android双击返回键退出程序

@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);
        }
    }