docs: public static <E> Set<E> difference(Set<E> first, Set<E> second) {
          if (first == null || first.size() == 0) {
              return new HashSet<>();
          }
  
          if (second == null || second.size() == 0) {
              return first;
          }
  
          Set<E> difference = new HashSet<>(first);
          difference.removeAll(second);
          return difference;
      }