Groups > comp.lang.java.programmer > #17732

Re: hashCode

Date	2012-08-12 03:46 -0700
From	Patricia Shanahan <pats@acm.org>
Newsgroups	comp.lang.java.programmer
Subject	Re: hashCode
References	<563f186a-edb3-4311-ae48-3af7decfce2c@googlegroups.com> <k03d6p$2bt$1@dont-email.me> <6ac6c252-c370-4f74-b29d-ce5368019977@googlegroups.com>
Message-ID	<icCdnU2SCaLnFLrNnZ2dnUVZ_h-dnZ2d@earthlink.com> (permalink)

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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

Thread

hashCode bob smith <bob@coolfone.comze.com> - 2012-08-10 08:47 -0700
  Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-10 12:13 -0400
    Re: hashCode markspace <-@.> - 2012-08-10 10:13 -0700
      Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-10 13:38 -0400
        Re: hashCode rossum <rossum48@coldmail.com> - 2012-08-11 10:36 +0100
  Re: hashCode Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-10 12:34 -0400
    Re: hashCode bob smith <bob@coolfone.comze.com> - 2012-08-10 15:22 -0700
      Re: hashCode Lew <lewbloch@gmail.com> - 2012-08-10 15:32 -0700
        Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-10 19:30 -0400
          Re: hashCode Lew <noone@lewscanon.com> - 2012-08-11 16:24 -0700
            Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-11 22:15 -0400
              Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-11 22:29 -0400
                Re: hashCode Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-11 22:43 -0400
                Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-11 22:54 -0400
                Re: hashCode Lew <noone@lewscanon.com> - 2012-08-11 21:46 -0700
                Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-12 16:53 -0400
                Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-12 17:00 -0400
      Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-10 19:27 -0400
        Re: hashCode Robert Klemme <shortcutter@googlemail.com> - 2012-08-12 17:06 +0200
          Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-12 16:59 -0400
            Re: hashCode Robert Klemme <shortcutter@googlemail.com> - 2012-08-13 19:17 +0200
              Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-19 19:42 -0400
                Re: hashCode Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2012-08-21 10:30 +0000
              Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-19 19:47 -0400
                Re: hashCode Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2012-08-21 10:43 +0000
                Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-27 19:04 -0400
                Re: hashCode Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-08-27 16:55 -0700
                Re: hashCode markspace <-@.> - 2012-08-27 17:03 -0700
                Re: hashCode Lew <lewbloch@gmail.com> - 2012-08-27 17:49 -0700
                Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-27 21:37 -0400
                Re: hashCode Patricia Shanahan <pats@acm.org> - 2012-08-27 18:58 -0700
                Re: hashCode markspace <-@.> - 2012-08-27 21:19 -0700
                Re: hashCode Patricia Shanahan <pats@acm.org> - 2012-08-28 01:06 -0700
                Re: hashCode markspace <-@.> - 2012-08-28 09:19 -0700
                Re: hashCode Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-08-28 16:33 -0700
                Re: hashCode markspace <-@.> - 2012-08-28 17:02 -0700
                Re: hashCode Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-08-29 11:06 -0700
                Re: hashCode Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-29 14:49 -0400
                Re: hashCode Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-08-29 13:40 -0700
                Re: hashCode Gene Wirchenko <genew@ocis.net> - 2012-08-29 18:02 -0700
                Re: hashCode Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2012-08-31 00:52 +0200
                Re: hashCode Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-30 21:43 -0400
                Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-30 21:52 -0400
                Re: hashCode Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2012-08-31 04:18 +0200
                Re: hashCode Jim Janney <jjanney@shell.xmission.com> - 2012-08-31 09:08 -0600
                Re: hashCode Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-31 11:38 -0400
                Re: hashCode Robert Klemme <shortcutter@googlemail.com> - 2012-08-31 17:55 +0200
                Re: hashCode Jim Janney <jjanney@shell.xmission.com> - 2012-08-31 09:56 -0600
                Re: hashCode Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-31 14:32 -0400
                Re: hashCode Jim Janney <jjanney@shell.xmission.com> - 2012-08-31 14:38 -0600
