无法将片段对象放入MainActivity : android

// MainActivity.java

import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentTransaction;

public class MainActivity extends AppCompatActivity {

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

        // 获取 FragmentManager 实例
        FragmentManager fragmentManager = getSupportFragmentManager();

        // 开始一个事务
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

        // 创建一个片段对象
        MyFragment myFragment = new MyFragment();

        // 将片段对象添加到容器视图中
        fragmentTransaction.add(R.id.fragment_container, myFragment);

        // 提交事务
        fragmentTransaction.commit();
    }
}

<!-- activity_main.xml -->

<FrameLayout
    android:id="@+id/fragment_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />