comp.lang.java.gui FAQ
From: Thomas Weidenfeller (nobody_at_ericsson.invalid)
Date: 02/09/04
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Date: Mon, 09 Feb 2004 10:25:58 +0100
Version: $Revision: 1.7 $
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Last-modified: $Date: 2004/02/09 09:19:08 $
comp.lang.java.gui FAQ
Table of Contents
1 Introduction
Q1.1 What is this, and what does it contain?
Q1.2 There are so many Java FAQs. Which is the right, official one?
Q1.3 I noticed broken links in the FAQ. Don't you verify them before
publishing?
Q1.4 What is AWT?
Q1.5 What is Swing?
Q1.6 What is SWT?
2 The Top 5 Questions
Q2.1 My GUI freezes. What to do?
Q2.2 How do I update the GUI from another thread, e.g. once I have
offloaded a time consuming task from the EDT to another thread?
Q2.3 I have arranged all my widgets nicely on a window. Then I changed
the OS / Java version / font / PLAF. Now everything is broken.
What's going on?
Q2.4 My graphics on a Canvas/JPanel/JComponent, etc. gets corrupted,
or I get a null pointer exception when trying to draw. How can I
avoid this?
Q2.5 How can I make a transparent or non-rectangular window?
3 Window / [J]Frame / [J]Dialog (Top-Level Containers)
Q3.1 How can I make sure a window is always on top of all other
windows using AWT or Swing?
Q3.2 How can I (de)iconify a window?
Q3.3 How can I replace/remove the icon in the title bar (window
decoration) of a [J]Frame?
Q3.4 How can I replace the icon in the title bar (window decoration)
of a [J]Dialog?
Q3.5 My modal dialog goes behind the main window. How can I make sure
it is in-front instead?
4 Other Common Questions
Q4.1 My GUI has rendering problems when the JMenu opens over my
top Panel ...
Q4.2 I append text to a JTextArea. How Do I make sure the text area is
always scrolled down to the end of the text?
Q4.3 How can I do this JavaScript thing on my web site?
5 The Newsgroup
Q5.1 What is this newsgroup's charter? What are acceptable topics?
Q5.2 Where can I find an archive of this group?
Q5.3 What is an SSCCE?
Q5.4 Why don't people like top-posting? What is top-posting?
Q5.5 Is there more about posting to newsgroups and asking questions?
6 Resources
6.1 Sun's Java Web Site
6.2 Icons
6.3 Misc. Examples, Tips and Tricks
6.4 Style Guides
6.5 SDK Documentation
6.6 More Swing
6.7 Online Magazines
6.8 Java 2D API
6.9 AWT
6.10 Java 3D API
6.11 General Java
6.12 More?
Q6.12.1 But I need more, more, more!
7 Improvement Suggestions
8 Acknowledgments
1 Introduction
~~~~~~~~~~~~~~
Q1.1 What is this, and what does it contain?
This is the FAQ for the comp.lang.java.gui newsgroup. It mostly
consists of a selection of pointers to resources regarding Swing and
AWT programming in Java.
Q1.2 There are so many Java FAQs. Which is the right, official one?
There is probably not THE FAQ. Everyone can start an FAQ, and many have
done so. See it as some benefit. There is a lot of information out
there.
For a list of other FAQs and FAQ lists :-) see
http://mindprod.com/jgloss/faqs.html
Q1.3 I noticed broken links in the FAQ. Don't you verify them before
publishing?
No, I don't. I rely on feedback from readers. Also, links might break
at any time, e.g. just seconds after a link has been verified.
Q1.4 What is AWT?
AWT (The Abstract Window Toolkit) is Sun's first Java GUI toolkit. It
is rather limited and uses the native GUI components of the operating
system.
Unless you have to support an old VM, Swing is usual the better choice
for a Java GUI toolkit.
Q1.5 What is Swing?
Swing is Sun's second attempt at a Java toolkit. It is rich in
functions and widgets, and is considered the standard Java GUI
toolkit. Nowadays it is bundled with the Java 2 Standard Edition.
Most parts of Swing are written in Java, especially, most of the GUI
components. Swing uses some parts of AWT in order to gain access to the
native GUI system for event handling and top-level containers.
Q1.6 What is SWT?
SWT is an alternative GUI toolkit from IBM. Unlike AWT and Swing, it is
not part of the Java 2 Standard Edition. You have to obtain it
separately for the platforms you want to support (it uses a native
library).
2 The Top 5 Questions
~~~~~~~~~~~~~~~~~~~~~
Q2.1 My GUI freezes. What to do?
Most likely you are blocking the event dispatching thread (EDT).
Offload time-consuming tasks from your event listeners to separate
threads.
