Internet Explorer.
5. (Sect. 16) Where can I find crypto libraries for Java?
[*] Cryptographic libraries are not part of the Java release because US
Government policy classifies strong cryptography under the same rules
as munitions. Its export is regulated under the International Traffic
in Arms Regulations. Many people regard this as a Kafka-esque (and
futile) attempt to stem the use of cryptography inside the US.
Other sites:
o The comprehensive and free "Cryptix" crypto library written in
Java is at:
http://www.cryptix.org
See also http://www.hi.is/~logir/logi.crypto/
o Another free crypto library for Java is at:
http://www.acme.com/java/software/Package-Acme.Crypto.html
It includes Blowfish, CRC16, CRC32, DES, DES3, IDEA, RC4, ROT13
(can they really call that "crypto"?), and more.
o Another pure Java Cryptography toolkit is at
http://www.freestylesoft.com/products/crypto/avalanche.html (free
for personal use).
o One commercial Java encryption source (from Ireland) is:
http://www.baltimore.ie/jcrypto.htm
o A complete crypto API for Java (with HTML documentation) at:
http://www.geocities.com/SiliconValley/Heights/8298
The library provides comprehensive and complete range of crypto
library and functions covering DES, 3DES, IDEA, Blowfish ...and
RSA, DH, DSA and PGP access to Java programmers. The crypto
functions are based on the C cryptlib, by Peter Gutmann. It would
be illegal to export this under current US government rules, but
the author of the code is outside the US, and not subject to US
export regulations. Download it today before it becomes illegal.
o Some Java crypto available with source under GPL is at
http://www.aba.net.au/solutions/crypto/jce.html
http://www.geocities.com/SiliconValley/1394/
o Also, data about Sun's Java Cryptography Extension (JCE) is
available at:
http://developer.java.sun.com/developer/earlyAccess/jdk12/jce.html
(This may not be exported outside the USA and Canada).
o An actual port of PGP v2.6.3i to Java is at:
http://tassun.math.nsc.ru
o A list of free crypto libraries, not necessarily in Java, and of
course the FBI will be all over you like hair on an ape if you try
to export any of these, is at:
http://www.homeport.org/~adam/crypto/
6. (Sect. 16) How do I find out what these terms mean?
[*] Read Bruce Schneier's excellent book "Applied Cryptography 2nd Ed."
for more info on what these terms mean. Read David Kahn's excellent
(and exhaustive) book "The Codebreakers" for more info on the history
and background of encryption.
7. (Sect. 16) Where is Javasoft's Security FAQ?
[*] Javasoft's security FAQ can be found at:
http://java.sun.com/sfaq/index.html
Other sites:
http://java.sun.com/products/jdk/1.1/docs/guide/security/
8. (Sect. 16) Where is that online book on Java Security?
[*] Gary McGraw and Ed Felten's book Securing Java can be found on the
web at
http://www.securingjava.com/
-------------------------------
17. For C and C++ Afficionados
1. (Sect. 17) How do I translate C/C++ into Java or vice-versa?
[*] In general it is not simple to translate C/C++ into Java, as Java
lacks the arbitrary pointer arithmetic of those languages. If your C
code does not use pointer arithmetic, automatic translation gets a lot
simpler. Try these URLs:
http://www.ist.co.uk (search for X-Designer 4.6: Java edition).
http://members.aol.com/laffra/c2j.html
http://www.ilog.com/
Going the other way there are currently three freely-available tools to
translate Java into C. It seems that these have been done for hacking
value, rather than practical purposes.
o j2c from Japan,
http://www.webcity.co.jp/info/andoh/java/j2c.html
o Toba from the Sumatra research project, translates 1.0.2 .class
files into .c source code
http://www.cs.arizona.edu/sumatra/toba
o JCC from Nik Shaylor.
http://www.geocities.com/CapeCanaveral/Hangar/4040/
None of them support the AWT yet, and both j2c and JCC have additional
restrictions.
There's a product to convert Visual Basic to Java. Details at
http://www.blackdirt.com and
http://www.javadelphi.com (also a Delphi-to-Java source converter)
and
http://www.tvobjects.com
There's a product to translate COBOL source to Java source, see
http://www.Synkronix.com/
This program dumps info about the class file:
http://www.professionals.com/~cmcmanis/java/dump/index.html
Chuck McManis was one of Sun's original Java implementors.
2. (Sect. 17) How are finalizers different from C++ destructors?
[*] Java objects are not explicitly deleted and do not have
destructors. Instead they are implicitly garbage collected when the JVM
realizes your program can no longer access them. Typically this
technology is _not_ based on reference counting and _will_ cope with
circular references.
Every object has a routine called finalize() which will be called
before the object is collected. This is Java's nearest equivalent to
C++'s destructor. However, it is not a good idea to rely on
finalization for the timely freeing of resources.
This is because garbage collection and hence finalization may be
arbitrarily delayed, and may never happen at all if the program
terminates before it runs out of memory. You should instead provide
your objects with methods similar to Graphics.dispose() to free
resources, and call the dispose() method explicitly when you have
finished using them - typically within the "finally" clause of a
"try/catch" block. You may then call your dispose() method from within
your finalize() method as a last-ditch attempt to free the resource if
someone forgets.
Alas, all this means the C++ idiom of "object construction is resource
aquisition" does not translate well to Java. However, note that 90% of
destructors in C++ are there to free memory, and the GC means you don't
need to do that in Java. As well as fixing an important source of bugs,
the GC is essential to Java's security model; without it you could
forge object references by preserving the reference after the object
has been deleted.
If your program appears to be crashing due to running out of some
system resource (like File, Window or Graphics handles), it probably
because the system is running out of handles before it has run out of
memory. Check that you have called the dispose() method (or equivalent)
on every object that uses system resources. You can help the GC a
little bit more by explicitly NULLing out references that you've
finished with.
3. (Sect. 17) What's the Java equivalent of sizeof()?
[*] There isn't one. sizeof() in C and C++ is used in three main
places:
1. To check on the size of a primitive type. In Java, the sizes of
primitive types are fixed in the language specification (a short
is _always_ 16 bits; an int is _always_ 32 bits, etc), so this is
no longer necessary.
2. In memory allocation (i.e. malloc (32 * (sizeof(int));) In Java
you always allocate a specific type of object, rather than a block
of raw memory that you will fill as you like. The system always
knows the size of the kind of objects you are allocating. So
sizeof is not needed.
3. In pointer arithmetic (i.e. p += sizeof (int)) Pointer arithmetic
of this type is not allowed in Java, so this isn't necessary,
either.
For all these reasons, there is no need for a Java sizeof() operator.
by having the object serialize itself to a ByteArrayOutputStream, and
looking at the bytearray.length.
That won't work because a lot of additional data is written when an
object is serialized. The additional data includes a description of the
class, any objects referenced by the serialized object, null references
(written as a single byte), etc. If you write another instance of the
same class, the amount of data written can differ dramatically. And if
you serialize the same object again, it isn't written at all -- even if
its data fields have changed! Instead, a one byte token and a four byte
"sequence number" that refer to the first writing are output. Using
Object Serialization to determine the size of an object does not
(except by coincidence) give the right answer.
4. (Sect. 17) Does Java have the equivalent of "const" arguments in C and
C++?
[*] Java 1.1 adds the ability to use the "final" keyword to make
arguments constant. When used to qualify a reference type, however,
this keyword indicates that the reference is constant, not that the
object or array referred to is constant. For example, the following
Java code:
void foo(final MyClass c, final int a[]) {
c.field = 7; // allowed
a[0] = 7; // allowed
c = new MyClass(); // final means this is NOT allowed
a = new int[13]; // final means this is NOT allowed
}
is roughly equivalent to the following C/C++ code:
void foo(MyClass * const c, int * const a) {
c->field = 7; // allowed
a[0] = 7; // allowed
c = new MyClass(); // const means this is NOT allowed
a = new int[13]; // const means this is NOT allowed
}
Java does not have any equivalent to the following C/C++ function
declarations:
void foo(const MyClass *c); // a pointer to a const class
void foo(const int *a); // a pointer to a const int
void foo(const int a[]); // a pointer to an array of const ints
5. (Sect. 17) Are there any hacks around this?
