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From: Lew <lewbloch@gmail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: equals(), Sets, Maps, and degrees of equality
Date: Fri, 11 Nov 2011 07:27:21 -0800 (PST)
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Andreas Leitgeb wrote:
> Sean Mitchell wrote:
>> 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?
> 
> Yep, I also noticed this lack in the context of "orthogonality".
> I just still haven't found any actual practical use for it.
> 
>> 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,

This is terrible modeling.

> In that case, I'd use a Map<String,Dog>, which would be keyed by the breed
> name itself, rather than by an arbitrary dog of that breed.
> 
> Probably, I'd even create a class Breed, that would encapsulate some more
> properties along with the name, and then use the Breed class as key.

+
This is the way to go.

> For the example at hand, this would allow filling the structure with breeds
> that are "requested" (initially associated with null Dog), then some other
> part of the code could pick an appropriate Dog for each requested Breed.
> 
>> But I may also want a Map<Dog, Owner> in which each Dog is made unique
>> by name.
> 
> That doesn't make sense to me, because the owner would rather be a property
> of the dog(*), than of its name. Who's the owner of "any dog called Rufus", btw?
> 
> *: natural language is sometimes quite at odds with modelling language. :-)

Andreas is on to something here.

-- 
Lew