因此，我试图建立一个应用程序与底部导航栏，将切换之间的片段。我做的编码根据一些文档。
我尝试了互联网上的每一个解决方案，但似乎没有一个适合我。
错误：

Caused by: java.lang.ClassCastException: com.example.jetpacklesson.MainFragment cannot be cast to androidx.navigation.fragment.NavHostFragment
        at com.example.jetpacklesson.MainActivity.onCreate(MainActivity.java:30)

下面是XML主活动代码

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/firstFragmentHolder"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:name="com.example.jetpacklesson.MainFragment"
        app:defaultNavHost = "true"
        app:layout_constraintBottom_toTopOf="@+id/bnv_bottomBar"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph = "@navigation/nav_graph"/>

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bnv_bottomBar"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:menu="@menu/nav_bottom_navigation" />
</androidx.constraintlayout.widget.ConstraintLayout>

和主要活动Java代码：

BottomNavigationView bnv_navigationBar;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        NavHostFragment navHostFragment = (NavHostFragment) getSupportFragmentManager().findFragmentById(R.id.firstFragmentHolder);
        NavController navController = navHostFragment.getNavController();
        bnv_navigationBar = findViewById(R.id.bnv_bottomBar);
        NavigationUI.setupWithNavController(bnv_navigationBar, navController);
    }