What should I use as a hash code of null?

Question

Assume we have a following trivial class:

public class Foo {

    public Integer bar;

}

And we want to build a "good" hashCode method for it. By "good" I would, for instance mean that there's a small probability of hash code collision in "real-life" cases.

In "real-life" for such a class I would reasonably expect Foos with bar set to null or 0. I'd even argue that these two are probably the most frequent values.

But let's take a look at what Eclipse, for instance, generates:

public class Foo {

    public Integer bar;

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((bar == null) ? 0 : bar.hashCode());
        return result;
    }
}

And it's not just Eclipse, it seems like using 0 as hashCode for null is a normal practice.

But this would produce identical hash codes for null and 0, wouldn't it? An as I'm assuming that null and 0 are probably the most frequent cases - this leads to a higher probabiliy of collission.

So here comes my question. What would be a good hashCode value for null?

Answer 1

From Joshua Bloch's excellent book Effective Java, 2nd Edition (page 49):

If the value of the field is null, return 0 (or some other constant, but 0 is traditional).

So you can use any constant of your choice, but usually, 0 is used as the hash code of null.

In your case, where 0 appear frequently, it might indeed be better to choose a different constant than 0 (one that doesn't appear as a valid value in your field) to avoid collisions.