Dear Tomaz, I can help you in finding out whether a Ball touches another Ball(atleast its a start) but I can't help you to make them bounce off each other as I myself don't know. Start by making a new method in the Ball class called touches(). It is to be implemented as follows:
Anyone who knows how to make them bounce off each other, PLEASE Help US!
Well, implement the touches method that Shyam suggests, and if two balls touch, you reverse the directions that the balls are heading in. You do know what determines the direction of a ball in the code above, right? (of course there's a little more to it because balls won't always hit square with each other, but for the sake of argument, let's assume they do)