I'm not sure what you mean by real Java. If you have programmed in Greenfoot, you have programmed Java. A class can be whatever you want it to be, give it fields for an image, a location (within some "world"), perhaps some bounds (could just be the bounds of the image). Then whatever form or window responsible for drawing the game environment can grab these classes' images to draw them given their location fields etc. Edit: Greenfoot is just a library and a little bit of overhead taking care of the window, drawing of game environment, execution of game objects, mouse interaction, etc. It is not a different language than that of Java

This was the tutorial I used and it was great Java Games Tutorial (I think you were asking for something like this) After I posted I saw you wanted something short..... but if you want something more in-depth just go here

By actual java I meant not greenfoot, but Just java. And I'm just wondering how the game keeps tracks of classes and interacts with them to create the game. I just want a basic explination, not a full tutorial. Although thank you JetLennit; the tutorial is very interesting! And bourn, I did not know that Greenfoot was that simple...

Keep a list of game objects, and tell each one to "act" every game cycle interval that you define. Or have them have their own threads is a possibility