I recently watched a bit of Notch\'s Ludum Dare live stream. He uses Eclipse\'s hotswap feature extensively while he works on his game. (here is a video of what I am referri
This may not be exactly what you're looking for but it's what I've been doing. Just throw in a break point while your game is running. Once it's stopped, you can edit things, remove the break point, and then hit F5 to continue.
For the most part, this allows you to edit things while the game is running and continue running the game afterwards but it doesn't work for everything. You can't add or remove class level things and also can't add/remove entire functions or the parameters. Mostly, I do this to test out different numbers and equations for smaller things that get hit every update.