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Groups > comp.lang.java.programmer > #20558

Re: Need clarification on Object.equals.

Date 2012-12-19 09:39 -0800
From Patricia Shanahan <pats@acm.org>
Newsgroups comp.lang.java.programmer
Subject Re: Need clarification on Object.equals.
References (3 earlier) <14r3pk5hnnuce.1fmlq04hracjd.dlg@40tude.net> <7f86e17c-72f6-4b87-948d-9ee8ad102390@googlegroups.com> <50d1e9c7$0$289$14726298@news.sunsite.dk> <50D1EEC2.6090607@telia.com> <50d1f40f$0$288$14726298@news.sunsite.dk>
Message-ID <nJ-dnU4EifleZkzNnZ2dnUVZ_gadnZ2d@earthlink.com> (permalink)

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On 12/19/2012 9:06 AM, Arne Vajhøj wrote:
> On 12/19/2012 11:43 AM, Lars Enderin wrote:
>> 2012-12-19 17:22, Arne Vajhøj skrev:
>>> On 12/19/2012 11:13 AM, FredK wrote:
>>>> On Tuesday, December 18, 2012 10:56:19 AM UTC-8, Peter Duniho wrote:
>>>>> On Tue, 18 Dec 2012 04:48:16 -0800, Roedy Green wrote:
>>>>>>> Node a = new Gate();
>>>>>>> Monitor b = new Monitor();
>>>>>>> System.out.println(a.equals(b)); // --> prints 'true'
>>>>>> Gate or one of its superclasses is implementing equals.
>>>>> And doing it incorrectly, I'd say.
>>>>
>>>> Why would you think it was done incorrectly?
>>>> For most classes, a.equals(b) is not the same as a==b.
>>>> Think about string:
>>>> string a = "abc");
>>>> string b = new string(a);
>>>> Clearly a==b is false, but a.equals(b) is true.
>>>>
>>>> Usually the equals() method returns true if the internal state of both
>>>> objects is the same.
>>>
>>> It is rather unusual to have objects of different classes considered
>>> equals.
>>>
>>> Your example is different.
>>
>> Both Gate and Monitor were extensions of AbstractSet, and empty, which
>> eventually led to Set#containsAll, and thus equals, returning true.
>
> Yes. We know that now. And the problem was solved by not
> extending that.
>
> But if the class should continue to extend from
> that it should probably have overridden equals again
> with an implementation that did use instanceof to test
> for type (it could have called super.equals after that
> if appropriate).

AbstractSet implements Set, so all its subclasses also implement Set.

That means they are bound by the Set contract for .equals, which treats
all classes that implement Set the same way. There might be a
performance advantage to overriding AbstractSet's .equals in some cases,
but its result would have to be preserved.

The right solution is, of course, the action the OP is already taking -
don't extend AbstractSet unless you really are implementing a Set, and
intend to follow the Set contract.

