Well - I don't have kde installed on my current PCs, but remember, that you can do it, if you do it while opening the java-application (I can't with gnome-nautilus).
Somewhere the action is specified to happen, by double-clicking a .class or a .jar file.
I guess right-clicking on .class-File opens a context-menu, where you may modify what to happen.
The action is to start java with this class, and to pass the arguments.
Something like:
perhaps with full name of path to java included.
I can't
test it, but I hope this will move you a step ahead.
[ August 13, 2005: Message edited by: Stefan Wagner ]