Question

Found this code on http://www.docjar.com/html/api/java/util/HashMap.java.html after searching for a HashMap implementation.

  264       static int hash(int h) {
  265           // This function ensures that hashCodes that differ only by
  266           // constant multiples at each bit position have a bounded
  267           // number of collisions (approximately 8 at default load factor).
  268           h ^= (h >>> 20) ^ (h >>> 12);
  269           return h ^ (h >>> 7) ^ (h >>> 4);
  270       }

谁能对此有所了解? 该评论告诉我们why,但我要理解,这可改善“低厚度”和,从而保证这些职位与“号”号号”号号号号号号号号号号号号号号号号号号号号号号号号号相撞。这些字面是什么?

Answer 1

为了使其具有任何意义,必须结合理解哈希马普如何分配物品以 b。这是选择一个锅炉指数的三维功能:

static int indexFor(int h, int length) {
    return h & (length-1);
}

So you can see, that with a default table size of 16, only the 4 least significant bits of the hash actually matter for allocating buckets! (16 - 1 = 15, which masks the hash by 1111b)

如果你洗衣,这显然是坏消息。恢复《刑法》职能:

10101100110101010101111010111111

01111100010111011001111010111111

11000000010100000001111010111111
//etc etc etc

Such a hash function would not likely be "bad" in any way that is visible to its author. But if you combine it with the way the map allocates buckets, boom, MapFail(tm).

If you keep in mind that h is a 32 bit number, those are not magic numbers at all. It is systematically xoring the most significant bits of the number rightward into the least significant bits. The purpose is so that "differences" in the number that occur anywhere "across" it when viewed in binary become visible down in the least significant bits.

串通是受约束的,因为拥有相同相关附属机构的不同数目现在受到很大约束,因为在双职工代表中任何地方出现的任何差异都压缩到 matter的轨道上。

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