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