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 don't think a nontrivial hashcode exists. Also, your equals() violates the general contract as stated in the API --- it is not transitive:

(1,2) equals (1,3)

(4,3) equals (1,3)

But (4,3) is not equal to (1,2).

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For the sake of completeness, I present to you the Skeet-Niko proof =)

Claim: The hashcode must be the trivial constant function.

Proof: Let (a,b) and (c,d) be two objects with distinct hashcodes, i.e. h(a,b) ≠ h(c,d). Consider the object (a,d). By the OP's definition, (a,d) is equal to (a,b), and (a,d) is equal to (c,d). It follows from the hashcode contract that h(a,d) = h(a,b) = h(c,d); a contradiction.