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From: Eric Sosman <esosman@ieee-dot-org.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: equals(), Sets, Maps, and degrees of equality
Date: Thu, 10 Nov 2011 23:24:48 -0500
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On 11/10/2011 10:07 PM, markspace wrote:
> On 11/10/2011 5:58 PM, Eric Sosman wrote:
>
>> markspace's idea of endowing each Dog
>> with a DogComparisonStrategy wouldn't work if the same Dog instance
>> could be a member of the Set *and* a key in the Map:
>
>
> Right. It's really not that different than making subclasses to handle
> different schemes.

     It seems strikingly different to me.  Your suggestion is that
a Dog could dictate which of several hashCode/equals pairs to use,
but any particular Dog instance could select only one pair at a time.
With inner classes or wrappers, a single Dog instance could present
multiple aspects simultaneously and could participate simultaneously
in multiple collections based on those different key-nesses.

     Look at it this way: In your formulation, the Dog instance says
"I am known by my breed" or "I am known by my name," and every Set<Dog>
or Map<Dog,Owner> has to accept the Dog's own decision.  With inner
classes or wrappers, the Dog says "I am a Beagle and my name is Snoopy,"
and each Set<Dog.Breed> or Map<Dog.Name,Owner> makes its own choice
about which aspect counts.  In one case the Dog dictates its aspect;
in the other the collection chooses the aspect it cares about.

-- 
Eric Sosman
esosman@ieee-dot-org.invalid