Show back button in Toolbar from Fragment in Android App

back_button_toolbar_fragment

# In this situation I have the MainActivity that calls FragmentA. This fragment implements OnBackStackChangedListener, 
# and checks the size of the backStack; if it's less than one, then it hides the UP button. The fragments B and C are called by
# the A, them just show/hide the button.

# MainActivity:

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case android.R.id.home:
                getSupportFragmentManager().popBackStack();
                return true;
        }
        return super.onOptionsItemSelected(item);
    }

    public void showUpButton() { getSupportActionBar().setDisplayHomeAsUpEnabled(true); }
    public void hideUpButton() { getSupportActionBar().setDisplayHomeAsUpEnabled(false); }
    
# Fragment A:
    
    public class FragmentA extends Fragment implements FragmentManager.OnBackStackChangedListener {
    
        @Override
        public void onCreate(@Nullable Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            // listen to backstack changes
            getActivity().getSupportFragmentManager().addOnBackStackChangedListener(this);
        }

        public void onBackStackChanged() {
            if(getActivity() != null) {
                // enable Up button only if there are entries on the backstack
                if(getActivity().getSupportFragmentManager().getBackStackEntryCount() < 1) {
                    ((MainActivity)getActivity()).hideUpButton();
                }
            }
        }

        ...
    }
    
# Fragments B and C:

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // displays the back button on toolbar
        ((MainActivity)getActivity()).showUpButton();
    }
    
    

can you please give more context.