Patricia

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Need clarification on Object.equals. "plewto@gmail.com" <plewto@gmail.com> - 2012-12-18 00:13 -0800
  Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-18 10:39 +0000
    Re: Need clarification on Object.equals. Roedy Green <see_website@mindprod.com.invalid> - 2012-12-18 04:48 -0800
      Re: Need clarification on Object.equals. Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-12-18 10:56 -0800
        Re: Need clarification on Object.equals. FredK <fred.l.kleinschmidt@gmail.com> - 2012-12-19 08:13 -0800
          Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 11:22 -0500
            Re: Need clarification on Object.equals. Lars Enderin <lars.enderin@telia.com> - 2012-12-19 17:43 +0100
              Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 12:06 -0500
                Re: Need clarification on Object.equals. Patricia Shanahan <pats@acm.org> - 2012-12-19 09:39 -0800
                Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 13:24 -0500
            Re: Need clarification on Object.equals. FredK <fred.l.kleinschmidt@gmail.com> - 2012-12-19 11:23 -0800
          Re: Need clarification on Object.equals. Patricia Shanahan <pats@acm.org> - 2012-12-19 08:25 -0800
    Re: Need clarification on Object.equals. plewto@gmail.com - 2012-12-18 10:56 -0800
      Re: Need clarification on Object.equals. Jim Janney <jjanney@shell.xmission.com> - 2012-12-18 14:24 -0700
        Re: Need clarification on Object.equals. Patricia Shanahan <pats@acm.org> - 2012-12-18 13:52 -0800
  Re: Need clarification on Object.equals. markspace <-@.> - 2012-12-18 10:24 -0800
    Re: Need clarification on Object.equals. plewto@gmail.com - 2012-12-18 10:48 -0800
      Re: Need clarification on Object.equals. David Lamb <dalamb@cs.queensu.ca> - 2012-12-18 14:01 -0500
        Re: Need clarification on Object.equals. plewto@gmail.com - 2012-12-18 12:14 -0800
          Re: Need clarification on Object.equals. plewto@gmail.com - 2012-12-18 12:17 -0800
            Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-18 22:16 -0500
              Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 10:20 +0000
                Re: Need clarification on Object.equals. Patricia Shanahan <pats@acm.org> - 2012-12-19 03:22 -0800
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 12:06 +0000
                Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 09:07 -0500
                Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 09:09 -0500
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 14:36 +0000
                Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 09:42 -0500
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 15:05 +0000
                Re: Need clarification on Object.equals. Lars Enderin <lars.enderin@telia.com> - 2012-12-19 16:15 +0100
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 15:47 +0000
                Re: Need clarification on Object.equals. Lars Enderin <lars.enderin@telia.com> - 2012-12-19 16:57 +0100
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 16:51 +0000
                Re: Need clarification on Object.equals. Lars Enderin <lars.enderin@telia.com> - 2012-12-19 18:12 +0100
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 17:27 +0000
                Re: Need clarification on Object.equals. Gene Wirchenko <genew@telus.net> - 2012-12-19 11:40 -0800
                Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 10:17 -0500
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 15:40 +0000
                Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 10:50 -0500
                Re: Need clarification on Object.equals. Lars Enderin <lars.enderin@telia.com> - 2012-12-19 17:01 +0100
                Re: Need clarification on Object.equals. Gene Wirchenko <genew@telus.net> - 2012-12-19 10:22 -0800
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 18:49 +0000
                Re: Need clarification on Object.equals. Gene Wirchenko <genew@telus.net> - 2012-12-19 11:41 -0800
                Re: Need clarification on Object.equals. Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-12-19 17:05 -0800
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-20 08:34 +0000
                Re: Need clarification on Object.equals. Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-12-20 07:27 -0800
                Re: Need clarification on Object.equals. Gene Wirchenko <genew@telus.net> - 2012-12-20 09:04 -0800
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-20 18:10 +0000
                Re: Need clarification on Object.equals. Lew <lewbloch@gmail.com> - 2012-12-20 11:08 -0800
                Re: Need clarification on Object.equals. Peter Dunihơ <Np0eStPeAdM@NnOwSlPiAnMk.com> - 2012-12-21 07:50 +0000
                Re: Need clarification on Object.equals. Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-12-21 07:30 -0800
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-21 17:09 +0000
                Re: Need clarification on Object.equals. Leŵ <lewbl0ch@gmail.com> - 2012-12-23 00:54 +0000
                Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-21 12:23 -0500
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-21 17:38 +0000
                Re: Need clarification on Object.equals. Arne Vajhoj <arne@vajhoj.dk> - 2012-12-23 00:50 +0000
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-21 08:39 +0000
                Re: Need clarification on Object.equals. Gene Wirchenko <genew@telus.net> - 2012-12-20 13:39 -0800
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-21 08:34 +0000
                Re: Need clarification on Object.equals. Leif Roar Moldskred <leifm@dimnakorr.com> - 2012-12-19 09:22 -0600
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 15:35 +0000
                Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 10:56 -0500
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 17:08 +0000
                Re: Need clarification on Object.equals. Joshua Cranmer <Pidgeot18@verizon.invalid> - 2012-12-19 08:58 -0600
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 15:32 +0000
                Re: Need clarification on Object.equals. Joshua Cranmer <Pidgeot18@verizon.invalid> - 2012-12-19 11:08 -0600
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 17:32 +0000
                Re: Need clarification on Object.equals. Lars Enderin <lars.enderin@telia.com> - 2012-12-19 18:41 +0100
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 17:58 +0000
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 18:12 +0000
                Re: Need clarification on Object.equals. Joshua Cranmer <Pidgeot18@verizon.invalid> - 2012-12-19 12:29 -0600
                Re: Need clarification on Object.equals. Joshua Cranmer <Pidgeot18@verizon.invalid> - 2012-12-19 12:00 -0600
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 18:06 +0000
                Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 13:13 -0500
                Re: Need clarification on Object.equals. Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-12-19 07:48 -0800
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 16:48 +0000
                Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 12:09 -0500
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 17:50 +0000
                Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 13:14 -0500
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 18:59 +0000
                Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 13:16 -0500
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 19:05 +0000
                Re: Need clarification on Object.equals. Gene Wirchenko <genew@telus.net> - 2012-12-19 11:45 -0800
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 20:00 +0000
                Re: Need clarification on Object.equals. Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-12-19 17:15 -0800
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-20 09:22 +0000
                Re: Need clarification on Object.equals. Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-12-20 07:36 -0800
                Re: Need clarification on Object.equals. Gene Wirchenko <genew@telus.net> - 2012-12-19 10:19 -0800
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-19 19:16 +0000
                Re: Need clarification on Object.equals. Gene Wirchenko <genew@telus.net> - 2012-12-19 11:48 -0800
                Re: Need clarification on Object.equals. Leif Roar Moldskred <leifm@dimnakorr.com> - 2012-12-19 14:12 -0600
                Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 09:04 -0500
                Re: Need clarification on Object.equals. Lew <lewbloch@gmail.com> - 2012-12-19 13:40 -0800
                Re: Need clarification on Object.equals. lipska the kat <lipskathekat@yahoo.co.uk> - 2012-12-20 09:30 +0000
          Re: Need clarification on Object.equals. Lew <lewbloch@gmail.com> - 2012-12-18 12:52 -0800
          Re: Need clarification on Object.equals. Joshua Cranmer <Pidgeot18@verizon.invalid> - 2012-12-19 09:27 -0600
            Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-19 10:44 -0500
      Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-18 22:11 -0500
  Re: Need clarification on Object.equals. Arne Vajhøj <arne@vajhoej.dk> - 2012-12-18 22:10 -0500

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