code: public void push(Fragment fragment) {
  
          Fragment top = peek();
          if (top != null) {
              manager.beginTransaction()
                  .setCustomAnimations(R.anim.enter_from_right, R.anim.exit_to_left, R.anim.enter_from_left, R.anim.exit_to_right)
                  .remove(top)
                  .add(containerId, fragment, indexToTag(manager.getBackStackEntryCount() + 1))
                  .addToBackStack(null)
                  .commit();
          }
          else {
              manager.beginTransaction()
                  .add(containerId, fragment, indexToTag(0))
                  .commit();
          }
  
          manager.executePendingTransactions();
      }
docs: Pushes a fragment to the top of the stack.