MainActivity.java -Fragment

public class MainActivity extends AppCompatActivity {
implements BottomNavigationView.OnNavigationItemSelectedListener
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // kita set default nya Home Fragment
        loadFragment(new HomeFragment());
        // inisialisasi BottomNavigaionView
        BottomNavigationView bottomNavigationView = findViewById(R.id.bn_main);
        // beri listener pada saat item/menu bottomnavigation terpilih
        bottomNavigationView.setOnNavigationItemSelectedListener(this);

   }

// method untuk load fragment yang sesuai
private boolean loadFragment(Fragment fragment) {
   if (fragment != null) {
      getSupportFragmentManager().beginTransaction()
         .replace(R.id.fl_container, fragment)
         .commit();
      return true;
   }
   return false;
}

// method listener untuk logika pemilihan
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem item) {
   Fragment fragment = null;
   switch (item.getItemId()){
      case R.id.home_menu:
         fragment = new HomeFragment();
         break;
      case R.id.search_menu:
         fragment = new SearchFragment();
         break;
      case R.id.favorite_menu:
         fragment = new FavoriteFragment();
         break;
      case R.id.account_menu:
         fragment = new AccountFragment();
         break;
   }
   return loadFragment(fragment);
}
}