Android - prevent to open previous fragment

Question

guys. This must be a silly question but i'm not managing to work this out. My scenario is: i have in my MainActivity a BottomNavigation which i navigate over three fragments. And my problem is that when the back button (from android bottom navigation toolbar) is pressed the previous fragment opens but i want the app to close. So my question is: how i manage to prevent the previous fragments to open?

PS: I know it has something to do with FragmentMananger back stack but i did not understand how to use it.

PS2: Sorry for bad english.

Answer 1

The fragments are on the backstack.

Edit:

Work with FragmentTransaction and use addToBackStack (null)

//  Create new fragment and transaction
    Fragment newFragment = new  ExampleFragment();
    FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack
    transaction.replace(R.id.fragment_container, newFragment);
    transaction.addToBackStack(null);

// Commit the transaction
    transaction.commit();