[*] Certainly! There are always hacks around stuff. One way of
enforcing constant values is to have two interfaces, a constant one and
a non-constant one, e.g.
public interface ConstFoo {
int getValue();
}
public interface Foo extends ConstFoo {
int getValue();
void setValue(int i);
}
Then when you want to receive a parameter that cannot be modified you
have:
void noChange(ConstFoo foo);
For a parameter that can be modified
void change(Foo foo);
6. (Sect. 17) How can I write C/C++ style assertions in Java?
[*] The two classes shown below provide an assertion facility in Java.
Set Assert.enabled to true to enable the assertions, and to false to
disable assertions in production code. The AssertionException is not
meant to be caught--instead, let it print a trace. Since the exception
is not meant to be caught, we just extend Error instead of
RuntimeException. As with RuntimeException, a method does not need to
declare that it throws Error. In addition programmers are less likely
to write "catch(Error) ..." than "catch(RuntimeException)".
With a good optimizing compiler there will be no run time overhead for
many uses of these assertions when Assert.enabled is set to false.
However, if the condition in the assertion may have side effects, the
condition code cannot be optimized away. For example, in the assertion
Assert.assert(size() <= maxSize, "Maximum size exceeded");
the call to size() cannot be optimized away unless the compiler can see
that the call has no side effects. C and C++ use the preprocessor to
guarantee that assertions will never cause overhead in production code.
Without a preprocessor, it seems the best we can do in Java is to write
Assert.assert(Assert.enabled && size() <= maxSize, "Too big");
Alternatively, use
if (Assert.enabled)
Assert.assert( size() <= maxSize, "Too big" );
In this case, when Assert.enabled is false, the method call can always
be optimized away totally, even if it has side effects. The relevant
sections of the JLS are Section 13.4.8, final Fields and Constants and
Section 14.19, Unreachable Statements. 13.4.8 requires that primitive
constants ("a field that is static, final, and initialized with a
compile-time constant expression") be inlined. So everywhere
Assert.enabled is refered it is replaced at compile time with its
value. Writing:
if (Assert.enabled) Assert.assert(size() <= maxSize, "Too big");
is exactly the same as writing:
if (false) Assert.assert(size() <= maxSize, "Too big");
... assuming Assert.enabled is false at compile time. Section 14.19
discusses compiling away such dead code. To sum up: the inlining of the
primitive constant is required by the spec. The subsequent optimization
of not generating code masked by (what turns into) an "if (false) ..."
is not required but is implemented by many existing Java compilers.
public class AssertionException extends Error {
public AssertionException(String s) {
super(s);
}
}
public final class Assert {
public static final boolean enabled = true;
public static final void assert(boolean b, String s) {
if (enabled && !b)
throw new AssertionException(s);
}
}
7. (Sect. 17) How do I do stuff like scanf and sscanf in C/C++? And how do
I do stuff like sprintf, e.g.
float x = 12345.6789;
printf("%6.3f/n", x);
[*] You can break a string like "5 loaves 2 fishes" into its parts by
using java.util.StringTokenizer. This is the Java equivalent of
sscanf().
StreamTokenizer does a similar thing on a file or any stream (i.e, what
scanf() and fscanf() do in C).
To do formatted character output, create a format string, and then use
that to format your binary value, e.g.
import java.text.*;
float fi = 1234.56789F;
DecimalFormat mydf = new DecimalFormat( "###0.000" );
mydf.setMinimumIntegerDigits(3); // for example
System.out.println( mydf.format(fi) );
gives:
1234.567
If you want to see a float print out as "0.0000001" instead of "1E-7",
use:
java.text.DecimalFormat myFmt = new
java.text.DecimalFormat("#,###,###,###.############");
System.out.println(myFmt.format(myFloat));
There are lots of different characters you can feed to the
DecimalFormat constructor, not just "0" and "#". See
$JAVAHOME/src/java/text/DecimalFormat.java source for details.
The always excellent Acme site has an sprintf() package written in Java
See http://www.acme.com.
8. (Sect. 17) What is the Java equivalent of C++'s "friend"?
[*] The keyword "friend" in C++ is a hack to allow a piece of code to
access the private member declarations of another class. In Java, you
would do this by labelling, not the friend, but the private members.
Instead of making them private, make them either protected or package
(no keyword) or public.
The four different Java protection levels are: private, package,
protected, and public.
o private members can only be accessed by the containing class and
internal classes.
o package (specified by omitting other keywords) is the default
level of protection; members are accessible from any class within
the package of the containing class.
o protected is package-level-access plus access to sub-classes of
the containing class. So "protected" is less protected than the
default.
o public fields in public classes are accessible from all classes.
9. (Sect. 17) Does anything like the C++ Standard Template Library exist
for Java?
[*] Yes, only it's better and simpler to use in Java. It's called the
Java Generic Library. This library (JGL) is freely downloadable from
http://www.objectspace.com/
It includes about a dozen nice data structures (including sets and
bags) and algorithms like unions, searching, and sorting.
It has over 100,000 users and 11 OEM distributors. [Some Java vendors
are bundling it with their next release]
10. (Sect. 17) What happens to post-increment when an exception is thrown?
[*] If you have the code:
array[i++] = foo();
and foo() throws an exception, i will be incremented anyway. This can
cause problems if sometimes foo() throws an exception and you don't
want i incremented in cases when it does.
This is a consequence of JLS 15.25.1 and 15.6.1 "the left-hand operand
of a binary operator appears to be fully evaluated before any part of
the right-hand operand is evaluated." (assignment is taken as a binary
operator). Note that this is not how C++ behaves.
-------------------------------
18. Java Idioms
See also the list of Java Design Patterns at
http://c2.com/cgi/wiki?JavaIdioms
1. (Sect. 18) What are the naming conventions in Java?
[*] The naming conventions are straightforward:
o Package names are guaranteed uniqueness by using the Internet
domain name in reverse order: com.javasoft.jag - the "com" or
"edu" (etc.) part used to be in upper case, but now lower case is
the recommendation.
o Class and interface names are descriptive nouns, with the first
letter of each word capitalized: PolarCoords. Interfaces are often
called "something-able", e.g. "Observable", "Runnable",
"Sortable".
o Object and data (field) names are nouns/noun phrases, with the
first letter lowercase, and the first letter of subsequent words
capitalized: currentLimit.
o Method names are verbs/verb phrases, with the first letter
lowercase, and the first letter of subsequent words capitalized:
calculateCurrentLimit.
o Constant (final) names are in caps: UPPER_LIMIT.
Other sites:
o Check out the section "Naming Conventions" in the language
specification:
http://java.sun.com/docs/books/jls/html/6.doc.html#11186
o Also take a look at Doug Lea's draft coding standard:
http://gee.cs.oswego.edu/dl/html/javaCodingStd.html
2. (Sect. 18) How do I convert a String to an int?
[*] There are several ways. The most straightforward is:
String mystring = numString.trim();
int i = Integer.parseInt(myString);
long l = Long.parseLong(myString)
or
String mystring = numString.trim();
i = Integer.parseInt(myString,myIntRadix);
Note 1: There is a gotcha with parseInt - it will throw a
NumberFormatException for String values in the range "80000000" to
"ffffffff". You might expect it to interpret them as negative, but it
does not. The values have to be "-80000000" .. "-ffffffff" to be
properly recognized as negative values. This is true for all radixes.
According to Bug Parade bug report 4068580, the proper way to generate
negative-valued hex Strings for eventual use by parseInt() is with
Integer.toString(i, 16). Once that high "sign bit" is on, without the
accompanying character, parseInt() says "too big".
