code: private String formatWordsFrequency(Map<String, AtomicInteger> frequency){
          StringBuilder str = new StringBuilder();
          if(frequency != null && !frequency.isEmpty()) {
              AtomicInteger c = new AtomicInteger();
              frequency
                      .entrySet()
                      .stream()
                      .sorted((a, b) -> b.getValue().get() - a.getValue().get())
                      .forEach(e -> str.append("\t").append(c.incrementAndGet()).append("、").append(e.getKey()).append("=").append(e.getValue()).append("\n"));
          }
          str.setLength(str.length()-1);
          return str.toString();
      }
docs: 格式化词频统计信息
  @param frequency 词频统计信息