code: public long distance(String a, String b)
      {
          SynonymItem itemA = get(a);
          if (itemA == null) return Long.MAX_VALUE / 3;
          SynonymItem itemB = get(b);
          if (itemB == null) return Long.MAX_VALUE / 3;
  
          return itemA.distance(itemB);
      }
docs: 语义距离
  
  @param a
  @param b
  @return