在HashMap中，通过对元素的hashCode进行一次hash运算得到一个hash值，然后再结合HashMap的size计算出元素插入table[]中的位置，为了避免哈希冲突，JDK在各个版本中都对hash算法算法进行了不断的优化，下面是JDK5至JDK8的演化过程。

JDK5

  1. static final int hash(Object key) {
  2.     int h;
  3.     return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
  4. }

JDK6

  1. static int hash(int h) {
  2.     // This function ensures that hashCodes that differ only by
  3.     // constant multiples at each bit position have a bounded
  4.     // number of collisions (approximately 8 at default load factor).
  5.     h ^= (h >>> 20) ^ (h >>> 12);
  6.     return h ^ (h >>> 7) ^ (h >>> 4);
  7. }

JDK7

  1. final int hash(Object k) {
  2.     int h = hashSeed;
  3.     if (0 != h && k instanceof String) {
  4.         return sun.misc.Hashing.stringHash32((String) k);
  5.     }
  6.     h ^= k.hashCode();
  7.     // This function ensures that hashCodes that differ only by
  8.     // constant multiples at each bit position have a bounded
  9.     // number of collisions (approximately 8 at default load factor).
  10.     h ^= (h >>> 20) ^ (h >>> 12);
  11.     return h ^ (h >>> 7) ^ (h >>> 4);
  12. }

JDK8

  1. static final int hash(Object var0) {
  2.     int var1;
  3.     return var0 == null ? 0 : (var1 = var0.hashCode()) ^ var1 >>> 16;
  4. }