code: private List<String> diff(Collection<String> first, Collection<String> second) {
          final ArrayList<String> list = new ArrayList<String>(first);
          list.removeAll(second);
          return list;
      }
docs: Returns a list of elements in the first collection which are not present in the second collection
  
  @param first  First collection
  @param second Second collection
  @return Difference between the two collections