如何从activity中调用片段中的字符串？

public class MyFragment extends Fragment {
    private Activity mActivity;

    @Override
    public void onAttach(@NonNull Context context) {
        super.onAttach(context);
        mActivity = (Activity) context;
    }

    public String getFragmentString() {
        return "这是片段中的字符串";
    }
}

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        MyFragment fragment = (MyFragment) getSupportFragmentManager().findFragmentById(R.id.fragment_container);
        String fragmentString = fragment.getFragmentString();

        // 使用获取到的字符串进行后续操作
        // ...
    }
}