You can do the necessary implementation by hand, or you can use
existing frameworks like the SwingWorker class from Sun. See the series
of articles in
http://java.sun.com/products/jfc/tsc/articles/threads/threads1.html
Q2.2 How do I update the GUI from another thread, e.g. once I have
offloaded a time consuming task from the EDT to another thread?
See the javax.swing.SwingUtilities.invokeLater() and
javax.swing.SwingUtilities.invokeAndWait() methods.
Usually you want invokeLater().
Again, see
http://java.sun.com/products/jfc/tsc/articles/threads/threads1.html
for more details.
Q2.3 I have arranged all my widgets nicely on a window. Then I changed
the OS / Java version / font / PLAF. Now everything is broken.
What's going on?
This sounds as if you don't use layout managers, but instead hard-coded
component sizes and widgets. If you want to avoid this problem, there
is no way around using layout managers, or implementing your own
geometry management from scratch.
Q2.4 My graphics on a Canvas/JPanel/JComponent, etc. gets corrupted,
or I get a null pointer exception when trying to draw. How can I
avoid this?
Do not use Component.getGraphics(). Instead, subclass and override the
paint() (AWT), or paintComponent() (Swing) method.
Component.getGraphics() simply can't work. Java uses a callback
mechanism for drawing graphics. You are not supposed to "push" graphics
information into a component using getGraphics(). Instead you are
supposed to wait until Java calls your paint()/paintComponent()
method. At that moment you are supposed to provide the Component with
the drawings you would like to do.
This mechanism is necessary so Java can support graphics systems which
don't remember window contents when it is obscured (e.g. overlayed by
another window). When the window becomes visible again, such graphics
systems have to ask the application to reconstruct the window content.
Therefore, paint()/paintComponent() is supposed to be the memory of a
component. getGraphics(), however, doesn't have any recollection of
previous drawing operations. So once a drawing done via getGraphics()
is lost, it can't be reconstructed. There is nothing in there that
stores the old drawing data, and there is nothing in AWT/Swing which
informs getGraphics() to do some re-drawing.
In addition, there are situations where Component.getGraphics() simply
returns null. This is a defined behavior of the method. And finally,
most users of getGraphics() forget to dispose the Graphics object after
usage.
See
http://java.sun.com/products/jfc/tsc/articles/painting/index.html
for more information.
Q2.5 How can I make a transparent or non-rectangular window?
You can't in a good, platform independent way.
Although particular Java components can be 'transparent', they will
always be contained inside a rectangular root component that is not
transparent.
One hack is to take a snapshot of the underlying screen region using
java.awt.Robot.createScreenCapture(rectangle). And then using that
snapshot as a background image for the window. If the background
changes, the illusion is gone.
An alternative for applets can be found here:
http://java.sun.com/docs/books/faq/src/app/MatchBackgroundExample.html
[I also remember to have seen a solution using JNI for windows.
Pointers are of course welcome.]
3 Window / [J]Frame / [J]Dialog (Top-Level Containers)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Q3.1 How can I make sure a window is always on top of all other
windows using AWT or Swing?
You can't. AWT and Swing don't provide this feature. All you can do is
to use a (modal) [J]Dialog, and make sure the [J]Dialog is provided
with the correct parent/owner in the constructor.
Q3.2 How can I (de)iconify a window?
Before Java 1.2 you had to revert to native calls.
Since Java 1.2 you can use [J]Frame.setState().
Since Java 1.4 you can use [J]Frame.setExtendedState(), too.
setExtendedState() provides more features than setState().
Q3.3 How can I replace/remove the icon in the title bar (window
decoration) of a [J]Frame?
Use setIconImage().
To revert to the platform's default icon use:
frame.setIconImage(null);
On some platforms this might remove the icon. Alternatively you can try
a transparent Image if you don't want to have an icon.
Q3.4 How can I replace the icon in the title bar (window decoration)
of a [J]Dialog?
There is only a partial solution to this problem, and it is not
recommended.
A dialog gets its icon from its parent frame. You can create a dummy
frame, set the icon of that dummy frame, and use it in the constructor
of the dialog as the dialog's owner:
JFrame dummy = new JFrame();
Image icon = ...
dummyFrame.setIconImage(icon);
JDialog dialog = new JDialog(dummy);
However, this is dangerous. Certain GUI behavior depends on a correct
[J]Frame (parent window) <-> [J]Dialog (child window) relation.
Introducing a dummy parent breaks this relation. Things which can go
wrong include (de)iconising of all windows of an application, and
ensuring a modal dialog is always placed on-top of the main window.
Q3.5 My modal dialog goes behind the main window. How can I make sure
it is in-front instead?
Make sure you have properly set up the 'owner' of the dialog in the
dialog's constructor. Don't use null, and don't use a dummy frame (to
set the dialog's icon).
