code: @Override
      public List<String> getKeywords(List<Term> termList, int size)
      {
          clear();
          add(termList);
          Collection<TermFrequency> topN = top(size);
          List<String> r = new ArrayList<String>(topN.size());
          for (TermFrequency termFrequency : topN)
          {
              r.add(termFrequency.getTerm());
          }
          return r;
      }
docs: 提取关键词（非线程安全）
  
  @param termList
  @param size
  @return