Note 2: There are similar methods for Byte, Short, and Long. Use
myString.trim() to get rid of unwanted spaces before the conversion.
Some of the parse methods can cope with spaces, others can't. They were
written by two different people.
int i = Integer.valueOf(my_str).intValue();
also works but involves the creation of an extra object. Note: the
pre-FCS JDK 1.2 documentation at one point said that parseDouble and
parseFloat methods were to be introduced, but this does not seem to be
the case (see bug 4160672). JDK 1.2.
float f = Float.valueOf(my_str).floatValue();
double d = Double.valueOf(my_str).doubleValue();
3. (Sect. 18) How do I convert an int to a string?
[*] Try any of these:
String s = String.valueOf(i);
or
String s = Integer.toString(i);
or
String s = Integer.toString(i, radix);
or
// briefer but may result in extra object allocation.
String s = "" + i;
Note: There are similar classes for Double, Float, Long, etc.
4. (Sect. 18) How do I print the hex value of an int?
[*] You can print the hex equivalent of an int with:
int i = 0xf1;
System.out.println("i is hex " + Integer.toHexString(i) );
OK, how do I read a hex string into an int?
int i = Integer.valueOf(myHexString, 16).intValue();
5. (Sect. 18) How can you send a function pointer as an argument?
[*] Simple answer: use a "callback". Make the parameter an interface
and pass an argument instance that implements that interface.
public interface CallShow { public void Show( ); }
public class ShowOff implements CallShow {
public void Show( ) { .... }
public class ShowOff2 implements CallShow {
public void Show( ) { .... }
public class UseShow {
CallShow savecallthis;
UseShow( CallShow withthis ) {
savecallthis = withthis;
}
void ReadyToShow( ) { savecallthis.Show( ); }
}
// in some other class that uses all this stuff:
UseShow use_1 = new UseShow( new ShowOff() );
UseShow use_2 = new UseShow( new Showoff2() );
and then the ReadyToShow() method on use_1 or use_2 will call the
appropriate method, as if you had stored a pointer to the method.
6. (Sect. 18) How do I execute a command from Java?
[*] Use
Runtime.getRuntime().exec( myCommandString )
where myCommandString is something like "/full/pathname/command". An
applet will need to be signed in order to allow this.
Note, there is a gotcha associated with reading output from commands.
When the runtime exec's the process, it passes to it 3 streams, for
stdin, stdout, and stderr; the out and err are buffered but the buffer
size isn't very big. When your process runs, it reads (if needed) from
in, and writes to out and err. If it doesn't write more than the
buffer-size, it can run to completion.
But if it tries to write more data to one or the other stream than the
buffer can hold, the write blocks, and your process hangs, waiting for
you to empty the buffer so it can write some more.
So after the exec call, get the streams, and read from them in a loop
until they both hit end-of-stream (don't block on either one, just read
whatever is available from each, each loop iteration). Then when the
streams have ended, call the process.waitFor() method to let it finish
dying.
7. (Sect. 18) How do I do I/O redirection in Java using exec()?
[*] This solution works on Unix platforms using either JDK 1.0.2, or
JDK 1.1. The trick is to use an array of Strings for the command line:
String[] command = {"/bin/sh", "-c", "/bin/ls > out.dat"};
If you don't do this, and simply use a single string, the shell will
see the -c and /bin/ls and ignore everything else after that. It only
expects a single argument after the -c.
import java.io.*;
import java.util.*;
class IoRedirect {
public static void main(String Argv[]) {
try {
String[] command = {"/bin/sh", "-c", "/bin/ls > out.dat"};
Process p = Runtime.getRuntime().exec(command);
p.waitFor();
System.out.println("return code: "+ p.exitValue());
} catch (IOException e) {
System.err.println("IO error: " + e);
} catch (InterruptedException e1) {
System.err.println("Exception: " + e1.getMessage());
}
}
}
8. (Sect. 18) So why can't I exec common DOS commands this way (as in
18.7)?
[*] The reason is that many of the DOS commands are not individual
programs, but merely "functions" of command.com. There is no DIR.EXE or
COPY.EXE for example. Instead, one executes the command processor
(shell) explicitly with a request to perform the built-in command, like
so:
Runtime.getRuntime().exec("command.com /c dir") for example. On NT, the
command interpreter is "cmd.exe", so the statement would be
Runtime.getRuntime().exec("cmd /c dir") And you could bring the command
output into the program with code like:
Process p = runtime.exec ("cmd /c dir");
DataInputStream procIn = new DataInputStream(p.getInputStream());
while ( true ) {
String line = procIn.readLine();
if ( line == null ) break;
// do something with lin
}
This situation occurs on any OS where some commands are actually
interpreted directly by the shell.
9. (Sect. 18) OK, how do I read the output of a command?
[*] As above (18.6, 18.7), adjusted like this:
BufferedReader pOut= new BufferedReader(
new InputStreamReader(p.getInputStream()));
try {
String s = pOut.readLine();
while (s != null) {
System.out.println(s);
s = pOut.readLine();
}
} catch (IOException e) { }
Another possibility is to read chunks of whatever length as they come
in:
...
p = r.exec(cmd);
InputStream is = p.getInputStream();
int len;
byte buf[] = new byte[1000];
try {
while( (len = is.read(buf)) != -1 ) {
String str = new String(buf,0,0,len);
System.out.println( "Process out: " + str );
}
} catch( java.io.EOFException eof ) { ...
} catch( java.io.IOException ioe ) { ... }
However, you cannot read output from some DOS commands on Windows
95/98. This is bug 4211683 at Sun, but it is actually a Microsoft
bug/poor design. Windows programs that explicitly use a handle to the
console ("CON" or "CON:") instead of stdin and stdout cannot be used as
subprocesses to Java. The Win95 FTP program is an example of such a
program. Windows provides no convenient way for Java to redirect the
input and output of such processes from the console to input and output
pipes. The only workaround is to use a different implementation of the
program that does not use the console directly.
Microsoft documents a difference in DOS Console stream handling between
95 and NT: see
http://support.microsoft.com/support/kb/articles/q150/9/56.asp
So capturing output from GNU ls on Win95 works, but the output from
"command.com /c dir" is not captured.
10. (Sect. 18) How do I compile code which has a cyclic dependency, i.e
class pkg1.X contains a reference to class pkg2.Y ?
[*] You throw both classes at the compiler at the same time.
javac pkg1/X.java pkg2/Y.java
11. (Sect. 18) What is the point of creating the temporary reference to
this.layoutMgr?
[*] This code is from the 1.0 AWT, and the programmer was probably
pretty skilled.
public synchronized void layout() {
LayoutManager layoutMgr = this.layoutMgr;
if (layoutMgr != null) {
layoutMgr.layoutContainer(this);
}
}
The code makes a local copy of a global variable for one or both of two
reasons.
The first reason is that accessing local variables can be faster than
accessing (non final) member variables. It's good for loops or where
there are many references in the source.
The second reason is so that even if other threads update the global,
this.layoutMgr = someOtherLayoutMgr;
This method will still have a pointer to the original layoutMgr.
If the local variable were omitted, and another thread used the
setLayout() method to change layoutMgr to null between when the layout
method checked for null and when it invoked layoutMgr's layoutContainer
method, a NullPointerException would result.
Note that the synchronized keyword on the layout method doesn't help
any, since setLayout (which could make such a dire change) isn't
synchronized. Synchronized methods only lock out other synchronized
methods on this object. (The unhelpful synchronized keyword on the
layout method is gone in JDK 1.1.)
There are two alternative solutions. One would be to make setLayout
synchronized and make layoutMgr private, so that it can't be set other
ways. This provides a stronger form of thread serialization, in that
you would never be able to see an old layout manager being used after
it had been replaced. However, it is slower. Another option that
provides no increase in thread serialization over the original would be
to catch the NullPointerException.
