Following in the same fashion of Pong and Wumpus World, I've written a simplified Space Invaders game. I've uploaded it to youtube, make sure you watch it in high quality and full screen, to avoid blur text: http://www.youtube.com/watch?v=wORlAZoxttA
It's not the complete game, but in it's current form is simpler than Pong or Wumpus World; so it's a better place to start learning. I've written it, with re-use in mind, such as the configuration classes and key rules, to be re-used in other games; although they will need refactoring first. For Invaders I've committed each stage of the game, as separate files, so it's easy to see the stages for yourself.