Generating hashCode from multiple fields? [duplicate]

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?

There are a ton of questions already on SO related to this. Was there something that wasn't clear? — Brian Roach
– Brian Roach, Commented May 29, 2013 at 22:11

Dan Grahn · Accepted Answer · 2015-04-20 16:43:20Z

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, 2015 at 16:43

Dan Grahn

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answered May 29, 2013 at 22:10

Sergey Kalinichenko

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4 Comments

meijuh Over a year ago

Should there be parentheses here? Seems the explanation does not match the code.

OrlandoL Over a year ago

No there is no parentheses

Trang Oul Over a year ago

Why 31 and not an even smaller prime, such as 2?

Sergey Kalinichenko Over a year ago

@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.

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