Threaded programming is hard! This idiom was probably put in place by
someone who got really bitten by this in the past.
12. (Sect. 18) What is the difference between "a & b" and "a && b" ?
[*] "a & b" takes two boolean operands, or two integer operands. It
always evaluates both operands. For booleans, it ANDs both operands
together producing a boolean result. For integer types, it bitwise ANDs
both operands together, producing a result that is the promoted type of
the operands (i.e. long, or int). "|" is the corresponding bitwise OR
operation. "^" is the corresponding bitwise XOR operation.
"a && b" is a "conditional AND" which only takes boolean operands. It
always avoids evaluating its second operand if possible. If a is
evaluated to false, the AND result must be "false" and the b operand is
not evaluated. This is sometimes called "short-circuited" evaluation.
"||" is the corresponding short-circuited OR operation.
Possible mnemonic: The longer operators "&&" or "||" try to shorten
themselves by not evaluating the second operator if they can.
13. (Sect. 18) If I create a thread, and then null out the reference to it,
what happens to the thread? Does it get interrupted or what?
[*] The code looks like this:
Thread t = new Thread( my_runnable_obj );
t.start();
...
t = null; // what happens to the thread?
The answer is that you may no longer have a reference to the thread,
but the JVM still does. Once a thread is started, and as long as it
keeps running, it is a root object. Root objects are the starting
points for "things in use" that the garbage collector uses.
14. (Sect. 18) How can a Java program determine the level of JDK support
given by the underlying VM? I.e. is it running in a JDK 1.0.2 or 1.1
VM?
[*] Look at the java.version system property with:
String ver = System.getProperty("java.version");
There isn't a lot of standardization on the string contents however.
Another possibility is to try { ... } to load a class that is unique to
one release, like this:
boolean isJDK1_1 = true;
try {
// java.awt.Cursor is available only in the 1.1.x JDK
Class cls = Class.forName("java.awt.Cursor");
} catch (Exception e) {
// we should have written 'ClassNotFoundException e',
// but Communicator generates security exception instead.
isJDK1_1 = false;
}
This approach has the advantage that it can be compiled by any version
compiler.
15. (Sect. 18) How can I set a system property?
[*] JDK 1.2 has
System.setProperty( "property", "new value" );
Until then, you can get all the properties, and set just the one you
want with code like this:
System.getProperties().put("property", "new value" );
16. (Sect. 18) How can I clone using serialization?
[*] Look at the code below, submitted by expert programmer John Dumas.
It uses serialization to write an object into a byte array, and reads
it back to reconstitute a fresh copy. This is a clever hack!
import java.io.ByteArrayOutputStream;
import java.io.ByteArrayInputStream;
import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
public class Cloner {
private Cloner() {}
public static Object cloneObject(Object o) throws Exception {
ByteArrayOutputStream bOut = new ByteArrayOutputStream();
ObjectOutputStream out = new ObjectOutputStream(bOut);
out.writeObject(o);
ByteArrayInputStream bIn =
new ByteArrayInputStream(bOut.toByteArray());
ObjectInputStream in = new ObjectInputStream(bIn);
return(in.readObject());
}
public static void main(String args[]) throws Exception {
java.util.Vector v = new java.util.Vector();
v.addElement(new StringBuffer("Hello"));
java.util.Vector vClone =
(java.util.Vector)Cloner.cloneObject(v);
// Changing the StringBuffer int the cloned vector has no
// effect on the original StringBuffer object --
// demonstrating that we have indeed done a deep copy
((StringBuffer)vClone.elementAt(0)).append(" world");
StringBuffer sb = (StringBuffer)v.elementAt(0);
System.out.println(sb.toString());
sb = (StringBuffer)vClone.elementAt(0);
System.out.println(sb.toString());
int array[] = { 1, 2, 3, 4, 5 };
int arrayClone[] = (int [])Cloner.cloneObject(array);
// Again, changes to an element in the cloned array do not
// have any effect on the original
arrayClone[0]++;
System.out.println(array[0]);
System.out.println(arrayClone[0]);
}
}
The main() routine is just a driver. All the cleverness is in the very
brief cloneObject(). It does a "deep" clone, which is what you usually
want (though Java gives you a "shallow" clone by default).
-------------------------------
19. Java GOTCHA'S
1. (Sect. 19) What is a "GOTCHA" (for non-English native speakers)?
[*] It is an abbreviation of "Got you!" It is the triumphant
exclamation that a bug or programming idiom makes as it traps the
unwary programmer. This section details some of the popular "gotcha's"
of Java.
See also the list of Java Gotcha's at
http://mindprod.com/gloss.html
2. (Sect. 19) Why can't I filter filenames with the accept() method?
[*] It's a known bug. FileDialog doesn't call FilenameFilter.accept().
The bug id is 4031440, and it can be seen at the Java Developer
Connection.
There is no way to implement FilenameFilter support with the current
reliance on the Win32 common file dialog. To support FilenameFilter,
the FileDialog needs to issue a callback for each file it wants to
display, which the FilenameFilter can veto. But the Win32 common
FileDialog doesn't have any way to issue callbacks. Instead it accepts
simple wildcard patterns for choosing files which match a certain
pattern. That's a reasonable alternative to FilenameFilters, but that
model isn't supported by the current Java API.
JFC has a JFileChooser class that is a better bet for you to use for
file dialogs. It works fine.
3. (Sect. 19) I changed a final value, and recompiled just the file that
it was in, and the entire rest of the program used the old value!
[*] This is the "expected" behavior. If you have this in one file
class Flags { final static boolean debug = true; }
and you change it to, and recompile just this file:
class Flags { final static boolean debug = false; }
Then the rest of your Java .class files will still see it as "true".
When you declare a "static final int" (or any other primitive), the
compiler turns that into a compile time constant whose value can be
substituted wherever it is used in your program. If you update the
value in the source file, you'll need to recompile every class that
uses it.
See Java Language Specification, section 13.4.8 "final Fields and
Constants": "We call a field that is static, final, and initialized
with a compile-time constant expression a primitive constant. If a
field is a primitive constant, then deleting the keyword final or
changing its value will not break compatibility with pre-existing
binaries by causing them not to run, but they will not see any new
value for the constant unless they are recompiled."
4. (Sect. 19) What is the "substring trap"?
[*] The "substring trap" is the name for a mistake that is all too easy
to make when using the substring() method of class String. The method
signature is:
public String substring(int beginIndex, int endIndex)
The name "endIndex" suggests that is the index where the Substring
ends.
But in fact, the substring extends only to the character at position
(endIndex-1)! It seems to be done this way so that
s.substring(0,s.length()) is equal to s. If so, the name of the second
parameter should be something like endInxLessOne or Length. But not the
confusing and misleading endIndex. Beware the substring trap.
5. (Sect. 19) Why does getGraphics() return null on my offscreen image?
[*] The following code
class MyFrame extends Frame {
MyFrame() {
Image offscreen = createImage(100,100);
Graphics offg = offscreen.getGraphics();
}
...
}
will usually not work, since the peer will not exist at this time.
Without the peer for the Frame, you cannot succeed in creating
offscreen Images. (There's no problem creating Produced Images without
a peer. Trying to draw them, of course, is another matter).
One "standard" form of offscreen code looks like this: (note the reuse
of the Graphics and Image objects for as long as possible)
class Gumble extends java.awt.Something {
private Image offi;
private Graphics offg;
public void update(Graphics g) {
if (g == null) return; // Paranoia
Dimension size = size();
if ( offi == null
|| offi.getWidth()!=size.width
|| offi.getHeight()!=size.height ) {
if (offg!=null) offg.dispose();
offi = createImage(size.width, size.height);
offg = offi.getGraphics();
// Regenerate offi here...
}
// If you use getClipBounds() here,
// check that for being null, too!
