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From: Jim Janney <jjanney@shell.xmission.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: hashCode
Date: Fri, 31 Aug 2012 09:08:35 -0600
Organization: an undertaking of great advantage, but nobody to know what it is
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Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> writes:

> On 31/08/2012 03:43, Eric Sosman allegedly wrote:
>> On 8/30/2012 6:52 PM, Daniele Futtorovic wrote:
>>> On 29/08/2012 20:49, Eric Sosman allegedly wrote:
>>>> 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);
>>>>>>
>>>> The difficulty is that an external Hasher would have no access to
>>>> private fields (...)
>>>
>>> This.
>> 
>>     ?
>> 
>>     package deal;
>>     public class ToBeHashed {
>>         private int noGettersForMeThanks;
>>         ...
>>     }
>> 
>>     package express;
>>     Hasher<ToBeHashed> hasher = new Hasher<ToBeHashed>() {
>>         public int hashCode(ToBeHashed obj) {
>>             // How do you obtain the value of
>>             // obj.noGettersForMeThanks
>>             // if that would be useful?
>>         }
>>         ...
>>     }
>> 
>> As an example of why a hasher might want access to a strictly-private
>> field, I offered String: How could a Hasher<String> (outside String
>> itself) use String's private `hash' element?  (And before you say
>> "Give String a getHash() method," ponder what hashCode() does.)
>> 
>
> Sorry for not being clear, Eric. I meant to emphasize how IMO that one
> single point you mention is the decisive argument that settles the issue.
>
> Imposing that classes should expose all information "relevant" (says
> who??) to hashing is utter rubbish IMNSHO.

Objects that compare equal must hash to the same value.  It follows that
if the hash function uses a value, so must the comparison method.
Relying on non-public information in the comparison method means
allowing two objects to compare non-equal even though there is no way,
using public information, to distinguish between them.  I can honestly
say I've never done that or felt a need to do that.

-- 
Jim Janney