I'm trying to implement a drop-in replacement for the JavaFX file dialog to incorporate the KDE file dialog, and my current implementation just currently involves shelling out to kdialog. It works, mostly, though there are some minor issues: you can't have only the title for the dialog, for example; setting a blank caption (what I consider to be the program name) does nothing to remove the KDialog title; also, it currently doesn't implement an overwrite confirmation for file saving, which means I'd have to create one after the file dialog is closed, which isn't the prettiest. Also, getting the winid for the JavaFX app to attach the dialog to the app isn't the most fun.

In order to have a better solution, I was wondering if the KDE bindings to Java are available anywhere, and are reasonably up to date, so that I can simply call KFileDIalog instead. If not, is it reasonably possible (i.e. not too much hoop jumping) to write a JNI/BridJ wrapper over KFileDialog and use that?

You need to look into Qt Jambi - however the last time that website was updated was in 2012, so it may not be usable anymore. In terms of KDE interfaces for Java though, i'm not aware of any, although it is possible they do exist. I'm not aware of the specific mechanics of writing a wrapper using JNI so I can't comment there i'm afraid.