如果主活动中存在片段，则在何处更新UI元素

public class MyFragment extends Fragment {
    private TextView textView;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_layout, container, false);
        textView = view.findViewById(R.id.text_view);
        return view;
    }

    public void updateUI(String newText) {
        textView.setText(newText);
    }
}

public class MainActivity extends AppCompatActivity {
    private MyFragment myFragment;

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

        myFragment = new MyFragment();
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.add(R.id.fragment_container, myFragment);
        fragmentTransaction.commit();
    }

    public void updateFragmentUI(String newText) {
        myFragment.updateUI(newText);
    }
}

public class MainActivity extends AppCompatActivity {
    // ...

    public void onClickButton(View view) {
        updateFragmentUI("New Text");
    }
}