code: private int hash(Object k) {
          int h = hashSeed;
  
  //        if ((0 != h) && (k instanceof String)) {
  //            return sun.misc.Hashing.stringHash32((String) k);
  //        }
  
          h ^= k.hashCode();
  
          // Spread bits to regularize both segment and index locations,
          // using variant of single-word Wang/Jenkins hash.
          h += (h <<  15) ^ 0xffffcd7d;
          h ^= (h >>> 10);
          h += (h <<   3);
          h ^= (h >>>  6);
          h += (h <<   2) + (h << 14);
          return h ^ (h >>> 16);
      }
docs: Applies a supplemental hash function to a given hashCode, which
  defends against poor quality hash functions.  This is critical
  because ConcurrentHashMap uses power-of-two length hash tables,
  that otherwise encounter collisions for hashCodes that do not
  differ in lower or upper bits.