// several implementations have been known to pass them....
g.drawImage(offi);
}
public void paint(Graphics g) {
update(g);
}
}
See also Question 8.7
6. (Sect. 19) The dynamic type of a method argument doesn't seem to be
used to choose an overridden method at runtime.
[*] Correct. Generally, if you invoke a method on an object, the
object's actual runtime type, not the type of the reference that you
used to reference it, determines which method is invoked. This is
regular polymorphism.
It's not the same for object parameters: the compiler decides at
compile time, depending on the types of the parameter expressions,
which method signature to use, and this is "hardwired" into the
bytecode. The compiler does not look at the object argument at runtime
and say "ah, this is a derived type, so I will choose the method that
takes the derived type as an argument."
This is best seen in a code example:
class Base { }
class Derived extends Base { }
public class foo {
public static void method(Base b) {
System.out.println("In the base method...");
}
public static void method(Derived d) {
System.out.println("In the derived method...");
}
public static void test(Base b) {
if (b instanceof Derived)
System.out.print("Derived: ");
else
System.out.print("Base: ");
method(b); // which method? method(base) or method(derived)?
}
public static void main(String args[]) {
Base b = new Base();
Derived d = new Derived();
System.out.println("test calls.");
test(b);
test(d);
}
}
Running the program gives an output of
test calls.
Base: In the base method...
Derived: In the base method...
See JLS section 15.11.4.4 and 15.11.3:
"If class S contains a declaration for a method named m with the same
descriptor (same number of parameters, the same parameter types, and
the same return type) required by the method invocation as determined
at compile time then this is the method to be invoked."
7. (Sect. 19) Why did I lose my updates when I changed data fields in a
graph that I was serializing?
[*] Quoting from the object serialization specification at:
http://www.javasoft.com/products/jdk/1.1/docs/
guide/serialization/spec/serial-arch.doc.html#4176
The writeObject method serializes the specified object and
traverses its references to other objects in the object graph
recursively to create a complete serialized representation of the
graph.
Within a stream, the first reference to any object results in the
object being serialized or externalized and the assignment of a
handle for that object. Subsequent references to that object are
encoded as the handle.
In other words, changing an object and then writing it again does not
really write it twice. Instead it just writes a reference back to the
first occurrence, losing any fields that have changed in the meantime.
There are three ways around this: (1) (inefficient) Reset (or close and
reopen) the stream, and start again by writing the new value of the
object. This is drastic -- you are throwing away all the serialization
that you have already done.
(2) (kludgey) Create a new object and write that.
(3) (could be a lot of work) Write your own protocol for object
serialization. Have something like a data stream where the contents of
an object are marked by special identifiers. Each "end" of the stream
can decide whether it will use a new object each time or reuse an
existing object.
8. (Sect. 19) When I click on a Java window frame, it doesn't close!
[*] You need to add the code to listen for a window closing event, and
take the appropriate action (hide the window, exit the program if the
top level frame, etc).
The window closing event handler is simple:
Frame mf = new Frame("binky");
mf.addWindowListener( new WindowAdapter() {
public void windowClosing(WindowEvent we) {
System.exit(0); // or setVisible(false); etc.
} });
This really should be the default behavior of an AWT Frame. So you'll
be delighted to hear that JavaSoft has "made it so" for the JFrame
Swing component. That leads to a slightly different problem. See
Question 4.3.3. See also Question 13.9.
9. (Sect. 19) What's the deal with "super"? How far back into parent
classes can I go?
[*] The most common use of super is the call "super()" to invoke a
constructor in the superclass. The keyword "super" is also used to
access fields of any superclass (not just the immediate superclass)
that are hidden by an identically named feature in the current class.
However there is no way to "chain" several super's together, and reach
back higher into the parent class hierarchy. E.g. do not think that
"super.x" means the "x of parent" and "super.super.x" means the "x of
grandparent". This is a very common mistake. There is no
"super.super.x". Looking at the generated byte code, if you have
class Parent { }
class Child extends Parent { }
then, in Child "super.someParentMethod();" means "invokespecial
X.someParentMethod()" in the JVM, not "invokevirtual".
Invokespecial means "call the exact method I am telling you."
Invokevirtual means "call the right method for whatever object this
is".
10. (Sect. 19) When I change some component (e.g. a new label on a button)
I don't see the change on the screen immediately even if I repaint().
[*] You need to add a call to the paintImmediately(x,y,w,h) method of
JComponent. That repaints the component completely before continuing
execution. On pre-JDK 1.2 systems, use:
invalidate();
validate();
They cause the component hierarchy to be marked as needing to be laid
out again, and the validate causes that to be done. It may be
expensive, but is a way of getting the peers to recalculate size and to
do what is needed to bring the display up to date. It has limitations:
it doesn't cause an immediate screen update when invoked from an event
handler, where paintImmediately() does.
11. (Sect. 19) Why aren't popup menus working cross-platform for me?
[*] On Windows, the pop-up trigger is a mouse release (except in
certain programs like Netscape Communicator). On Unix, the pop-up
trigger is a mouse press.
Therefore you need to ask the question isPopupTrigger() in both the
mousePressed() and mouseReleased() methods when implementing the
MouseListener interface. Alternatively override Component's
processMouseEvent as a central place for handling mouse input.
12. (Sect. 19) Why aren't newlines working cross-platform for me?
[*] Code like this:
if (c == '\n')
fin = true;
is not cross-platform. On Unix the line terminator is "\n", on Windows,
it is frequently "\r\n", on the Mac it is "\r".
The call System.getProperty("line.separator") will return a string
containing the platform-specific line separator character(s), and you
then need to compare it according to how your data is formatted (e.g.
compare 2 characters or one). There is also a property for the
separator character in file pathnames, and other values too.
This can screw-up your networking programs too. Most ASCII based
protocols like HTTP expect \r\n to terminate a request. They will hang
on a request from a Mac that only sends \r.
13. (Sect. 19) Why didn't my text display in my GUI? Is the Inset wrong?
[*] The most common Inset problem is not an Inset problem at all, but
rather that people just assume the x,y location of a
Graphics.drawString() actually refers to the top left part of the
string image. In fact it refers to the baseline. So you'll need to take
g.drawString("Hello World",0,getFontMetrics(getFont()).getAscent());
14. (Sect. 19) Why did my polygon come out the wrong shape?
[*] This question and answer comes directly off
comp.lang.java.programmer, and deserves to be immortalized for
posterity.
When I use fillPolygon with the following points I get two inverted
triangles instead of a rectangle. Why?
int xPoints[] = {71, 78, 71, 78};
int yPoints[] = {147, 147, 130, 130};
g.fillPolygon(xPoints, ypoints, xPoints.length);
Developer Felix Pahl supplied the answer in limerick form:
o A developer (for details bored her)
didn't follow the polygon's border
so instead of right angles
she got two triangles
'cause the endpoints were in the wrong order!
You must put the points in the order you would encounter them in if you
went round the polygon's border. The filling algorithm is doing the
right thing! Try drawing the points on paper to see:
71,130 78,130
O--------O
| |
| |
| |
o--------o
71,147 78,147
Under JDK1.1, the two endpoints are connected automatically and you
would order the array elements as:
int xPoints[] = { 71, 78, 78, 71};
int yPoints[] = {130, 130, 147, 147};
Under JDK1.0.2, you have to explicitly connect the two endpoints, and
you would write the array elements as:
int xPoints[] = { 71, 78, 78, 71, 71};
int yPoints[] = {130, 130, 147, 147, 130};
15. (Sect. 19) Why can't I see all the components I added to a Frame?
[*] If you have code like:
Frame myframe = new Frame("Child Frame");
myframe.resize(512,384);
myframe.add(new Label("Child"));
myframe.show();
The default layout manager for Frame is BorderLayout. Components
positioned with a BorderLayout should include a positioning constant to
be correct. If you don't include one, "-1" is assumed, which causes the
component to go to the end of the list, and possibly be buried (in
terms of z-order) under any other components you add. So, change the
add to
myframe.add("Center", new Label("Child"));
and all will be well.
