Thank you! The Tower class needs an object of the body in the constructor to be created. So the body is saved in the Tower, so that the Tower can get the coordinates of the tower. It is like this: public class Tower{ private Body tank; public Tower(Body tank){ this.tank = tank; } public void act(){ follow(); } public void follow(){ setLocation(tank.getX(), tank.getY()); } } There is much more stuff, too, for example to shoot. If you have more questions ask, it's no problem. :)