Java HashMap 哈希函数 [英] Java HashMap Hashing function

问题描述

我正在通过 Java 的 HashMap hash() 实现，如下所示

I was going through Java's HashMap hash() implementation , its like below

final int hash(Object k) {
            // some checks
            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);
                   // >>> is Unsigned right shift
    }

我不知道为什么要添加下面的代码，这样做有什么好处?

I am not sure why the below code is added , and what advantage is gained by same ?

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

或者，如果我从实现中删除上述代码，让我重新提出我的问题，有什么缺点?我了解它如何避免碰撞的机会，但不确定确切"如何?

Or Let me re-frame my question if i remove above code from implementation what is the disadvantage ? I understand some how its avoiding chances of collision but not sure "exactly" how ?

有人可以通过举个例子帮助我理解，并解释它在使用和不使用上述代码的情况下如何工作?

can some one help me understand by giving an example , and explain how will it work with and without the above code ?

推荐答案

Java 哈希表实现不是将表的大小设置为素数大小，而是两倍大小.这允许它使用快速位掩码而不是昂贵的余数运算，这通常是一件好事，但缺点是特别糟糕的散列函数可能比平时有更多的冲突.您引用的代码以最小化额外冲突的方式混合了哈希的位.

The Java hash table implementation sizes the table not to a prime size, but to a power of two size. This allows it to use fast bit masking instead of expensive remainder operations, which is generally a good thing, but the drawback is that particularly bad hash functions might have more collisions than usual. The code you cite mixes the bits of the hash in a way that minimizes the extra collisions.

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