STEP 1:
BASE ACTIVITY
    private BottomNavigationView.OnNavigationItemSelectedListener navigationItemSelectedListener = new BottomNavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            baseViewModel.navButtonPressed(item.getItemId());
            return true;
        }
    };

////////////////////////////////////
STEP 2:

BASE VIEWMODEL

public void navButtonPressed(int itemId) {
        switch (itemId) {
            case R.id.bottomNav_discover:
                getNavEvent().setValue(Constants.OPENED_DISCOVER);
                break;
            case R.id.bottomNav_community:
                getNavEvent().setValue(Constants.OPENED_COMMUNITY);
                break;
            case R.id.bottomNav_library:
                getNavEvent().setValue(Constants.OPENED_LIBRARY);
                break;
            case R.id.bottomNav_personal:
                getNavEvent().setValue(Constants.OPENED_PERSONAL);
                break;
        }
    }

////////////////////////////////////
STEP 3:
BASE ACTIVITY:

@Override
    protected void onCreate(Bundle savedInstanceState) {

        final Observer<Integer> navEventObserver = new Observer<Integer>() {
            @Override
            public void onChanged(Integer integer) {
                switch (integer) {
                    case Constants.OPENED_DISCOVER:
                        getSupportFragmentManager().beginTransaction().replace(R.id.base_fragment_container, new DiscoverFragment()).commit();
                        break;
                    case Constants.OPENED_COMMUNITY:
                        getSupportFragmentManager().beginTransaction().replace(R.id.base_fragment_container, new CommunityFragment()).commit();
                        break;
                    case Constants.OPENED_LIBRARY:
                        getSupportFragmentManager().beginTransaction().replace(R.id.base_fragment_container, new LibraryFragment()).commit();
                        break;
                    case Constants.OPENED_PERSONAL:
                        getSupportFragmentManager().beginTransaction().replace(R.id.base_fragment_container, new PersonalFragment()).commit();
                        break;
                   }
            }
        };

        baseViewModel.getNavEvent().observe(this, navEventObserver);
    }