兩次退出程序

@Override
    public boolean onKeyUp(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            long secondTime = System.currentTimeMillis();
            if (secondTime - firstTime > 800) {//如果兩次按鍵時間間隔大於800毫秒，則不退出
                Toast.makeText(MainActivity.this, "再按一次退出程序...",
                        Toast.LENGTH_SHORT).show();
                firstTime = secondTime;//更新firstTime
                return true;
            } else {
                System.exit(0);//否則退出程序
            }
        }
        return super.onKeyUp(keyCode, event);
    }