Android 再按一次退出程序

private long exitTime = 0;@Overridepublic boolean onKeyDown(int keyCode, KeyEvent event) {    if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN) {        if ((System.currentTimeMillis() - exitTime) > 3000) {            Toast.makeText(getApplicationContext(), "再按一次后退键退出应用程序", Toast.LENGTH_SHORT).show();            exitTime = System.currentTimeMillis();        } else {            finish();            System.exit(0);        }        return true;    }    return super.onKeyDown(keyCode, event);}