在写hashcode时为什么要使用一个31这样的数

[孤好梦中杀人]

为什么在写hashcode时为什么要使用一个31这样的数，我用100，11，17是不是一样呢？有什么不一样的地方呢？

[coolbamboo2008]

31好像是什么东西的一个最小约数。有道理的（好像是hash表用来区分不同的空间的），忘记了，一般也没人去关注这个的。楼主愿意写几都行。

[coooliang]

哪里看到的，贴来看下。

[solomon5926]

引用 2 楼 cl61917380 的回复:

哪里看到的，贴来看下。

源代码里面有，好像是HashMap还是HashTable的，这个值应该是经验值，没有很明确的理论证明的

[小绵羊]

According to Joshua Bloch's Effective Java (a book that can't be recommended enough, and which I bought thanks to continual mentions on stackoverflow): The value 31 was chosen because it is an odd prime. If it were even and the multiplication overflowed, information would be lost, as multiplication by 2 is equivalent to shifting. The advantage of using a prime is less clear, but it is traditional. A nice property of 31 is that the multiplication can be replaced by a shift and a subtraction for better performance: 31 * i == (i << 5) - i. Modern VMs do this sort of optimization automatically. 31是素数 31*N 可以变成 N<<5-N 且5bit