public function onKeyboardEvent(event:KeyboardEvent):void {
     switch(event.keyCode) {
      case Keyboard.UP :
      ball.y -= 10;
      break;
       
      case Keyboard.DOWN :
      ball.y += 10;
      break;
      
      case Keyboard.LEFT :
      ball.x -= 10;
      break;
      
      case Keyboard.RIGHT :
      ball.x += 10;
      break;
      
      default:
      break;
     }
    }
   }
  }