I have written a Java application that opens a dialog (FileChooser) which runs fine on Windows but when I try and run it on the Pi, the dialog fails to appear. What may be wrong?
JavaFX on the Pi runs on a single primary JavaFX stage. The implementation on Pi does not allow additional stages. As such, you can't create secondary windows or dialogs that open in their own windows. This is simply a restriction in the current release. The release at which this restriction is present is at least:
$ java -version
java version "1.8.0"
Java(TM) SE Runtime Environment (build 1.8.0-b132)
Java HotSpot(TM) Client VM (build 25.0-b70, mixed mode)