You can check for collision in either of the two object classes. Normally, though, it is done in the object that will be remaining in the world (and it makes sense to do it here). Of course, if it is a random decision as to which one stays, it really does not matter. The reason it is normally done in the one that stays is that oftentimes the collision is recorded somehow (either a counter or a scoreboard), and once you remove the one object, it no longer has any reference to the world. As far as coding, use the getOneIntersectingObject(Class class) method to see if one of those type objects have collided with the instance object in question. Let us say you have a Monster object and a Shot object, then in the Monster class: (maxShotCount would be an instance integer variable for the Monster class that was initialized to a number representing the number of shots required to eliminate the monster).