Just unburied a rigid-body physics engine that I made in 2012!
https://github.com/BlueThen/Gazelle-Java-Physics-Engine
It's probably uncompilable right now, but I might return to it and polish it up a bit. Meanwhile, I'll be adding a page to my portfolio on Gazelle!
If your Java happens to be out-of-date enough to allow these applets, you can see some demos at:
http://bluethen.com/secretApplets/asteroids/ (wasd to move, space to shoot)
and http://bluethen.com/secretApplets/gazelle/ (click and drag)