MainActivity

package com.example.moviecatalogue;

import android.content.Context;
import android.support.annotation.NonNull;
import android.support.design.widget.BottomNavigationView;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.MenuItem;

import com.example.moviecatalogue.View.MovieFragment;
import com.example.moviecatalogue.View.SettingFragment;
import com.example.moviecatalogue.View.TvShowFragment;

import static com.example.moviecatalogue.ViewModel.MainViewModel.KEY_MOVIES;


public class MainActivity extends AppCompatActivity {

    private Fragment pageContent = new MovieFragment();
    private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
        = new BottomNavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
            switch (menuItem.getItemId()) {
                case R.id.movie_nav:
                    //Fragment Initialization
                    MovieFragment movieFragment = new MovieFragment();
                    //Fragment Transaction
                    FragmentTransaction fragmentMovieTransaction = getSupportFragmentManager().beginTransaction();
                    fragmentMovieTransaction.replace(R.id.fragment_container, movieFragment, movieFragment.getClass().getSimpleName());
                    fragmentMovieTransaction.commit();
                    return true;
                case R.id.tvshow_nav:
                    //Fragment Initialization
                    TvShowFragment tvShowFragment = new TvShowFragment();
                    //Fragment Transaction
                    FragmentTransaction fragmentTvShowTransaction = getSupportFragmentManager().beginTransaction();
                    fragmentTvShowTransaction.replace(R.id.fragment_container, tvShowFragment, tvShowFragment.getClass().getSimpleName());
                    fragmentTvShowTransaction.commit();
                    return true;
                case R.id.setting_nav:
                    //Fragment Initialization
                    SettingFragment settingFragment = new SettingFragment();
                    //Fragment Transaction
                    FragmentTransaction fragmentsettingTransaction = getSupportFragmentManager().beginTransaction();
                    fragmentsettingTransaction.replace(R.id.fragment_container, settingFragment);
                    fragmentsettingTransaction.commit();
                    return true;
            }
            return false;


        }

    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        MovieFragment movieFragment = new MovieFragment();

        FragmentTransaction fragmentMvTransaction = getSupportFragmentManager().beginTransaction();
        fragmentMvTransaction.replace(R.id.fragment_container, movieFragment, movieFragment.getClass().getSimpleName());
        fragmentMvTransaction.commit();

        BottomNavigationView bottomNav = findViewById(R.id.nav_view);
        bottomNav.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);

        if (savedInstanceState == null) {
            bottomNav.setSelectedItemId(R.id.movie_nav);
        } else {
            pageContent = getSupportFragmentManager().getFragment(savedInstanceState, KEY_MOVIES);

            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, pageContent).commit();
        }
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        if (pageContent.isAdded()){
            getSupportFragmentManager().putFragment(outState, KEY_MOVIES, pageContent);
        }
    }
}