Java 5中Java 7 for HashMap的变化

Question

我不是Java专家,只是体验Java 5和Java 7上以下程序输出的变化.任何人都知道Java 7中HashMap实现的变化是什么吗？

import java.util.HashMap;
import java.util.Map;

public class HashMapDemo {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<String, String>();
        map.put("1", "111");
        map.put("a", "aaa");
        map.put("A", "AAA");

        map.put("D", "DDD");
        map.put("d", "ddd");
        map.put("0", "000");

        map.put("B", "BBB");
        map.put("b", "bbb");
        map.put("2", "222");

        for(Map.Entry<String, String> entry : map.entrySet()){
            System.out.println(entry.getKey()+ "  "+entry.getValue());
        }
    }
}

Java 7上的输出

D  DDD
2  222
d  ddd
1  111
0  000
b  bbb
A  AAA
B  BBB
a  aaa

Java 5上的输出

0  000
1  111
a  aaa
A  AAA
B  BBB
b  bbb
2  222
D  DDD
d  ddd

Answer 1

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);
}