code: private Map<String, AtomicInteger> frequency(List<Word> words){
          Map<String, AtomicInteger> frequency =new HashMap<>();
          words.forEach(word->{
              frequency.computeIfAbsent(word.getText(), k -> new AtomicInteger()).incrementAndGet();
          });
          return frequency;
      }
docs: 统计词频
  @param words 词列表
  @return 词频统计结果