code: public TreeSet<TermFrequency> values()
      {
          TreeSet<TermFrequency> set = new TreeSet<TermFrequency>(Collections.reverseOrder());
  
          for (Map.Entry<String, TermFrequency> entry : entrySet())
          {
              set.add(entry.getValue());
          }
  
          return set;
      }
docs: 按照频率从高到低排序的条目
  @return