16. (Sect. 19) Why do I get the wrong results when I compare two Strings
together?
if (s1 == s2)
is giving me funny results.
[*] The comparison using "==" on objects, like Strings, is asking the
question "do these two objects have the same reference?". That is, do
they have the same address, and hence are not two object but one? What
you most probably meant is "do these two Strings have the same
contents?" which you can express this way:
if ( s1.equals(s2) )
This is a very, very easy mistake to make and impossible to spot until
you have had it explained to you.
People talk about "interning" a String. That means calling the intern()
method on a String. This places the String in the runtime constant pool
if it was not already there. The compiler is required to intern() all
literal Strings. If you intern() all your Strings as well then all
duplicates are shared and comparisons can be done by the (much faster)
address comparison rather than content comparison. It's a performance
optimization. See also Q3.22.
Note that this comparison error also occurs with other objects, not
just Strings. The code:
if (getBackground() == Color.black)
is a test for object identity, rather than content identity. It will
work if you originally setBackground(Color.black). To avoid difficult
debugging in the future, you almost certainly want to say
if (getBackground().equals( Color.black ) )
or even (in this visual case) compare the darkness of the RGB values of
the pixels.
17. (Sect. 19) Why doesn't final prevent my object from changing?
[*] You have code like this
final StringBuffer s = new StringBuffer("don't change me");
// ...
s.append(", but I did");
System.out.println(s);
And the new value of s is "don't change me, but I did". The reason is
that the "final" modifier makes the reference variable (here, s) final,
not the object that s points to. It means that the reference variable
cannot be changed to point to some other StringBuffer. The state of the
StringBuffer can still be modified by calling methods on it or directly
assigning to its public fields.
The right way to think about final is that it prevents you assigning to
that particular variable. The only way to make the fields of an object
constant (unchanging) is to make all its data fields private, and not
provide any set methods for them, only get methods. Even that won't
stop other objects of the same class adjusting it.
18. (Sect. 19) Why can't the compiler find my package?
[*] When trying to compile a file in a package you get a compiler error
like:
DBTest.java:10: Class database.Table not found in type declaration.
The file Table.java and DBTest.java are in the same directory. They
both have "package database;" at the top of the file. The current
directory is included in the classpath.
The reason is that when compiling packages, you have to be at the 'top'
of the directory/package hierarchy. So to compile both Table.java and
DBTest.java, you have to be in the directory that contains the database
directory (i.e. where the package hierarchy starts), and just:
javac database/Table.java
javac database/DBTest.java
and it should all compile fine.
19. (Sect. 19) I have a program with keyboard input and a button. When the
user hits the space bar, the button gets pressed as it is in focus!
[*] The VM sets the focus on the first traversible object in the UI. If
you want the button not to be assigned focus by default, you must
subclass the button and override the isFocusTraversable() method to
return false.
Another approach is to manually set the focus on some other component
(say the Frame) when you show the window. To do so you have to jump
through hoops to outsmart the VM that is trying to set it on the
button. One approach is to listen for the windowActivated event and set
a Swing Timer to do a requestFocus() on the frame about 0.1 seconds
after the activated message. This seems convoluted, but it is the only
thing found that consistently works cross platform.
Another reader suggests that if the frame normally gets the focus
first, you can override its gotFocus() event and set the focus to the
component you want. Don't forget to return true!
20. (Sect. 19) What's the hidden size limitation of String serialization?
[*] If you wish to Serialize a string, be alert to the restriction
that:
The size of the String, when UTF-encoded, must be < 64Kb
So for robust code you have to examine the String once to ensure that
it will be less than 64Kb after encoding, and then have the JVM
effectively repeat that work in the process of encoding, when you write
it to an ObjectOutputStream.
A possible workaround that is to strip the string down to "byte[]" and
pass it around in RMI that way. The code with this restriction is in
DataOutputStream
public final void writeUTF(String str)
...
[perform the size-after-conversion-to-UTF computation]
...
if (utflen > 65535)
throw new UTFDataFormatException();
...
RMI relies on serialization, so RMI has the same String size
limitation.
21. (Sect. 19) When I change a field in just one object in my array, that
field changes in all the objects in my array!
[*] Here the problem is probably that you have initialized the array
with N references to the same one object.
This is easy to overlook, because arrays in Java only contain
references to objects, not objects. (Or they can contain primitives).
22. (Sect. 19) Do DrawRect and FillRect work on rectangles of the same
size?
[*] No. java.awt.Graphics.drawRect draws a rectangle that's 1 pixel
wider and 1 pixel taller than a rectangle drawn by fillRect.
23. (Sect. 19) My executable Jar files don't work!
[*] It is possible to exactly follow the instructions given in the
documentation and yet still get the following error message when
attempting to run the .jar file: "Failed to load Main-Class manifest
attribute from myapp.jar"
There has to be a carriage return after the Main-Class definition in
the manifest file, otherwise it does not work! Example, in a file
called manifest.txt:
Main-Class: MyClass // does not work
Main-Class: MyClass
// does work
Then, when the manifest file is merged into the jar file like this:
jar cmfv manifest.txt myapp.jar *.class
it works fine.
24. (Sect. 19) Why do I get the error message "Wrong number of arguments in
constructor"
[*] Almost certainly, you have written your constructor with a return
type or void or something. I.e. you have written:
public MyClass Myclass() { ...}
instead of:
public Myclass() { ...}
Giving a constructor a return type makes it into a regular method that
happens to have the same name as the class. The language should ideally
outlaw this, but it doesn't.
-------------------------------
20. Further Resources
1. (Sect. 20) Will Java ever be fast enough for games like Quake?
[*] Some people originally thought not.
Please see the site http://fragisland.fragzone.se/ which has the Frag
Island game (a quake-style game) written in 100% Java. You play it as
an applet, by browsing the above site. Watch out -- it's noisy!
2. (Sect. 20) Are there any commercial/shareware/free Java libraries?
[*] Take a look at the Java Collection Framework, a group of classes
that are part of Java 1.2. These classes implement general-purpose data
structures, and they will become widely used.
The documentation for JDK 1.2 explains that the Collection Framework
defines three kinds of things:
o Standard interfaces representing data structures of various kinds
for you to implement. Since these are interfaces, you can use them
in your code before you have implemented them.
o Partial implementations of those interfaces, saving you some work.
o Complete implementations, ready to use for data in your programs.
The standard interfaces are Collection, Set, List and Map, plus the
more specialised SortedSet and SortedMap. Lists have duplicate elements
whereas Sets do not. Finer distinctions such as immutability are
defined in the implementor classes, enforced by throwing runtime
exceptions. See the JDK 1.2 documentation for a full discussion.
For more about sorting prior to JDK 1.2, look at the class SortDemo in
the demo directory of the JDK. Alternatively, use one of the several
classic sorts available from Roedy Green. They are supplied free with
heavily commented Java source code.
See "QuickSort", "HeapSort" and "RadixSort" in the Java glossary at
http://mindprod.com/index.html.
Also, try the Java Generic Library. This library (JGL) is freely
downloadable from http://www.objectspace.com/
Also Visual Engineering has JChart at: http://www.ve.com. No licensing
fees.
Visual Numerics has its Java Numeric Library available for download at
http://www.vni.com/products/wpd/jnl/jnl_1_0.html. They offer the JNL as
a proposed standard library for numerical functions missing from Java.
3. (Sect. 20) Why doesn't somebody write a shell in Java? Then they could
use it on all platforms!
[*] Somebody has done just that. Look at http://www.jsh.net/
4. (Sect. 20) Are there any URLs for other libraries?
[*] Indeed, there are. The Java3D Repository http://java3d.sdsc.edu/
5. (Sect. 20) Are there any URLs for regular expression handlers in Java?
