1. What do QtJava and KDEJava actually do?

They wrap most of the classes in Qt and KDE so that they can be used to write Java programs, which look and behave identically to a C++ version. The Qt date, time, string and list classes are replaced with their Java equivalents.

2. What would you say are the best features of the binding?

It uses KDOC as the parser engine, so it has access to the documentation comments of the KDE methods. It translates these comments into Javadoc format, and has a go at translating the code snippets into 'cod java' - ie C++ with those funny symbols like '&', '*', '::' (that you never quite mastered ;-) ) replaced with java style '.'s.

3. What made you decide to write this, and how long did it take?

I wrote some code to patch KDevelop and KDOC 1.0 to use Objective-C for gnustep development in late 1999. After I'd done that, someone posted a question to comp.lang.objective-c asking about whether there were any Objective-C bindings for Qt. I thought that if I could do some bindings, I'd have a really nice complete Objective-C programming environment.

I started using Roberto Alsina's QtC for the C bindings and shell scripts for the Objective-C side (you can't call C++ directly from Objective-C, so you have to do two sets of bindings). I completed a prototype version good enough to run Qt Scribble in Objective-C in about 3 months. But the work was very labour intensive and I couldn't see how I could manage to get the whole Qt/KDE api done.

I wrote a patch to add Objective-C support to kdoc 2.0, so I got familiar with the Perl code. Mathieu Chouinard emailed me asking about how work on the Objective-C bindings was going. I told him I'd started to hack Roberto's Python scripts, but it was still taking a lot of work to get anything wrapped. Then it occured to me that it would be easier to make KDOC generate the same code as the python stuff, than it would be to make the Python parse as well as KDOC. After two months (less time than the prototype took), we had two KDOC-based converters, for C and Objective-C, for Qt 2.2 wrapped and working, and I was about to start on KDE 2.0.

But once I realised I could the same thing for Java/JNI at the end of October or so, I just downed tools on Objective-C and ran for it. Er.. 24 by 7. I haven't still been programming when the birds start singing since late 1988 or so! I got Qt 2.2 done before Christmas, then had six weeks off starting again in February. So KDE 2.0 has taken a bit less than a month to get this far.

Probably about a man year of 40 hour weeks for C/Objective-C/Java in total.

4. Who do you think will find this code most useful?

One group might be those people who would like to write apps with full Qt or KDE look and feel, but prefer to code in Java over C++ (because it's quicker). Another group might be those taking existing Java code and wrapping it to get a fully functional KDE app. A third group might use the Qt side to write apps which will run cross-platform, both on standard Wintel PC's (a port to Windows should be straightforward) and Linux, *BSD, etc.

5. How much of the Qt and KDE APIs have you covered?

280 Qt classes and over 400 KDE classes - pretty complete.

6. How hard do you think it will be to extend the binding to cover the new features in KDE 2.1?

The manual edits applied to get the bindings working are in the form of a patch. So the KDE 2.1 headers need to be put through the converter, then the patch applied. Finally, the diffs added to the KDE CVS since the initial check-in must be applied, and any essential new 2.1 classes added to the project.

7. How does the signal/slot mechanism of Qt translate into Java?

Here is an example of how to connect a signal to a slot in QtJava:

 QObject.connect( _colormenu,
                  SIGNAL( "activated( int )" ),
                  this,
                  SLOT( "slotColorMenu( int )" ) );

If the underlying C++ instance of '_colormenu' has an 'activated(int)' signal, the Java slot is connected to that. Otherwise, the runtime assumes that you want to connect to a Java signal and it connects the slot to that instead.

Emitting a signal looks like this:

 emit("colorChanged", darkGreen());

8. How does this compare with AWT and Swing?

A very interesting question! I've never done any AWT or Swing programming (my background is Objective-C with NeXTSTEP). It doesn't appeal to me because the subclassing and managment of event listeners looks clunky and doesn't make use of Java's dynamic runtime. Swing code appears to need 5 times as many classes and twice the bulk, yet offers less functionality, compared to Apple's Cocoa or Trolltech's Qt.

9. How hard is it for developers to add Java support for their own Qt widgets?

The KDOC converter is included in the Koala KDE bindings project, so they can run their widget headers through that and convert them. One day per widget or so would be my estimate

10. Could this code be used to embed Java code in a C++ KDE application?

Yes, you can mix C++ and Java. The objects don't always have to be instantiated from within the Java environment. If you allocate an object instance on the C++ side, and then you refer to it from within Java, a Java version is created automatically. However, when the Java version is subsequently garbage collected, the C++ instance isn't freed by the Java runtime. It would still exist over in the C++ environment.

However, if you instantiate an instance on the Java side with a 'new' command then the C++ is freed on Java garbarge collection.

11. Where do you plan to go from here?

I plan to finish the Objective-C and C bindings for KDE, finish adding Objective-C and Java support to KDevelop 2.0 and contribute bug fixes/enhancements to the code just released.

12. Is there anything else you'd like to say?

I haven't given the KDOC author a namecheck - which I think he deserves. So a big hand to Sirtaj Singh Kang from me is deserved. Also I forgot to mention there is a patch to the Qt Designer uic tool to generate Java from exactly the same .ui files as C++.

I'd like to thank Richard for creating this cool new binding, and for finding the time to answer my questions. You can get the code by checking out the kdebindings module from the CVS; it's not included in the packages for KDE 2.1 because it's so new. I have added some information to the Java language bindings section of the developer site, including an example of using the bindings for people to have a look at. People interested in using Java with KDE might want to join the kde-java mailing list.

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ScribbleWindow.java 8KB (9205 bytes)