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Re: hashCode

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From Eric Sosman <esosman@ieee-dot-org.invalid>
Newsgroups comp.lang.java.programmer
Subject Re: hashCode
Date Wed, 29 Aug 2012 14:49:15 -0400
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On 8/29/2012 2:06 PM, Daniel Pitts wrote:
> On 8/28/12 5:02 PM, markspace wrote:
>> On 8/28/2012 4:33 PM, Daniel Pitts wrote:
>>>
>>> interface Hasher<Type> {
>>>     int hash(Type t);
>>
>> Not really seeing how this is a good idea.  How would you implement this?
>>
>>
>>> So, that would change HashMap to take a Hasher<? super K> instance in
>>> its constructor.
>>
>> This is the problem; Map (and HashMap) were desired to be spec'd as
>> taking Object, not a subclass.
> Actually, they are Generic, so they are not spec'd to take Object, but
> to take a specific subtype defined at compile time. At least, now that
> they have the addition of Generics. Pre-generics, they still had
> Comparators which had the same behavior that I'm describing, but instead
> of defining buckets, they define an ordering. See below.

     Actually, "take" is insufficiently specific.  A Map<K,V>
has a put() method with K,V parameters, and a putAll() method
with a Map<? extends K, ? extends V> parameter.  To that extent,
Map<K,V> "takes" K.

     But Map<K,V> *also* has get() and remove() and containsKey()
methods with Object parameters, not K parameters.  (It also has a
containsValue() method taking Object, not V.)  So insofar as
these methods are concerned, Map<K,V> "takes" Object.

>>> A default Hasher<Object> could be implemented to use
>>> System.identityHashCode and == for the common use-case.
>>
>>
>> Again not seeing how you'd actually use that to put an object in a Map.
>
> Example usage:
> ++++
> // MyKeyHasher implements Hasher<MyKey>
> Map<MyKey, MyValue> map=new HashMap<MyKey,MyValue>(new MyKeyHasher());
>
> map.put(myFirstKey, myFirstValue);
> map.put(mySecondKey, mySecondValue);
> ++++

     This could sort of work, but not very well.  As I wrote some
<hunt, hunt, hunt, ah!> seventeen days ago in this thread:

	I don't think a HashCalculator interface along
	the lines of Comparable would save the day.

The difficulty is that an external Hasher would have no access to
private fields of MyKey.  That may seem a small drawback, since it
is rare to have a contributor to an object's "value" that is not
at the very least accessible through a getter.  The Hasher might
need to make method calls where today's built-in hashCode() just
makes field references, but -- hey, how bad could that be?

     IMHO, it could be pretty bad.  Take java.lang.String, for
example: As things stand today hashCode() inspects the "value"
of the String, and everything it uses would be accessible to a
Hasher<String>.  But hashCode() then caches the computed value
in a private field within String to avoid recomputing it on every
subsequent call!  Could Hasher<String> do the same?  How?

     Okay, so the implementor of String perceives the problem and
decides that String itself will provide a default Hasher<String>
implementation.  (This might be a static nested class, but it'd
probably be more efficient to have String implement Hasher<String>
directly, so every String is its own Hasher.)  And the implementor
of BigInteger does the same, and so does the implementor of URL,
and of File, and of -- Hey, wait a minute!  We're right back where
we began, except with more overhead and more verbiage!

-- 
Eric Sosman
esosman@ieee-dot-org.invalid

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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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