I have a question about Android viewpager.

public class MainActivity extends AppCompatActivity {
MovieDetailFragment movieDetailFragment;
MovieInfoFragment movieInfoFragment;

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



    movieDetailFragment = new MovieDetailFragment();
    movieInfoFragment = new MovieInfoFragment();

    getSupportFragmentManager().beginTransaction().add(R.id.container, movieInfoFragment).commit();
}

public void replaceMovieInfoFragment(){
    getSupportFragmentManager().beginTransaction().replace(R.id.container, movieDetailFragment).addToBackStack(null).commit();
}
}

public class MovieInfoFragment extends Fragment{

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    ViewGroup rootView = (ViewGroup) inflater.inflate(R.layout.movie_info_fragment, container, false);

    ViewPager viewPager = rootView.findViewById(R.id.pager);
    viewPager.setOffscreenPageLimit(5);
    ViewPagerAdapter adapter = new ViewPagerAdapter(getFragmentManager());
    ViewPager1 viewPager1 = new ViewPager1();
    adapter.addItems(viewPager1);
    ViewPager2 viewPager2 = new ViewPager2();
    adapter.addItems(viewPager2);
    ViewPager3 viewPager3 = new ViewPager3();
    adapter.addItems(viewPager3);
    ViewPager4 viewPager4 = new ViewPager4();
    adapter.addItems(viewPager4);
    ViewPager5 viewPager5 = new ViewPager5();
    adapter.addItems(viewPager5);

    viewPager.setAdapter(adapter);



    return rootView;
}

public class ViewPagerAdapter extends FragmentStatePagerAdapter {
    ArrayList<Fragment> items = new ArrayList<Fragment>();


    public void addItems(Fragment fragment) {
        items.add(fragment);
    }

    public ViewPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        return items.get(position);
    }

    @Override
    public int getCount() {
        return items.size();
    }
}

public class ViewPager1 extends Fragment{

@Override
public void onDetach() {
    super.onDetach();
}

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    ViewGroup rootView = (ViewGroup) inflater.inflate(R.layout.view_pager1, container, false);
    Button button = rootView.findViewById(R.id.button2);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            ((MainActivity)getActivity()).replaceMovieInfoFragment();
        }
    });

    return rootView;
}
}