<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  android:id="@+id/container"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical">
  // 容器，承載fragment
  <FrameLayout
    android:id="@+id/nav_host_fragment"
    android:name="androidx.navigation.fragment.NavHostFragment"
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_weight="1" />
  // BottomNavigationView
  <com.google.android.material.bottomnavigation.BottomNavigationView
    android:id="@+id/nav_view"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="?android:attr/windowBackground"
    app:menu="@menu/bottom_nav_menu_test" />
</LinearLayout>

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
  <item
    android:id="@+id/navigation_test1"
    android:icon="@drawable/ic_home_black_24dp"
    android:title="測(cè)試一" />
  <item
    android:id="@+id/navigation_test2"
    android:icon="@drawable/ic_dashboard_black_24dp"
    android:title="測(cè)試二" />
  <item
    android:id="@+id/navigation_test3"
    android:icon="@drawable/ic_notifications_black_24dp"
    android:title="測(cè)試三" />
</menu>

public class TestActivity extends AppCompatActivity {
  List<Fragment> mFragments = new ArrayList<>();
  test1Fragment t1f = new test1Fragment();
  test2Fragment t2f = new test2Fragment();
  test3Fragment t3f = new test3Fragment();
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_test);
    BottomNavigationView navView = findViewById(R.id.nav_view);
    mFragments.add(t1f);
    mFragments.add(t2f);
    mFragments.add(t3f);
    // navView 點(diǎn)擊事件
    navView.setOnNavigationItemSelectedListener((item)->{
      switchFragment(item.getItemId());
      return true;
    });
  }
  private void switchFragment(int id) {
    Fragment fragment = null;
    switch (id) {
      case R.id.navigation_test1:
        fragment = mFragments.get(0);
        break;
      case R.id.navigation_test2:
        fragment = mFragments.get(1);
        break;
      case R.id.navigation_test3:
        fragment = mFragments.get(2);
        break;
      default:
        break;
    }
    if (fragment != null) {
      getSupportFragmentManager().beginTransaction().replace(R.id.nav_host_fragment,fragment).commit();
    }
  }
}