code: public static <T> Collection<T> intersectWithModify(Collection<T> first,
                                                          Collection<T> second) {
          E.checkNotNull(first, "first");
          E.checkNotNull(second, "second");
          first.retainAll(second);
          return first;
      }
docs: Find the intersection of two collections, note that the first collection
  will be modified and store the results
  @param first the first collection, it will be modified
  @param second the second collection
  @param <T> element type of collection
  @return intersection of the two collections