[*] There is one from ORO Inc. They dissolved as a corporation, but one
of the founders maintains the software at http://www.oroinc.com/.
For other sources, see
http://www.meurrens.org/ip-Links/Java/regex/index.html
And don't forget to check out Lava -- a set of Java classes designed to
support programmers who develop console-mode applications and/or C
programmers who are converting to Java. The first release of Lava has
printf and other text formatting, encryption, parsing and miscellaneous
I/O. Lava can be downloaded from http://www.newbie.net/sharky/lava/
Also consider the Java version of the Unix find command. It offers
Regex filename matching, mindepth, maxdepth, symlink follow / no
follow, file type matching all cross-platform. The package is at
http://www.rule-of-eight.com/en/components/
6. (Sect. 20) Are there any installers for Java? Preferably
platform-neutral ones.
[*] There are several possibilities.
o InstallAnywhere 2 from ZeroG software. See
http://www.zerog.com/html/installanywhere_2.html
They have a free version for shareware authors.
o InstallShield makes a Java version of their installation package.
See http://www.installshield.com/java/default.asp
o IBM offers a comprehensive Java installer through its alphaworks
site http//www.alphaworks.ibm.com/formula/installtoolkit
o Denova offers the J'Express installer. There is a free trial
version available for download at http://www.denova.com
Freeware authors can receive a completely free license to
J'Express Standard. Simply fill in the application at
http://www.denova.com/JExpress/freeware.html.
7. (Sect. 20) What is "Jazilla"?
[*] Jazilla is Mozilla (Netscape Communicator free source) ported to
Java. In other words, a free source browser that supports Java and
Javascript, written in Java!
You can get more information, and volunteer to help with the project at
http://www.jazilla.org/
8. (Sect. 20) Where can I get Java for my Palm Pilot PDA?
[*] There is a translator allowing you to compile Java programs for the
Palm Pilot PDA! This is an astonishing piece of work as the Pilot has
such a small memory footprint. The translator is in an early stage of
development, but is available at:
http://www.cs.washington.edu/homes/mcdirmid/ghost
Try it, or even better, volunteer to help with the project. Details at
the website above.
There is a Java-PalmPilot Project called "jSyncManager" which allows
PalmPilot synchronization and jConduit development in pure Java. See
http://yaztromo.idirect.com/java-pilot.html.
If you can run Java 1.1 and Sun's Communications API, you can run
jSyncManager.
9. (Sect. 20) What is "Dippy Bird" and where can I get it?
[*] Dippy Bird is Java documentation in WinHelp format, which can be
used directly on Windows desktops, and has a searching utility. The
developer of the Dippybird project, Bill Bercik, has stopped further
work on the project due to lack of time and funds. Instead you can use
http://www.confluent.fr/javadoc/JavadocE.htm which has a more up to
date Java WinHelp doc.
You can get still get the Dippy Bird download at
http://www.dippybird.com/jdk111.exe (JDK 1.1). Note that on NT 4.0 you
need to change the generated shortcut to point to NT's 32-bit WinHelp.
10. (Sect. 20) Are there any Java graphing tools?
[*] Take a look at GraphMaker -- a complete full-featured Java
application for creating and searching graphs. It is available under
GPL with source, and uses the latest Swing JFC features. See
http://home.earthlink.net/~nfiedler/graph/
11. (Sect. 20) Where can I get icons for use with Java?
[*] Public spirited programmer and Java supporter Dean S. Jones has
created a collection of over 100 icons for use in Java freeware. They
are available on the Java Lobby site at
http://webart.javalobby.org/jlicons/.
12. (Sect. 20) What is "UML"?
[*] UML is the Unified Modeling Language. It is unified in the sense
that it draws together ideas from a couple of earlier software design
languages. UML is an emerging standard for diagrams of object-oriented
classes. It was devised by Grady Booch, Ivar Jacobsen, and James
Rumbaugh, and it unifies several popular existing notations.
A UML product is available from Rational Software, who also offer a
tutorial CD for free. See http://www.rational.com/uml/index.jtmpl There
are some whitepapers too, but there don't seem to be any free online
tutorials.
See http://www.togetherj.com/ for information on Together/J. That is a
platform-independent UML product that supports round-trip engineering
for Java. Whiteboard version is free.
13. (Sect. 20) Where can I get info on Java college courses?
[*] The JCampus site at http://www.jcampus.org has links and
connections to Java CS Dept. courses, assignments, academic papers and
Java-related events. JCampus is a non-profit, online Community for CS
Dept. professors, students and staff who are teaching, learning and
using the Java programming language.
14. (Sect. 20) What is the Java IFAQ?
[*] It is the Java list of Infrequently Answered Questions, a FAQ
maintained by Peter Norvig, author of the book "Artificial Intelligence
- A Modern Approach". Take a look at the Java IFAQ at
http://www.norvig.com/java-iaq.html There's a lot of good information
in that document.
15. (Sect. 20) Are there any Java tools for PDF?
[*] PDF (Portable Document Format) is the text publishing format
defined by Adobe. Acrobat is the technology to display and print PDF
files. Abode supplies the client (document reading) software for free.
There is a PDF toolkit written in Java at http://www.etymon.com. Even
better it is GPL'd. It is more a toolkit for programmers embedding PDF
in their products, than an end-user technology though. It doesn't have
a GUI for displaying PDF for example.
16. (Sect. 20) Are there any Java info search tools?
[*] IBM has a very good search engine for java developers
http://www.ibm.com/java
17. (Sect. 20) What other languages compile to bytecode?
[*] Quite a lot of languages compile to Java bytecode, more than 60 at
the last count. See the webpage
http://grunge.cs.tu-berlin.de/~tolk/vmlanguages.html
18. (Sect. 20) Has anyone written a Java-to-RPC interface, to talk to
legacy code?
[*] See www.distinct.com. It implements a subset of RPC, and is a
commercial, supported product. You can review RFCs 1831 and 1832 for
information on the full protocol. Java uses the same endianness as
RPC's external data representation (network byte order), so all the
Java file reads/writes can be used directly.
Netbula recently released Java RPC, a fully compatible port of ONC RPC
to Java. The binary package can be downloaded for evaluation from
http://netbula.com/javarpc/
The specifications are in RFC 1831 (the RPC protocol spec) and RFP 1832
(the XDR spec).
19. (Sect. 20) Are there any automated tools for Javadoc?
[*] Yes. See http://www.mindspring.com/~chroma/docwiz/docwizApplet.html
for a Java development tool called DocWiz. It is the easiest way to add
JavaDoc comments to your Java code.
20. (Sect. 20) Is there any information on XML and Java?
[*] Yes. See http://developerlife.com
It shows you how to use:
o the Sun parser (Sun ProjectX EA2)
o the DOM (org.w3c.dom.*) interfaces
o JFC/Swing with XML
o Servlets with XML
and more.
21. (Sect. 20) How can I find the format of a .class file/a JPG/a PNG
file/any file?
[*] There is a great website that maintains descriptions and links to
descriptions of hundreds of file formats. The site is at:
http://www.wotsit.org/
It shows you how the files are structured, and makes it a lot simpler
for you to write code that creates/decodes such a file.
22. (Sect. 20) Why not start up one copy of the JVM and share it among many
Java programs?
[*] If the JVM takes about 15MB (say) in overhead, and a program takes
3MB, then starting up a fresh JVM for each program is slow and
wasteful. If a single copy of the JVM and libraries could instead be
shared among multiple Java applications there would be less overhead
and everything would run faster.
That observation was the guiding force behind the echidna project to
support a JVM that could run multiple applications. See the site
http://www.javagroup.org/echidna/ for more details.
23. (Sect. 20) How can I edit sound files?
[*] The JaWavedit java code lets you edit .wav and .au files. It can be
found at http://www.bome.com/JaWavedit/. It's free to use.
