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From: Sean Mitchell <sean@mitchwood.com>
Newsgroups: comp.lang.java.programmer
Subject: equals(), Sets, Maps, and degrees of equality
Date: Wed, 9 Nov 2011 18:33:02 -0800 (PST)
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Anyone ever run into the case where you wish an Object could have more than one equals(), or that Set and Map implementations would let you pass in something like a closure to determine key equality?

It seems to me that objects can be equal in varying degrees. Let's consider a class Dog:

public class Dog {
  String breed;
  String name;
  String age;
}

I may want to have a Set<Dog>, which holds only one Dog of each breed, irrespective of name of age. In this case my equals()/hashcode() would only consider breed.

But I may also want a Mag<Dog, Owner> in which each Dog is made unique by name.

And of course, there is the most intuitive case where I want to use equals() to see if the instances map on all three fields.

I suppose I could create a wrapper class for each purpose which only overrides equals() and hashcode(), but that seems very unsatisfying and inefficient.

I'm interested in how other people have dealt with this. Surprisingly, I have not been able to Google up much on this subject. 

Thoughts?