                Re: hashCode Lew <lewbloch@gmail.com> - 2012-08-31 15:33 -0700
                Re: hashCode Jim Janney <jjanney@shell.xmission.com> - 2012-08-31 16:41 -0600
                Re: hashCode Lew <lewbloch@gmail.com> - 2012-08-31 16:26 -0700
                Re: hashCode Jim Janney <jjanney@shell.xmission.com> - 2012-09-02 11:54 -0600
                Re: hashCode Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2012-09-03 00:47 +0200
                Re: hashCode Jim Janney <jjanney@shell.xmission.com> - 2012-09-03 21:44 -0600
                Re: hashCode Jim Janney <jjanney@shell.xmission.com> - 2012-08-31 09:08 -0600
                Re: hashCode Robert Klemme <shortcutter@googlemail.com> - 2012-08-31 17:58 +0200
                Re: hashCode markspace <-@.> - 2012-08-29 11:51 -0700
                Re: hashCode Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-08-29 13:28 -0700
                Re: hashCode markspace <-@.> - 2012-08-29 16:05 -0700
                Re: hashCode Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-08-29 16:23 -0700
                Re: hashCode Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-29 20:56 -0400
                Re: hashCode Jan Burse <janburse@fastmail.fm> - 2012-08-30 11:19 +0200
                Re: hashCode Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-08-30 10:03 -0700
                Re: hashCode Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2012-08-30 18:34 +0000
                Re: hashCode Jim Janney <jjanney@shell.xmission.com> - 2012-08-30 08:11 -0600
                Re: hashCode Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-08-30 10:06 -0700
                Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-30 19:16 -0400
                Re: hashCode Lew <lewbloch@gmail.com> - 2012-08-28 13:58 -0700
                Re: hashCode Lew <lewbloch@gmail.com> - 2012-08-28 13:56 -0700
                Re: hashCode Patricia Shanahan <pats@acm.org> - 2012-08-28 14:07 -0700
                Re: hashCode Lew <lewbloch@gmail.com> - 2012-08-28 14:38 -0700
                Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-27 21:12 -0400
      Re: hashCode Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-11 07:58 -0400
        Re: hashCode Lew <noone@lewscanon.com> - 2012-08-11 16:29 -0700
          Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-11 22:16 -0400
      Re: hashCode Patricia Shanahan <pats@acm.org> - 2012-08-12 03:46 -0700
      Re: hashCode Joshua Cranmer <Pidgeot18@verizon.invalid> - 2012-08-12 12:23 -0400
        Re: hashCode Patricia Shanahan <pats@acm.org> - 2012-08-12 09:40 -0700
          Re: hashCode Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-12 13:59 -0400
            Re: hashCode Patricia Shanahan <pats@acm.org> - 2012-08-12 11:17 -0700
          Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-12 17:02 -0400
        Re: hashCode Jan Burse <janburse@fastmail.fm> - 2012-08-12 19:03 +0200
  Re: hashCode Roedy Green <see_website@mindprod.com.invalid> - 2012-08-10 12:17 -0700
    Re: hashCode Lew <lewbloch@gmail.com> - 2012-08-10 12:45 -0700
      Re: hashCode Roedy Green <see_website@mindprod.com.invalid> - 2012-08-11 04:54 -0700
        Re: hashCode Joerg Meier <joergmmeier@arcor.de> - 2012-08-11 18:25 +0200
          Re: hashCode Mike Winter <usenet@michael-winter.me.invalid> - 2012-08-11 18:53 +0100
        Re: hashCode Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-08-11 09:56 -0700
        Re: hashCode Jan Burse <janburse@fastmail.fm> - 2012-08-11 18:58 +0200
        Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-11 22:40 -0400
      Re: hashCode rossum <rossum48@coldmail.com> - 2012-08-11 18:47 +0100
    Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-10 19:25 -0400
      Re: hashCode Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-11 08:00 -0400
        Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-11 09:49 -0400
  Re: hashCode Jan Burse <janburse@fastmail.fm> - 2012-08-11 15:33 +0200
    Re: hashCode Jan Burse <janburse@fastmail.fm> - 2012-08-11 15:34 +0200
    Re: hashCode Lew <noone@lewscanon.com> - 2012-08-11 16:34 -0700
      Re: hashCode Lew <noone@lewscanon.com> - 2012-08-11 16:37 -0700
      Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-11 22:19 -0400
        Re: hashCode Lew <noone@lewscanon.com> - 2012-08-11 21:48 -0700
    Re: hashCode Jan Burse <janburse@fastmail.fm> - 2012-08-12 12:08 +0200
      Re: hashCode Jan Burse <janburse@fastmail.fm> - 2012-08-12 12:18 +0200
      Re: hashCode Lew <noone@lewscanon.com> - 2012-08-12 11:27 -0700
        Re: hashCode Arne Vajhøj <arne@vajhoej.dk> - 2012-08-12 17:11 -0400

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