以下是mainActivity的代码

public class MainActivity extends AppCompatActivity {

    @BindView(R.id.bottomNavView)
    public BottomNavigationView bottomNavigationView;
    public static final String TAG = "taobao";
    private HomeFragment homeFragment;
    private ChoicenessFragment choicenessFragment;
    private RedPacketFragment redPacketFragmentFragment;
    SearchFragment searchFragment;
    private FragmentManager fm;

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

        initFragment();
        initListener(); 

    }

    private void initFragment() {
        homeFragment = new HomeFragment();
        choicenessFragment = new ChoicenessFragment();
        redPacketFragmentFragment = new RedPacketFragment();
        searchFragment = new SearchFragment();
        fm = getSupportFragmentManager();
//        switchFragment(homeFragment);
    }

    private void initListener() {
        //设置监听
        bottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                Log.d(TAG, "title -- >" + item.getTitle());
                int itemId = item.getItemId();
                if (itemId == R.id.home_item_bottom_nav) {
//                    Log.d(TAG,"提示 == > 切换到主页");
                    LogUtils.d(MainActivity.class, "提示 == > 切换到主页");
                    switchFragment(homeFragment);
                } else if (itemId == R.id.choiceness_item_bottom_nav) {
                    switchFragment(choicenessFragment);
                    Log.d(TAG, "提示 == > 切换到精选");
                } else if (itemId == R.id.redpacket_item_bottom_nav) {
                    switchFragment(redPacketFragmentFragment);
                    Log.d(TAG, "提示 == > 切换到特惠");
                } else if (itemId == R.id.search_item_bottom_nav) {
                    switchFragment(searchFragment);
                    Log.d(TAG, "提示 == > 切换到搜索");
                }
                return true;

            }
        });

    }

    private void switchFragment(BaseFragment tagetFragment) {
        FragmentTransaction transaction = fm.beginTransaction();
        transaction.replace(R.id.framelayout,tagetFragment);
        transaction.commit();
    }
}

我发现在执行initFragment()就会加载出主页这个字样在fragment上

我的主页面布局文件

<?xml version="1.0" encoding="utf-8"?>
<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=".ui.activity.MainActivity">

    <FrameLayout
        android:id="@+id/framelayout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintBottom_toBottomOf="@+id/bottomNavView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

    </FrameLayout>

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bottomNavView"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:background="@drawable/shape_round_white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:menu="@menu/menu" />

    <fragment
        android:id="@+id/fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toTopOf="@+id/bottomNavView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/bottom_nav" />
</androidx.constraintlayout.widget.ConstraintLayout>

只要在home_fragment 中对应的布局文件设计布局，都会影响到每一个fragment。

以下是我切换fragment出现的log