Android 再按一次退出程序

private long mExitTime = 0;

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK  && event.getAction() == KeyEvent.ACTION_DOWN) {
        if ((System.currentTimeMillis() - mExitTime) > 2000) {
            CommonUtil.showToast("再按一次退出程序", MainActivity.this);
            mExitTime = System.currentTimeMillis();
        } else {
            finish();
            System.exit(0);
        }
        return true;
    }
    return super.onKeyDown(keyCode, event);
}