4 Other Common Questions
~~~~~~~~~~~~~~~~~~~~~~~~
Q4.1 My GUI has rendering problems when the JMenu opens over my
top Panel ...
Swing (JMenu) and AWT (Panel) components do not mix well.
See
http://java.sun.com/products/jfc/tsc/articles/mixing/index.html
for things you have to pay attention to.
Q4.2 I append text to a JTextArea. How Do I make sure the text area is
always scrolled down to the end of the text?
textarea.setCaretPosition(textarea.getDocument().getLength());
Q4.3 How can I do this JavaScript thing on my web site?
Java is not JavaScript. Try news:comp.lang.javascript or another
suitable JavaScript newsgroup.
5 The Newsgroup
~~~~~~~~~~~~~~~
Q5.1 What is this newsgroup's charter? What are acceptable topics?
The charter became official, when the voting for creating the
newsgroup passed. This was published in:
http://groups.google.com/groups?selm=860665862.3170%40isc.org
Here is an excerpt (Note, "all groups" refers to all the Java groups
from that voting, including comp.lang.java.gui):
CHARTER: all groups
The normal practice should be that most articles are posted to one
single, correct group ONLY. Cross-posting is only appropriate when the
problem is hard to categorize or when it legitimately concerns more
than one group. Answers should be posted to a single group only once
the nature of the problem has been ascertained. Many articles of this
sort should go to comp.lang.java.help (only).
It is not appropriate to post binary class files or long (longer than
one or two screenfuls) source listings on any of these groups. Instead,
the post should reference a WWW or FTP site (short source snippets to
demonstrate a particular point or problem are fine).
END CHARTER.
[...]
CHARTER: comp.lang.java.gui
This unmoderated group is for any and all discussion relating to
GUI toolkits or window frameworks in Java. Topics include the AWT,
Netscape's IFC, Microsoft's planned AFC, Visix's Vibe toolkit, among
others. The newsgroup will also be the appropriate place for
discussion of the JDK event model, mouse and keyboard issues,
bugs in windowing code, and graphics programming in Java. If it
concerns something that can be seen on the screen, it belongs in this
group.
END CHARTER.
One will note the list of ancient Java GUI technologies, and the
absence of Swing. It is save to say, that nowadays most discussions are
about Swing, and a few about Java.
Over the time, it has turned out that certain types of postings are not
welcome in the newsgroup, even if Java related. This includes:
* Postings which just contain a statement like "Help! It does not
work!", without any additional information, like the exact error
message, source code, or even a hint what "it" is supposed to mean.
* Posting your homework.
* Advocacy.
* Postings urging the readers to help. Especially in conjunction with
whining.
* Public and hidden advertising. You think you are too clever to be
caught? Well, read this thread in our sister group c.l.j.programmer
first, and watch how some business lost all its reputation:
http://groups.google.com/groups?threadm=58882783.0307010403.
67c54a5f%40posting.google.com
* Postings demonstrating unwillingness to learn are not welcome, too.
And learning starts by reading the API documentation.
Q5.2 Where can I find an archive of this group?
See e.g. http://groups.google.com/groups?group=comp.lang.java.gui
Q5.3 What is an SSCCE?
Short/small, self contained, compilable, example (source code).
It would be best if you provide such short (see the group's charter)
example source code in our first request for help. When asked for one,
please don't complain that your source code is to large, to tricky, to
secret for being cut down to a reasonable size and posted. You have the
problem, and you asked in a public forum, so it is in your interest to
provide the requested information.
For more information about hacking some example code together, go to
http://www.physci.org/codes/sscce.jsp
Q5.4 Why don't people like top-posting? What is top-posting?
See the following Question & Answer (well, Answer & Question) section:
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on Usenet and in e-mail?
-- Common Usenet signature
Q5.5 Is there more about posting to newsgroups and asking questions?
Yes, see e.g.
http://www.catb.org/~esr/faqs/smart-questions.html
http://www.yoda.arachsys.com/java/newsgroups.html
for making the most out of a newsgroup (ignore the hacker slang) [The
author of the first document has requested a notice that he is not a
help desk for your problems].
Also see the newsgroups:
news:news.newusers.questions
news:news.answers
And RFC 1855. E.g. at
http://www.faqs.org/rfcs/rfc1855.html
6 Resources
~~~~~~~~~~~
There are many good on-line resources regarding Java GUI programming
out there. The following list is limited by intention to resources
which get recommended often on c.l.j.g, including resources from
regulars.
