code: private static int hash(Object k) {
  	if (k instanceof KeyImpl) return ((KeyImpl) k).wangJenkinsHash();
  	// Spread bits to regularize both segment and index locations,
  	// using variant of single-word Wang/Jenkins hash.
  	int h = k.hashCode();
  
  	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.