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Re: hashCode

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  To: bob smith
From: Patricia Shanahan <pats@acm.org>

On 8/10/2012 3:22 PM, bob smith wrote: ...
> Better in the sense that you would never HAVE to override hashCode.
>
> Now, there are cases where you HAVE to override it, or your code is very
broken.
>

I've decided to go back to this message, because I feel the key issue is the 
circumstances under which hashCode would need to be overridden if Object's 
version returned a constant, compared to the current situation.

If Object's hashCode returned a constant, in practice anyone using an object as 
a key in a hash structure would want it overridden with one that has at least 
some chance of using multiple buckets. Without that property, a HashMap is an 
over-complicated, space-wasting cousin of a linked list.

The problem with this is that the programmer who knows that Widget instances 
are going to be used as HashMap keys does not necessarily control the Widget 
implementation. The programmer writing Widget has no idea whether it will ever 
be used as a HashMap key, and therefore no way of knowing whether it is safe, 
assuming Widget inherits the Object equals, to also inherit the Object 
hashCode.

Now compare to the current situation. The programmer implementing Widget 
decides whether to inherit a superclass equals or to write a Widget-specific 
equals. That programmer can assume the superclass has a hashCode that would be 
effective for a HashMap key, and only has to override hashCode if they are 
overriding equals.

In practice, it is a long time since I've written a hashCode manually. 
Generally, when I decide to override equals I tell Eclipse to generate an 
equals/hashCode pair based on the fields that control whether two instances are 
equal. Overriding hashCode is no additional work given that I would be telling 
Eclipse to generate an equals based on those fields anyway.

To me, the current situation seems "better".

Patricia

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hashCode "bob smith" <bob.smith@1:261/38.remove-t9h-this> - 2012-08-10 18:39 +0000
  Re: hashCode "Arne Vajhøj" <arne.vajhøj@1:261/38.remove-t9h-this> - 2012-08-10 18:39 +0000
    Re: hashCode "markspace" <markspace@1:261/38.remove-t9h-this> - 2012-08-10 18:39 +0000
      Re: hashCode "Arne Vajhøj" <arne.vajhøj@1:261/38.remove-t9h-this> - 2012-08-10 18:39 +0000
        Re: hashCode "rossum" <rossum@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
  Re: hashCode "Eric Sosman" <eric.sosman@1:261/38.remove-t9h-this> - 2012-08-10 18:39 +0000
    Re: hashCode "bob smith" <bob.smith@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
      Re: hashCode "Lew" <lew@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
        Re: hashCode "Arne Vajhøj" <arne.vajhøj@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
          Re: hashCode "Lew" <lew@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
            Re: hashCode "Arne Vajhøj" <������
høj@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
              Re: hashCode "Arne Vajhøj" <������
høj@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
                Re: hashCode "Eric Sosman" <eric.sosman@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
                Re: hashCode "Arne Vajhøj" <������
høj@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
      Re: hashCode "Eric Sosman" <eric.sosman@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
        Re: hashCode "Lew" <lew@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
          Re: hashCode "Arne Vajhøj" <������
høj@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
      Re: hashCode "Arne Vajhøj" <arne.vajhøj@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
        Re: hashCode "Robert Klemme" <robert.klemme@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
          Re: hashCode "Arne Vajhøj" <arne.vajhøj@1:261/38.remove-nlb-this> - 2012-08-13 18:36 +0000
            Re: hashCode "Robert Klemme" <robert.klemme@1:261/38.remove-nlb-this> - 2012-08-13 18:36 +0000
              Re: hashCode "Arne Vajhøj" <arne.vajhøj@1:261/38.remove-fzq-this> - 2012-08-20 18:58 +0000
              Re: hashCode "Arne Vajhøj" <arne.vajhøj@1:261/38.remove-fzq-this> - 2012-08-20 18:58 +0000
      Re: hashCode "Patricia Shanahan" <patricia.shanahan@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
      Re: hashCode "Joshua Cranmer" <joshua.cranmer@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
        Re: hashCode "Patricia Shanahan" <patricia.shanahan@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
          Re: hashCode "Eric Sosman" <eric.sosman@1:261/38.remove-6fd-this> - 2012-08-12 20:00 +0000
            Re: hashCode "Patricia Shanahan" <patricia.shanahan@1:261/38.remove-6fd-this> - 2012-08-12 20:00 +0000
          Re: hashCode "Arne Vajhøj" <������
høj@1:261/38.remove-nlb-this> - 2012-08-13 18:36 +0000
        Re: hashCode "Jan Burse" <jan.burse@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
  Re: hashCode "Roedy Green" <roedy.green@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
    Re: hashCode "Lew" <lew@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
      Re: hashCode "Roedy Green" <roedy.green@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
        Re: hashCode "Joerg Meier" <joerg.meier@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
          Re: hashCode "Mike Winter" <mike.winter@1:261/38.remove-ya-this> - 2012-08-11 19:19 +0000
        Re: hashCode "Peter Duniho" <peter.duniho@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
        Re: hashCode "Jan Burse" <jan.burse@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
        Re: hashCode "Arne Vajhøj" <arne.vajhøj@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
      Re: hashCode "rossum" <rossum@1:261/38.remove-ya-this> - 2012-08-11 19:19 +0000
    Re: hashCode "Arne Vajhøj" <arne.vajhøj@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
      Re: hashCode "Eric Sosman" <eric.sosman@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
        Re: hashCode "Arne Vajhøj" <arne.vajhøj@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
  Re: hashCode "Jan Burse" <jan.burse@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
    Re: hashCode "Jan Burse" <jan.burse@1:261/38.remove-pjg-this> - 2012-08-11 18:17 +0000
    Re: hashCode "Lew" <lew@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
      Re: hashCode "Lew" <lew@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
      Re: hashCode "Arne Vajhøj" <������
høj@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
        Re: hashCode "Lew" <lew@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
    Re: hashCode "Jan Burse" <jan.burse@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
      Re: hashCode "Jan Burse" <jan.burse@1:261/38.remove-m2z-this> - 2012-08-12 18:58 +0000
      Re: hashCode "Lew" <lew@1:261/38.remove-6fd-this> - 2012-08-12 20:00 +0000
        Re: hashCode "Arne Vajhøj" <������
høj@1:261/38.remove-nlb-this> - 2012-08-13 18:36 +0000

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