java 7 - Objects.hash() vs Objects.hashCode(), clarification needed

Question

in Java 7 we have

o.hashCode();
Objects.hashCode(o);

    Objects.hash(o);

The first 2 are roughly the same with the null point check, but what is last one?

When a single object reference is supplied, the returned value does not equal the hash code of that object reference.

Why is that? I mean, we don't need 3 methods that do the same thing, I understand that, but why do we need Objects.hash() at all? When would you chose to use one vs another?

Answer 1

See the documentation for hashCode and hash. hash takes Object... while hashCode takes Object. The example given is:

@Override public int hashCode() {
    return Objects.hash(x, y, z);
}

• Objects.hash(Object... values) should be used in cases when you want a hash of a sequence of objects, e.g. when defining your own hashCode method and want a simply-coded hash for multiple values that make up the identity of your object.
• Objects.hashCode(Object o) should be used when you want the hash of a single object, without throwing if the object is null.
• Object::hashCode() should be used when you want the hash of a single object, and will throw an exception if the object is null.

Note that hash(o) and hashCode(o) won't necessarily return the same thing! If you're doing it for a single object, you should probably use hashCode.