Question

I was just wondering why is that primes are used in a class’s hashCode() method? For example, when using Eclipse to generate my hashCode() method there is always the prime number 31 used:

public int hashCode() {
     final int prime = 31;
     //...
}

References:

Here is a good primer on Hashcode and article on how hashing works that I found (C# but the concepts are transferrable):
Eric Lippert’s Guidelines and rules for GetHashCode()

Answer 1

Because you want the number you are multiplying by and the number of buckets you are inserting into to have orthogonal prime factorizations.

Suppose there are 8 buckets to insert into. If the number you are using to multiply by is some multiple of 8, then the bucket inserted into will only be determined by the least significant entry (the one not multiplied at all). Similar entries will collide. Not good for a hash function.

31 is a large enough prime that the number of buckets is unlikely to be divisible by it (and in fact, modern java HashMap implementations keep the number of buckets to a power of 2).