Java:一个“主要"的数或“二的幂"作为 HashMap 的大小? [英] Java: A "prime" number or a "power of two" as HashMap size?

问题描述

许多书籍和教程都说哈希表的大小必须是素数，才能在所有桶中均匀分布密钥.但是 Java 的 HashMap 总是使用 2 的幂的大小.不应该使用素数吗?哈希表大小是素数"还是2 的幂"哪个更好?

Many books and tutorials say that the size of a hash table must be a prime to evenly distribute the keys in all the buckets. But Java's HashMap always uses a size that is a power of two. Shouldn't it be using a prime? What's better, a "prime" or a "power of two" as the hash table size?

推荐答案

使用 2 的幂可以有效地屏蔽哈希码的最高位.因此，在这种情况下，质量差的哈希函数可能会表现得特别糟糕.

Using a power of two effectively masks out top bits of the hash code. Thus a poor-quality hash function might perform particularly badly in this scenario.

Java 的 HashMap 通过不信任对象的 hashCode() 实现和 对其结果应用第二级散列:

Java's HashMap mitigates this by mistrusting the object's hashCode() implementation and applying a second level of hashing to its result:

对给定的 hashCode 应用补充散列函数，以防止质量差的散列函数.这很关键，因为 HashMap 使用长度为二的幂的哈希表，否则会遇到低位没有差异的 hashCode 的冲突.

Applies a supplemental hash function to a given hashCode, which defends against poor quality hash functions. This is critical because HashMap uses power-of-two length hash tables, that otherwise encounter collisions for hashCodes that do not differ in lower bits.

如果你有一个很好的散列函数，或者做一些类似于 HashMap 所做的事情，你是否使用素数等作为表大小都没有关系.

If you have a good hash function, or do something similar to what HashMap does, it does not matter whether you use prime numbers etc as the table size.

另一方面，如果散列函数未知或质量差，那么使用质数将是更安全的选择.但是，它会使动态大小的表格难以实现，因为突然之间，您需要能够生成素数，而不是仅仅将大小乘以一个常数因子.

If, on the other hand, the hash function is of unknown or poor quality, then using a prime number would be a safer bet. It will, however, make dynamically-sized tables tricker to implement, since all of a sudden you need to be able to produce prime numbers instead of just multiplying the size by a constant factor.

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