MainFragment.java

public final class MainFragment extends Fragment {

    public MainFragment() {
        super(R.layout.fragment_main);
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        BottomNavigationView navigation = requireViewById(view, R.id.navigation);
        navigation.setSelectedItemId(R.id.navigation_browse);
        navigation.setOnNavigationItemSelectedListener(this::onNavigationItemSelected);

        FragmentManager childFragmentManager = getChildFragmentManager();
        if (childFragmentManager.findFragmentById(R.id.fragmentContainer) == null) {
            BrowseFragment initialFragment = new BrowseFragment();
            String fragmentTag = BrowseFragment.class.getSimpleName();
            childFragmentManager.beginTransaction()
                    .add(R.id.fragmentContainer, initialFragment, fragmentTag)
                    .setPrimaryNavigationFragment(initialFragment)
                    .commit();
        }
    }

    private boolean onNavigationItemSelected(@NonNull MenuItem item) {
        FragmentManager fragmentManager = getChildFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

        // detach previous fragment if any
        Fragment detachingFragment = fragmentManager.getPrimaryNavigationFragment();
        if (detachingFragment != null) {
            fragmentTransaction.detach(detachingFragment);
        }

        int itemId = item.getItemId();
        String fragmentTag = getFragmentTag(itemId);
        Fragment attachingFragment = fragmentManager.findFragmentByTag(fragmentTag);
        if (attachingFragment == null) {
            // add fragment if it has not been added
            attachingFragment = getFragment(itemId);
            fragmentTransaction.add(R.id.fragmentContainer, attachingFragment, fragmentTag);
        } else {
            // attach fragment if it has already been added
            fragmentTransaction.attach(attachingFragment);
        }
        fragmentTransaction.setPrimaryNavigationFragment(attachingFragment);

        fragmentTransaction.setReorderingAllowed(true);
        fragmentTransaction.commit();
        return true;
    }

    private String getFragmentTag(@IdRes int itemId) {
        switch (itemId) {
            case R.id.navigation_downloads:
                return DownloadsFragment.class.getSimpleName();
            case R.id.navigation_browse:
                return BrowseFragment.class.getSimpleName();
            case R.id.navigation_queue:
                return QueueFragment.class.getSimpleName();
            default:
                throw new IllegalArgumentException("No corresponding fragment for given itemId");
        }
    }

    private Fragment getFragment(@IdRes int itemId) {
        switch (itemId) {
            case R.id.navigation_downloads:
                return new DownloadsFragment();
            case R.id.navigation_browse:
                return new BrowseFragment();
            case R.id.navigation_queue:
                return new QueueFragment();
            default:
                throw new IllegalArgumentException("No corresponding fragment for given itemId");
        }
    }
}