public void undoChangeBallType() {
   if (undoBallType != null && undo) {
     LevelScreen screen = levelScreenManager.getScreen();
     BallShooter shooter = screen.getShooter();
     shooter.changeBallType(undoBallType);
     undo = false;
   }
 }
 public void changeBallType(Ball.BallType type) {
   if (gameManager.hasItem(type)) {
     LevelScreen screen = levelScreenManager.getScreen();
     BallShooter shooter = screen.getShooter();
     shooter.changeBallType(type);
     undo = true;
   }
 }