Since I can't add a comment, I'll add an answer. I'd recommend NOT using threads for moving characters, etc. For file loading, playing sounds, and even server side networking it works well but any sort of AI/Graphics, it tends to cause endless headaches.
Also, as other people have mentioned, Java may not be the best choice. The simple reason that people say this is that it's difficult. The amount of support/tutorials for Java game development is rather lacking when compared to C++ (GameDev.net is great resource, but it's moved its focus more towards professional development over time), Flash (decent tutorial on Kongregate as well as being a spot that could host the game for you), or even C# (using XNA). Java game development has improved over the years and you can do some amazing things with it, but it may end up being more difficult going that route. No matter which language you choose, definitely look into using a graphics/game engine as it will cut down on the amount of work on your end and allow you to hopefully create a better game.