實作 Bottom Navigation View
===

public class MainActivity extends AppCompatActivity {

    private BottomNavigationView navView;
    private NavigationBarView.OnItemSelectedListener navListenr = new NavigationBarView.OnItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {

            item.setChecked(true);
            Fragment fragment = null;

            switch (item.getItemId()) {
                case R.id.navigation_home:
                    fragment = new HomeFragment();
                    switchFragment(fragment);
                    setTitle(R.string.title_home);
                    break;

                case R.id.navigation_dashboard:
                    fragment = new DashboardFragment();
                    switchFragment(fragment);
                    setTitle(R.string.title_dashboard);
                    break;

                case R.id.navigation_notifications:
                    fragment = new NotificationsFragment();
                    switchFragment(fragment);
                    setTitle(R.string.title_notifications);
                    break;
            }

            return false;
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        initBottomNav();

        initBody();
    }

    private void initBottomNav() {
        navView = findViewById(R.id.nav_view);

        navView.setOnItemSelectedListener(navListenr);
    }

    private void initBody() {
        MenuItem homeItem = navView.getMenu().findItem(R.id.navigation_home);
        navListenr.onNavigationItemSelected(homeItem);
    }

    private void switchFragment(Fragment fragment) {
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.replace(R.id.framelay, fragment);
        fragmentTransaction.commit();
    }

}

## Ref.
傳統方法（需自定監聽器切換畫面）
[How to implement BottomNavigationView with Fragments in Android Studio](https://www.youtube.com/watch?v=UMZZHHJ37bo)

傳統方法（不需自定監聽器）
[BottomNavigationView with Navigation Component - Android Studio Tutorial](https://www.youtube.com/watch?v=Chso6xrJ6aU)

###### tags: `Bottom Navigation View`