How can go to the previous fragment in backstack?

Question

I have a MainActivity in the onCreate method i add a fragment like this :

        getSupportFragmentManager().beginTransaction()
                .replace(R.id.content_frame, new FragmentA(), FRAGMENT_TAG).addToBackStack("main")
                .commit();

In FragmentA there is a lisview and when user clicked on an item i remove FragmentA and add FragmentB (In FragmentA there is a FragmentPagerAdapter and in FragmentPagerAdapter is a ListFragment in ListFragment i called below code )

 listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
     getActivity().getSupportFragmentManager().beginTransaction()
                        .replace(R.id.content_frame, new FragmentB(), MainActivity.FRAGMENT_TAG).addToBackStack("main")
                        .commit();
};

Now in FragmentB there is button and whenever user clicked on it i want to go back to the FragmentA . I try to call popBackStack() in button listener but nothing happen

 FragmentManager fragmentManager = getActivity().getSupportFragmentManager();
 fragmentManager.popBackStack();

Or i also try this below code but it gives me nullpointer exception

   FragmentManager fragmentManager = getActivity().getSupportFragmentManager();
        String fragmentTag = fragmentManager.getBackStackEntryAt(fragmentManager.getBackStackEntryCount() - 1).getName();
        Fragment currentFragment =getActivity().getSupportFragmentManager()
                .findFragmentByTag(fragmentTag);
        fragmentManager.beginTransaction().replace(R.id.content_frame,currentFragment).commit();
        fragmentManager.popBackStack();

Answer 1

如果要转到BackStack的上一个Fragment ，则只需执行onBackPressed() 。