java - Hashmap的hash()函数

Question

在jdk7版本的hashmap下有这么一个函数

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);
}

下面这段代码的作用？

h ^= (h >>> 20) ^ (h >>> 12);
    return h ^ (h >>> 7) ^ (h >>> 4);

Answer 1

就是为了让每一位都参与运算，让相近的数最后通过hash能分散开并减少碰撞。

http://stackoverflow.com/questions/9335169/understanding-strange-java-hash-function