Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!aioe.org!eternal-september.org!feeder.eternal-september.org!.POSTED!not-for-mail
From: markspace <-@.>
Newsgroups: comp.lang.java.programmer
Subject: Re: equals(), Sets, Maps, and degrees of equality
Date: Wed, 09 Nov 2011 22:43:38 -0800
Organization: A noiseless patient Spider
Lines: 44
Message-ID: <j9frqs$pbk$1@dont-email.me>
References: <24123649.762.1320892382934.JavaMail.geo-discussion-forums@vbmh5> <j9ffe1$3k9$1@dont-email.me>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Thu, 10 Nov 2011 06:43:40 +0000 (UTC)
Injection-Info: mx04.eternal-september.org; posting-host="XjIWM99mD7Ijfdu600oVPA"; logging-data="25972"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX18FHbgrijPIb7iiOWLFCnMQ86+mXaZ3xmo="
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:8.0) Gecko/20111105 Thunderbird/8.0
In-Reply-To: <j9ffe1$3k9$1@dont-email.me>
Cancel-Lock: sha1:m3PEGnwzm69dTI0vw2tUUprl848=
Xref: x330-a1.tempe.blueboxinc.net comp.lang.java.programmer:9821

On 11/9/2011 7:11 PM, Eric Sosman wrote:

> I'd be inclined to go with the inner class, or perhaps with a
> wrapper if there's a reason you can't modify Dog. YMMV.


I wonder if it's possible to make something like a wrapper, where 
instead of one per object you just have a singleton of special cases.

public class Dog {
   String name;
   String breed;
   float weight;

   DogComparisonStrategy compare;

   public boolean equals( Object o ) {
     return compare.equals( this, o );
   }

   public int hashcode() {
      return compare.hashcode( this );
   }

}

class CompareByWeight implements DogComparisonStrategy {

   public boolean equals( Dog d1, Object o ) {
     if( !(o instanceof Dog) ) return false;
     Dog d2 = (Dog) o;
     return d2.weight == d1.weight;
   }

   public int hashcode( Dog d ) {
     return Float.getFloatbits( d.weight );
   }
}


At least you might not need a million of the things, unlike wrapper. 
Obviously, you have to be able to modify Dog for this to work.