Generating hashCode from multiple fields?

Question

How do I generate a hashCode from two fields in my class?

For example, I want Pair classes with the same objects V to have the same hashCode:

public class Pair<V> {
    V from, to;
}

Should I multiply their hashCodes together? Add them? Multiply them with a prime?

a look at how [lombok](http://projectlombok.org/features/EqualsAndHashCode.html) implements it might help — soulcheck, May 29 '13 at 22:11
There are a **ton** of questions already on SO related to this. Was there something that wasn't clear? — Brian Roach, May 29 '13 at 22:11

score 13 · Accepted Answer · edited Apr 20 '15 at 16:43

13

One way to do it is adding the hash code of the first field to hash code of the second field, multiplied by a small prime number, like this:

public int hashCode() {
    return 31 * from.hashCode() + to.hashCode();
}

edited Apr 20 '15 at 16:43

Dan Grahn

answered May 29 '13 at 22:10

5

Should there be parentheses here? Seems the explanation does not match the code. – meijuh Jul 16 '14 at 15:38
No there is no parentheses – OrlandoL Aug 13 '19 at 19:59
Why 31 and not an even smaller prime, such as 2? – Trang Oul Feb 21 '22 at 12:08
@TrangOul There is no particular reason to use any specific prime. I would not pick 2, though, because it's the only even prime, so multiplying by it in the binary representation would amount to a simple shift. In other words, the result would be equivalent to `(from.hashCode() << 1) + to.hashCode()`. Although there is nothing particularly wrong with this approach, generally one would prefer more "mixing up" among the bits of the individual hash codes. – Sergey Kalinichenko Feb 21 '22 at 16:01

1 Answers1