hashCode implementations

[Jeff Storey]

Often I see in hashcode implementation examples multiplying some member variable by 37 (or some prime number).

For example,

public int hashCode() {
  return 37*a.hashCode() + b.hashCode();
}

What is the purpose of this? My guess would be this is to help the case when you have two objects such that

(object1.a.hashCode() = object2.b.hashCode()) && (object1.b.hashCode() = object2.a.hashcode());

Without that multiplier, both would have the same hashcode (which is okay, but maybe less efficient). Is there another purpose to this?

Thanks,
Jeff

[Remko Strating]

From what I understand the prime numbers are used for avoiding collisions.

See the following link for a detailed explanation

Hashtable

[David O'Meara]

True, although we say "reduce collisions" rather than "avoid collisions". Collisions happen, just hopefully not too often.

Nice link though, worth reading.

[Campbell Ritchie]

Originally posted by Jeff Storey:

(object1.a.hashCode() = object2.b.hashCode()) && (object1.b.hashCode() = object2.a.hashcode());

You did mean . . .

(object1.a.hashCode() [B]==[/B] object2.b.hashCode()) && (object1.b.hashCode() [B]==[/B] object2.a.hashcode());

. . . didn't you?