Show Back Arrow in Fragment

Question

I want to show Back Arrow in Fragments ToolBar. Im trying to write this code: ((AppCompatActivity) getActivity()).getSupportActionBar().setDisplayHomeAsUpEnabled(true);

but AS writes: method invokation .. may produce "java.lang.NullPointerException". What about getSupportActionBar(), AS writes cannot resolve method. Whats wrong? Here is my code:

public class AddFilterFrag extends android.app.Fragment  {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);       
        setHasOptionsMenu(true);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View rootView = inflater.inflate(R.layout.add_filter_layout, container, false);
        ((AppCompatActivity) getActivity()).getSupportActionBar().setDisplayHomeAsUpEnabled(true);

        return rootView;
    }
 @Override
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        super.onCreateOptionsMenu(menu, inflater);

       inflater.inflate(R.menu.fragment_menu, menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        if (item.getItemId() == android.R.id.home) {
            //something to do
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

Answer 1

This is just a warning you can safely ignore.

Background:

The Android sdk has Annotations to help developers avoid common mistakes. One of those annotations is @Nullable. The method getSupportActionBar() is annotated as such, as it is possible that null is returned from that method. One case might be, that the developer used the theme Theme.AppCompat.Light.NoActionBar thus disabling the ActionBar.

If you have an ActionBar or a Toolbar in your layout (and set it properly) that method will never return null. You can ignore the warning like this:

//noinspection ConstantConditions  
((AppCompatActivity) getActivity()).getSupportActionBar().setDisplayHomeAsUpEnabled(true);

Or if you want to be absolutely safe:

ActionBar ab = ((AppCompatActivity) getActivity()).getSupportActionBar();  

if(ab != null){  
    ab.setDisplayHomeAsUpEnabled(true);  
}  

Alternatively in Kotlin:

(activity as? AppCompatActivity)?.supportActionBar?.setDisplayHomeAsUpEnabled(true)

Answer 2

1. Frist add method to your activity that call fragment.

   public void changeToolbar(){ //Do your job }

2. call it in fragment

   ((YourActivity)getActivity).changeToolbar();

Answer 3

while using navigation graphs use navController to do that

On the activity thats holding the host fragment do this

lateinit var navController : NavController

In oncreateView

navController = this.findNavController(R.id.nav_host_fragment)
        NavigationUI.setupActionBarWithNavController(this, navController)

override onSupportNavigateUp

override fun onSupportNavigateUp(): Boolean {
        return navController.navigateUp()
    }

that handles the up arrow by itself.