My problem it's that I created a default Bottom Navigation Activity and I want to call a fragment method from MainActivity. The problem is that I 'm not able to take a reference of the fragment. I tried all type of solution here in stackOverFlow.

a lot of method is deprecated.

//MAIN ACTIVITY

 public class MainActivity extends AppCompatActivity  {

    private ActivityMainBinding binding;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());

        BottomNavigationView navView = findViewById(R.id.nav_view);
        // Passing each menu ID as a set of Ids because each
        // menu should be considered as top level destinations.
        AppBarConfiguration appBarConfiguration = new AppBarConfiguration.Builder(
                R.id.navigation_home, R.id.navigation_dashboard, R.id.navigation_notifications)
                .build();
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment_activity_main);
        NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration);
        NavigationUI.setupWithNavController(binding.navView, navController);

        FragmentManager fm = getSupportFragmentManager();
        HomeFragment fragment = (HomeFragment) fm.findFragmentById(R.id.navigation_home);
        fragment.sayHi();
    }

//FRAGMENT

public class HomeFragment extends Fragment {

    private HomeViewModel homeViewModel;
    private FragmentHomeBinding binding;

    public View onCreateView(@NonNull LayoutInflater inflater,
                             ViewGroup container, Bundle savedInstanceState) {
        homeViewModel =
                new ViewModelProvider(this).get(HomeViewModel.class);

        binding = FragmentHomeBinding.inflate(inflater, container, false);
        View root = binding.getRoot();

        final TextView textView = binding.textHome;
        homeViewModel.getText().observe(getViewLifecycleOwner(), new Observer<String>() {
            @Override
            public void onChanged(@Nullable String s) {
                textView.setText(s);
            }
        });
        return root;
    }
    public void sayHi(){
        Log.d("hi", "hi");
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        binding = null;
    }
}