Java 5中Java 7 for HashMap的变化

散列算法已更改.这意味着您不能依赖迭代顺序java.util.HashMap.这不应该让人感到意外,JDK从来没有给出任何这样的保证.如果订单对您很重要,请使用TreeMap或LinkedHashMap.

JDK5 HashMap:

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

JDK7 HashMap:

final int hash(Object k) {
  int h = hashSeed;
  if (0 != h && k instanceof String) {
     return sun.misc.Hashing.stringHash32((String) k);
  }

  h ^= k.hashCode();
  // This function ensures that hashCodes that differ only by
  // constant multiples at each bit position have a bounded
  // number of collisions (approximately 8 at default load factor).
  h ^= (h >>> 20) ^ (h >>> 12);
  return h ^ (h >>> 7) ^ (h >>> 4);
}