If you are inspired to write freeware like this yourself, many file
formats are explained and described at http://www.wotsit.org
-------------------------------
21. Acknowledgements
A jolly little song that explains how to solve commonly-encountered problems
in Java.
The FAQ Melody
by Antranig Basman.
On the First Day of Christmas, my true-love said to me:
Read the F-A-Q.
On the Second Day of Christmas, my true-love said to me:
My Image isn't drawing;
Read the F-A-Q.
On the Third Day of Christmas, my true-love said to me:
My Pixels are not grabbing,
My Image isn't drawing,
Read the F-A-Q.
On the Fourth Day of Christmas, my true-love said to me:
My Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.
On the Fifth Day of Christmas, my true-love said to me:
Null - Pointer - Exception!
My Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.
On the Sixth Day of Christmas, my true-love said to me:
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.
On the Seventh Day of Christmas, my true-love said to me:
J++ don't mind it,
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.
On the Eighth Day of Christmas, my true-love said to me:
Threads they are a-blocking,
J++ don't mind it,
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.
On the Ninth Day of Christmas, my true-love said to me:
Dialogs-a-hanging,
Threads they are a-blocking,
J++ don't mind it,
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.
On the Tenth Day of Christmas, my true-love said to me:
Time-zone's in Pacific,
Dialogs-a-hanging,
Threads they are a-blocking,
J++ don't mind it,
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.
On the Eleventh Day of Christmas, my true-love said to me:
Docs are not specific,
Time-zone's in Pacific,
Dialogs-a-hanging,
Threads they are a-blocking,
J++ don't mind it,
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.
On the Twelfth Day of Christmas, my true-love said to me:
File I/O's horrific,
Docs are not specific,
Time-zone's in Pacific,
Dialogs-a-hanging,
Threads they are a-blocking,
J++ don't mind it,
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing;
You Should Read The-e F-A-Q!
FAQ copyright 1997, 1998, 1999 by Peter van der Linden. Contributions and
help from:
Matt Kennel, Patric Jonsson, Brad Van Tighem, Tony Hursh, Glenn L
Vanderburg, Peter Jones, John McDowall, Jim Driscoll, Uday, Dave Harris,
Bill Wilkinson, Tom Valesky, Dan Drake, Giles Thomas, Mitch Baltuch, Guy
Ruth Hammond, Gordon Keith, Jason Brome, Shani Kerr, Steve Chapel, Timothy
Wolters, Robert Lynch, Jake Cormier, Sean C Sullivan, Joseph A. Millar, Jim
Frost, Jim Balter, Jeff Bauer, John Kochmar, Carl Burke, William Stubbs,
Mark Smith, Volker Turau, Real Gagnon, Russell Gold, Max Hailperin, Bill
Tschumy, Marco Nijdam, Marc Pawlowsky, Laurence Vanhelsuwe, Ian Macgregor,
Mike Faulkner, Rich Koch, Will Clark, Govind Seshadri, Rich Simkin, Ian
Stiles, Kieren, Darren Christie, Tom Lane, Michael Jungmann, Rob Mayoff,
George Ruban, Tom McCann, David Hopwood, Thomas Phan, Kai Stuke, Rolf
Howarth, Derek Snider, David Boydston, Andy Godwin, John F. Dumas, Doug
Bell, David J. Biesack, Tiger Quimpo, Martin Hugh Rogers, Brian Krahmer, Ian
Burrell, Nikki Locke, Bin Li, Jackson Thompson, Steve Odendahl, Greg Smith,
Jeffrey C. Ollie, Mark Halvin, Jeremy Cook, Lak Ming Lam, Peter S. Morris,
Mark Halvin, Juergen Keil, Alex Stewart, Mike Abney, Rodney Stephenson, Mark
Gritter, Satish Talim, Tamminen Eero, Alexander Gridnev, Eric Hodges, Jamey
Graham, Will Lockhart, Scott Plante, Tom Sanfilippo, Jan Newmarch, Sean
Breslin, Stuart D. Gathman, [email protected], C Matthew Curtin, Tor Iver
Wilhelmsen, A.N.Pryke, Phil Race, David Holmes, David Rodal, Dominique
Plante, Trent Jarvi, Ingrid Biery, Gopal Unni Krishnan, Grant Lewis, Tov Are
Jacobsen, Gary McGath, Marty Hall, Will Forster, Colin Mummery, Darin
McBride, Mayank Shah, Jens Alfke, Glen Stampoultzis, Philip Brown, Peter
Steiner, Kurt Spaugh, Rasmus Ekman, Jonathan Revusky, Ken Kalish, Dave
Sanders, Bill Hyden, James Cloughley, Philip "diodes" Gustafson, Paul
Kinnucan, Juan ValdИz, Antranig Basman, Felix Pahl, David N. Still, Simon
Arthur, Mark Hammond, Dan Kegel, Thomas Weidenfeller, Pavel Shvartsman,
Christen Monberg, George Reese, Ian Macgregor, John Sublett, David
Zimmerman, Tony Dahlman, Druid, Chris Kelly, Patricia Shanahan, Paul Hill,
Lyne Lamoureux, Don Kennedy, Alec Muffett, Andrew Mickish, Pavel Shvartsman,
Neil of Parkway Consultants, Chris Thiessen, David Michaels, Bob Sutherland,
Michael Allen Latta, Joshy, Eric Albert, Wes Isberg, Lisa Retief, Michael
Park, Dave Postill, Thomas Weidenfeller, Konstantin Laufer, HЕkan
Gustavsson, James Stauffer, Reuben Firmin, David Lim, Eamonn Maher, Craig
West, Pavel Shvartsman, Jay Dunning, Kevin Swan, Grant Gainey, Dan Schmitt,
Benjamin Goldberger, Jake Hamby, Yaakov Itzhaki, Robert Lynch, Laura Werner,
Tomm Hoeft, King Dale, Joe Halpin, Daniel M. Pomerantz, Kevin Kelley,
[email protected], Peter Seibel, Werner Zsolt, Hank Stuck, David M.
Karr, John J. Bartholdi, III, Garry Turkington, John Bokma, Frank McCreedy,
Nat Pryce, Jeff Luszcz, Brent Callaghan, Neil Allen, Joe Preston, Tim Bell,
Rajesh, Jeffrey Galyan, Nick Matsakis, Larry Hamel, David Lamb, Mr Tines,
Bill McHardy, Phillip Lord, Jon K. Hellan
------------------------------------------------------------------------
I am maintaining a FAQ list to address specifically programming issues
(not a general tutorial on Java). Please mail suggested FAQ entries
including answer to faqidea on the site afu.com.
Question with answer gets you a credit in the FAQ.
Peter van der Linden, Sun Certified Java Programmer.
------------------------------------------------------------------------
Cross references
Most cross reference links inside this document are still to be filled in
after the great FAQ re-org. If you'd like to contribute a few, send me the
new text for the NAME= and the HREF=, and I'll fold them in as time permits.
Look at the FAQ source for the style to follow.
Copyright
Copyright (c), 1997,1998,1999 Peter van der Linden. Permission to copy all
or part of this work is granted for individual use, and for copies within a
scholastic or academic setting. Copies may not be made or distributed for
resale. The no warranty, and copyright notice must be retained verbatim and
be displayed conspicuously. You need written authorization before you can
include this FAQ in a book and/or a CDROM archive, and/or make a
translation, and/or publish/mirror on a website (scholastic and academic use
excepted). If anyone needs other permissions that aren't covered by the
above, please contact the author.
No Warranty
This work is provided on an "as is" basis. The copyright holder makes no
warranty whatsoever, either express or implied, regarding the work,
including warranties with respect to merchantability or fitness for any
purpose. Furthermore the author has been known to wear socks that don't
match his pants, and to commit other egregious lapses of good fashion sense.
------------------------------------------------------------------------
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