当活动重新启动时，如何恢复活动的片段？

public class MainActivity extends AppCompatActivity {
    private static final String KEY_FRAGMENT_TAG = "fragment_tag";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        if (savedInstanceState == null) {
            // 第一次创建活动时，添加片段
            Fragment fragment = new MyFragment();
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.fragment_container, fragment, KEY_FRAGMENT_TAG)
                    .commit();
        } else {
            // 恢复之前保存的片段
            Fragment fragment = getSupportFragmentManager().findFragmentByTag(KEY_FRAGMENT_TAG);
            getSupportFragmentManager().beginTransaction()
                    .replace(R.id.fragment_container, fragment, KEY_FRAGMENT_TAG)
                    .commit();
        }
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        // 保存活动的状态信息
        // 例如：outState.putString("key", value);
    }

    @Override
    protected void onRestoreInstanceState(Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
        // 恢复之前保存的状态信息
        // 例如：String value = savedInstanceState.getString("key");
    }
}