一个ROBOT模拟按键问题

      GraphicsEnvironment environment = GraphicsEnvironment.getLocalGraphicsEnvironment();
                       GraphicsDevice screen = environment.getDefaultScreenDevice();                       try
                       {
                           Robot robot = new Robot(screen);
   robot.keyPress(KeyEvent.VK_ALT);
   robot.keyPress(KeyEvent.VK_F4);
   robot.keyRelease(KeyEvent.VK_ALT);
   robot.keyRelease(KeyEvent.VK_F4);
   robot.delay(1000);
                       }
                       catch (AWTException ex)
                       {
                           ex.printStackTrace();
                       }请教：这段代码为什么不能模拟窗口关闭？也不是焦点的问题。