Method to add fragment to activity. It will take care of already registered fragment, and poping fragment if they are in back stack

MainActivity

    private void changeFragment(Fragment frag, boolean saveInBackstack) {
        String backStateName = ((Object) frag).getClass().getName();

        try {
            FragmentManager manager = getSupportFragmentManager();
            boolean fragmentPopped = manager.popBackStackImmediate(backStateName, 0);

            if (!fragmentPopped && manager.findFragmentByTag(backStateName) == null) { //fragment not in back stack, create it.
                FragmentTransaction transaction = manager.beginTransaction();
                transaction.replace(R.id.fragment_container, frag, ((Object) frag).getClass().getName());

                if (saveInBackstack) {
                    Log.d(LOG_TAG, "Change Fragment : addToBackTack");
                    transaction.addToBackStack(backStateName);
                } else {
                    Log.d(LOG_TAG, "Change Fragment : NO addToBackTack");
                }

                transaction.commit();
            } else {
                Log.d(LOG_TAG, "Change Fragment : nothing to do");
                // custom effect if fragment is already instanciated
            }
        } catch (IllegalStateException exception) {
            Log.e(LOG_TAG, "Unable to commit fragment, could be activity as been killed in background. " + exception.toString());
        }
    }