如何将Android Studio现有的两个活动转换为两个片段，以便创建导航栏？

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

Fragment1 fragment1 = new Fragment1();
Fragment2 fragment2 = new Fragment2();

FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
fragmentTransaction.add(R.id.fragment_container, fragment1);
fragmentTransaction.commit();

<com.google.android.material.bottomnavigation.BottomNavigationView
    android:id="@+id/bottom_navigation"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_gravity="bottom"
    app:menu="@menu/bottom_navigation_menu" />

BottomNavigationView bottomNavigationView = findViewById(R.id.bottom_navigation);
bottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem item) {
        Fragment selectedFragment = null;

        switch (item.getItemId()) {
            case R.id.fragment1:
                selectedFragment = new Fragment1();
                break;
            case R.id.fragment2:
                selectedFragment = new Fragment2();
                break;
        }

        getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, selectedFragment).commit();
        return true;
    }
});