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Groups > comp.lang.java.programmer > #17716

Re: hashCode

From Lew <noone@lewscanon.com>
Newsgroups comp.lang.java.programmer
Subject Re: hashCode
Date 2012-08-11 16:29 -0700
Organization albasani.net
Message-ID <k06psm$h3a$1@news.albasani.net> (permalink)
References <563f186a-edb3-4311-ae48-3af7decfce2c@googlegroups.com> <k03d6p$2bt$1@dont-email.me> <6ac6c252-c370-4f74-b29d-ce5368019977@googlegroups.com> <k05hd9$uhc$1@dont-email.me>

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Eric Sosman wrote:
>      Okay: Then returning a constant 1 (or 42 or 0 or whatever)
> would in fact satisfy the letter of the law regarding hashCode():

Not if you consider all aspects of what the Javadocs promise.

See my post upthread.

> Whenever x.equals(y) is true, x.hashCode() == y.hashCode().  In
> your example this would be trivially true because x,y,z,... all
> have the same hashCode() value, whether they're equal or not --
> You have lived up to the letter of the law.

No, because the law requires that the method support 'HashMap', which in turn 
calls for "properly" hashed objects.

>      Of course, such a hashCode() would make all those hash-based
> containers pretty much useless: They would work in the sense that
> they would get the Right Answer, but they'd be abominably slow,

Indeed.

> with expected performance of O(N) instead of O(1).  See
> <http://www.cs.rice.edu/~scrosby/hash/CrosbyWallach_UsenixSec2003/>
> for a survey of some denial-of-service attacks that work by driving
> hash tables from O(1) to O(N), resulting in catastrophic failure
> of the attacked system.
>
>      In other words, the letter of the law on hashCode() is a bare
> minimum that guarantees correct functioning, but it is not enough
> to guarantee usability.  Why isn't the law more specific?  Because

Actually, if you consider all that the Javadocs tell you, this "letter of the 
law" to which you refer is like saying the sequence "ABC" constitutes all of 
"the ABCs".

> nobody knows how to write "hashCode() must be correct *and* usable"
> in terms that would cover all the classes all the Java programmers
> have dreamed up and will dream up.  Your hashCode() meets the bare
> minimum requirement, but is not "usable."  The actual hashCode()
> provided by Object also meets the bare minimum requirement, and *is*
> usable as it stands, until (and unless; you don't HAVE to) you
> choose to implement other equals() semantics, and a hashCode() to
> match them.

As Arne states, "correct" means "fulfills the specification". The 
specification for Java API methods is the standard Javadocs, which do impose 
performance considerations on 'hashCode()'.

One understands that the spec isn't always fully enforceable by the compiler. 
[1] It is correct that the compiler will allow 'return 1;'. It is not correct 
that that fulfills the specification.

[1] Doesn't one?

-- 
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg

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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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