Personally, I would recommend picking up one of the java engines and playing around with it. For 3d there is jpct or 3dzzd that I know about that make pretty good online games. For 2d jgame. There are probably a lot more, but that's the ones I know about. I've only actually used jpct.
I was going to use an engine for JAVA, but I decided to try to start from scratch making a JAVA game. Sorry, I should have included it in my original post.