Does anybody know any good 2D game engines for Java? I'd like one that at least tries to keep up-to-date. I tried using JGame, but it didn't have the flexibility I required. I tried using Slick2D, but it doesn't offer an easy solution for collision detection. Instead, most people suggest you loop through all of your entities and check if you entered another object that way, but O(n) seems like a really, really horrible way to go about that when you plan on having several enemies, boundaries, bullets flying everywhere...
Also, the creator of Slick2D hardly ever updates anymore, a lot of the tutorials in the Wiki aren't even completed. I'm not blaming him, I understand what it feels like when you don't have time, but surely there's a framework out there like what I'm looking for?
The Lightweight Java Game Library sounds very interesting. I've never used myself, but they have a nice list of projects using their engine, and from what I'm quite sure I saw in their files, the Revenge of the Titans game uses this lib.
The Slick lib also seems interesting, it works on top of LWJGL.