Why FragmentManager's getBackStackEntryCount() return zero?

private static void changeFragment(Fragment f, boolean init) {
        FragmentTransaction ft = fm.beginTransaction();
        ft.replace(R.id.info_content, f,f.getClass().getName());
        if(!init){
            ft.addToBackStack(null);
        }
        ft.commit();
    }

when I want to get the stack cout by call fm.getBackStackEntryCount(), it returns zero?