code: public List<Map.Entry<K, Float>> nearest(K key, int size)
      {
          Vector vector = storage.get(key);
          if (vector == null)
          {
              return Collections.emptyList();
          }
          return nearest(key, vector, size);
      }
docs: 查询与key最相似的元素
  
  @param key  键
  @param size topN个
  @return 键值对列表, 键是相似词语, 值是相似度, 按相似度降序排列