There were several attempts at porting Java to AmigaOS.
I think there's a Kaffee port somewhere, one old one has rudimentary support for some AWT classes and that was it.
There was another recent Java port but it currently insn't completed. Again, I believe the GUI parts aren't fully coded yet.
Your best bet, at the moment, is linux. Otherwise, if you have an XServer for Amiga, you might be able to compile Kaffee and have it use the Xserver? (Not 100% certain on that one though.)
Steph