What's a good example of an open-source Java-game?

_{(source: freecol.org)}

FreeCol - based on Sid Meier's Colonization.

This doesn't directly answer your question, but if you ever decide you want to try writing a game in JavaFX, the JavaFX site has a bunch of sample code, including a couple of games (as of current writing).

I wonder if http://sourceforge.net/projects/creepsmash/ would be a good choice. It was built by a team of computer science students in order to simulate a "real" commercial-style project. Thus, they used things like version control, build tools, and unit testing. Probably a better project to learn from then most, since that was its purpose. Yet the game is rather entertaining and plenty of people play it for fun. Not sure how much of its documentation is in English, though.