在写hashcode时为什么要使用一个31这样的数
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？ --------------------编程问答-------------------- 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。 --------------------编程问答-------------------- 哪里看到的，贴来看下。 --------------------编程问答--------------------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 --------------------编程问答-------------------- 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
补充：Java , Java SE
