groovy implementation 'androidx.appcompat:appcompat:1.3.0' implementation 'androidx.fragment:fragment:1.4.0' <androidx.fragment.app.FragmentContainerView android:id="@+id/fragment_container" android:layout_width="match_parent" android:layout_height="match_parent" app:layout_behavior="@string/appbar_scrolling_view_behavior" /> FragmentContainerView fragmentContainer = findViewById(R.id.fragment_container); FragmentManager fragmentManager = getSupportFragmentManager(); FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction(); MyFragment myFragment = new MyFragment(); fragmentTransaction.add(R.id.fragment_container, myFragment); fragmentTransaction.commit(); FragmentManager fragmentManager = getSupportFragmentManager(); FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction(); MyFragment myFragment = new MyFragment(); fragmentTransaction.add(R.id.fragment_container, myFragment); fragmentTransaction.addToBackStack(null); fragmentTransaction.commit(); FragmentManager fragmentManager = getSupportFragmentManager(); FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction(); MyFragment myFragment = new MyFragment(); fragmentTransaction.replace(R.id.fragment_container, myFragment); fragmentTransaction.addToBackStack(null); fragmentTransaction.commit(); MyActivity myActivity = (MyActivity) getActivity(); MyFragment myFragment = (MyFragment) getSupportFragmentManager().findFragmentById(R.id.fragment_container); myFragment.myPublicMethod();