@Override    public boolean onKeyDown(int keyCode, KeyEvent event) {        // TODO Auto-generated method stub        if(keyCode == KeyEvent.KEYCODE_BACK)        {            ExitClick();        }        return false;    }    private static Boolean isExit = false;            private void ExitClick() {          Timer tExit = null;          if (isExit == false) {              isExit = true; // 准备退出              Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();              tExit = new Timer();              tExit.schedule(new TimerTask() {                  @Override                  public void run() {                      isExit = false; // 取消退出                  }              }, 2000); // 如果2秒钟内没有按下返回键，则启动定时器取消掉刚才执行的任务                } else {              finish();              System.exit(0);          }      }