6.1 Sun's Java Web Site
Sun's Swing Quick-Start Tutorial:
http://java.sun.com/docs/books/tutorial/uiswing/mini/index.html
Sun's Swing Tutorial:
http://java.sun.com/docs/books/tutorial/uiswing/index.html
Sun's Swing Connection (TSC) (Swing developer's site):
http://java.sun.com/products/jfc/tsc/index.html
The TSC article index (Swing architecture, Swing and threads, painting
architecture, etc.):
http://java.sun.com/products/jfc/tsc/articles/
6.2 Icons
It is suggested to check the licenses of each icon collection before
using them in any products. Several of the following packages come with
open source / free software licenses.
Sun's well hidden set of Icons for Swing's Metal LnF:
http://developer.java.sun.com/developer/techDocs/hi/repository/
More icons:
http://sourceforge.net/projects/icon-collection/
(the link to javalobby.org on that page is broken)
The Ximian Open-Office icons (very nice):
http://developer.ximian.com/themes/icons/ooo-icons.html
Gnome icons:
http://tigert.gimp.org/gnome/gnome-stock/
SVG BlueSphere Icon Theme:
http://svgicons.sourceforge.net/
KDE Icons:
http://www.buzzard.org.uk/jonathan/kde-icons.html
6.3 Misc. Examples, Tips and Tricks
A collection of examples for doing nice things with Swing components
(some are a little bit outdated):
http://www2.gol.com/users/tame/swing/examples/SwingExamples.html
[offline, Andrew Thompson is working to resurrect them at a new
site]
Christian Kaufhold's Java and Swing info (including JTable info):
Jeanette's notes (including JTable remarks):
http://www.mycgiserver.com/~Kleopatra/swing/swingentry.html
Marco Schmidt's Java resource page (Java imaging information)
http://www.geocities.com/marcoschmidt.geo/java.html
Karsten Lentsch's company web page:
6.4 Style Guides
Java Look and Feel Design Guidelines:
http://java.sun.com/products/jlf/ed2/book/index.html
Java Look and Feel Design Guidelines: Advanced Topics:
http://java.sun.com/products/jlf/at/book/index.html
6.5 SDK Documentation
GUI Information in the SDK documentation (commonly overlooked):
See your local SDK installation, or
Abstract Window Toolkit (AWT)
http://java.sun.com/j2se/1.4.2/docs/guide/awt/index.html
Swing
http://java.sun.com/j2se/1.4.2/docs/guide/swing/index.html
2D Graphics and Imaging
http://java.sun.com/j2se/1.4.2/docs/guide/2d/index.html
Image I/O
http://java.sun.com/j2se/1.4.2/docs/guide/imageio/index.html
Print Service
http://java.sun.com/j2se/1.4.2/docs/guide/jps/index.html
Input Method Framework
http://java.sun.com/j2se/1.4.2/docs/guide/imf/index.html
Accessibility
http://java.sun.com/j2se/1.4.2/docs/guide/access/index.html
Drag-and-Drop data
http://java.sun.com/j2se/1.4.2/docs/guide/dragndrop/index.html
Swing Examples in the SDK:
See the directory demo/jfc in your Java SDK installation.
6.6 More Swing
Swing class-hierarchy chart
http://www.holub.com/goodies/java.swing.html
jGuru Swing FAQ
http://www.jguru.com/faq/Swing
CodeGuru Swing Examples
http://www.codeguru.com/java/Swing/index.shtml
6.7 Online Magazines
Regular GUI articles can be found in
6.8 Java 2D API
The Programmer's Guide
http://java.sun.com/j2se/1.4.2/docs/guide/2d/spec/j2d-bookTOC.html
6.9 AWT
Sun's AWT FAQ
http://java.sun.com/docs/books/faq/faqawt.html
6.10 Java 3D API
There is a separate newsgroup. see
news:comp.lang.java.3d
http://groups.google.com/groups?group=comp.lang.java.3d
6.11 General Java
The Java Tutorial:
http://java.sun.com/docs/books/tutorial/
All kinds of tutorials:
http://developer.java.sun.com/developer/onlineTraining/
Roedy's Java Glossary:
http://www.mindprod.com/jgloss/jgloss.html
6.12 More?
Q6.12.1 But I need more, more, more!
Learn how to use a search engine like
7 Improvement Suggestions
~~~~~~~~~~~~~~~~~~~~~~~~~
Please mail suggestions, corrections, updates, etc. to user "cljg_faq"
on the "gmx.de" site. Your "Subject:" line must contain the string
"[cljg]" somewhere. Otherwise your mail will be discarded
automatically. In addition, the address is heavily spam-protected.
If you suggest a new entry, please also provide the answer, not only
the question.
8 Acknowledgments
~~~~~~~~~~~~~~~~~
This FAQ contains contributions and help from:
Andrew Thompson, David Postill, Manish Hatwalne
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