Android连续点击两次返回键退出App

private long firstTime = 0;

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    long secondTime = System.currentTimeMillis();

    if (keyCode == KeyEvent.KEYCODE_BACK) {
        if (secondTime - firstTime < 2000) {
            System.exit(0);
        } else {
            Toast.makeText(getApplicationContext(), "再按一次返回键退出", Toast.LENGTH_SHORT).show();
            firstTime = System.currentTimeMillis();
        }

        return true;
    }

    return super.onKeyDown(keyCode, event);
}