首先新增一個Fragment，名稱為BlankFragment

再新增一個Package，名稱為menuFragment

然後在menuFragment底下，新增三個HomepageFragment、CharacterFragment、PlotFragment
再到drawable新增三張.png的圖片，檔名分別為cat、pikachu、plot

修改menuFragment底下三個Fragment的layout，這裡示範其中一個(其他兩個改掉background裡的圖片就好)

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".menuFragment.CharacterFragment">

    <!-- TODO: Update blank fragment layout -->
    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@drawable/pikachu"/>

</FrameLayout>

MainActivity:
使用BottomNavigationView控制Fragment

public class MainActivity extends AppCompatActivity {

    final FragmentManager fragmentManager = getSupportFragmentManager();

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

        BottomNavigationView bottomNavigationView = findViewById(R.id.nav);

        // 取消元件自帶的動畫
        bottomNavigationView.getMenu().setGroupCheckable(0, false, false);

        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        BlankFragment fragment = new BlankFragment();
        fragmentTransaction.add(R.id.frag, fragment);
        fragmentTransaction.commit();

        bottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                switch (item.getItemId()) {
                    case R.id.homePage:
                        FragmentTransaction homePageTransaction = fragmentManager.beginTransaction();
                        homePageTransaction.replace(R.id.frag, new HomepageFragment());
                        homePageTransaction.commit();
                        break;
                    case R.id.character:
                        FragmentTransaction characterTransaction = fragmentManager.beginTransaction();
                        characterTransaction.replace(R.id.frag, new CharacterFragment());
                        characterTransaction.commit();
                        break;
                    case R.id.plot:
                        FragmentTransaction plotTransaction = fragmentManager.beginTransaction();
                        plotTransaction.replace(R.id.frag, new PlotFragment());
                        plotTransaction.commit();
                        break;
                }
                return true;
            }
        });
    }
}

謝謝大家願意花時間閱讀，小弟弟我在此鞠躬