hashCode() method when equals() is based on multiple independent fields

Question

i have a class whose equality is based on 2 fields such that if either one is equal then the objects of this type are considered equal. how can i write a hashCode() function for

Answer 1

I think you can't. The reason is, your equals() method is not transitive.

Transitivity means for three non-null x, y, z, if x.equals(y), y.equals(z), then x.equals(z). In your example, an object x={id: 1, name: "ha"}, y={id: 1, name: "foo"}, z={id: 2, name: "bar"} have this property (x.equals(y) and y.equals(z)). However, x.equals(z) is false. Every equals() method should have this property, see the Java API docs.

Back to hashing functions: Every function yields an equivalence defined by f(x)==f(y). That means if you are interested in comparison of the function values and want it to return true if x==y (and possibly in other cases), you'll receive a transitive relation, which means you have to consider at least a transitive closure of objects' equivalence. In your case, the transitive closure is the trivial relation (everything equals to anything). Which means you can't distinguish different objects by any function.