@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);  
        }  
    }