code: public boolean saveKeyTo(String path)
      {
          LinkedList<String> keyList = new LinkedList<String>();
          for (Map.Entry<String, TermFrequency> entry : trie.entrySet())
          {
              keyList.add(entry.getKey());
          }
          return IOUtil.saveCollectionToTxt(keyList, path);
      }
docs: 仅仅将值保存到文件
  @param path
  @return