public void actionPerformed (ActionEvent event){  
			   for(int k=0; k <= usableIndex; k++) {
				   objects[k].moveX();
				   objects[k].moveY();
				   objects[k].updateCenter();
				   if (objects[k].getX() <= 0 || objects[k].getX() >= WIDTH-objects[k].getImageSize()){
					   objects[k].changeXDirection();
					   objects[k].moveX();
				   }
				   if (objects[k].getY() <= 0 || objects[k].getY() >= HEIGHT-objects[k].getImageSize()){
					   objects[k].changeYDirection();
					   objects[k].moveY();
				   } 
			   }
			   checkCollision();
			   repaint();
		   }