All these posts and no one mentions JCreator? It's probably one of the best IDEs I've ever used for Java, and it can even be installed on Linux (using Wine of course)

http://www.jcreator.com/

Another IDE I use regularly in Linux is KDevelop, which is a KDE based IDE, and I used it to write code on my fave languages, the only flipside to it is that I have to compile the code in console, but that's really nothing to me.

http://www.kdevelop.org/