点击两次返回键退出程序

private Long firstTime = Long.valueOf(0);@Override    public boolean onKeyUp(int keyCode, KeyEvent event) {        if (keyCode == KeyEvent.KEYCODE_BACK)        {            long secondTime = System.currentTimeMillis();            if (secondTime - firstTime > 800)            {//如果两次按键时间间隔大于800毫秒，则不退出                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);//否则退出程序            }        }        return super.onKeyUp(keyCode, event);    }