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-   -   hashCode (http://www.velocityreviews.com/forums/t949561-hashcode.html)

bob smith 08-10-2012 06:39 PM

hashCode
 
From: bob smith <bob@coolfone.comze.com>

Is it always technically correct to override the hashCode function like so:

@Override
public int hashCode() {
return 1;
}

Would it be potentially better if that was Object's implementation?

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Arne Vajh°j 08-10-2012 06:39 PM

Re: hashCode
 
To: bob smith
From: Arne Vajhoj <arne@vajhoej.dk>

On 8/10/2012 11:47 AM, bob smith wrote:
> Is it always technically correct to override the hashCode function like so:
>
> @Override
> public int hashCode() {
> return 1;
> }


It meets the minimum contractual obligation for that method.

But performance of anything using the hash capability (like HashMap<>) would be
very bad.

> Would it be potentially better if that was Object's implementation?


It has a different behavior that may not be valid if you override equals.

Arne

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Eric Sosman 08-10-2012 06:39 PM

Re: hashCode
 
To: bob smith
From: Eric Sosman <esosman@ieee-dot-org.invalid>

On 8/10/2012 11:47 AM, bob smith wrote:
> Is it always technically correct to override the hashCode function like so:
>
> @Override
> public int hashCode() {
> return 1;
> }
>
> Would it be potentially better if that was Object's implementation?


Define "better."

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

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markspace 08-10-2012 06:39 PM

Re: hashCode
 
To: Arne Vajh°j
From: markspace <-@.>

On 8/10/2012 9:13 AM, Arne Vajhoj wrote:
> On 8/10/2012 11:47 AM, bob smith wrote:
>> Is it always technically correct to override the hashCode function
>> like so:
>>
>> @Override
>> public int hashCode() {
>> return 1;
>> }

>
> It meets the minimum contractual obligation for that method.
>
> But performance of anything using the hash capability (like HashMap<>)
> would be very bad.
>
>> Would it be potentially better if that was Object's implementation?

>
> It has a different behavior that may not be valid if you override equals.



I think at this point we are doing Bob's homework for him.

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Arne Vajh°j 08-10-2012 06:39 PM

Re: hashCode
 
To: markspace
From: Arne Vajhoj <arne@vajhoej.dk>

On 8/10/2012 1:13 PM, markspace wrote:
> On 8/10/2012 9:13 AM, Arne Vajhoj wrote:
>> On 8/10/2012 11:47 AM, bob smith wrote:
>>> Is it always technically correct to override the hashCode function
>>> like so:
>>>
>>> @Override
>>> public int hashCode() {
>>> return 1;
>>> }

>>
>> It meets the minimum contractual obligation for that method.
>>
>> But performance of anything using the hash capability (like HashMap<>)
>> would be very bad.
>>
>>> Would it be potentially better if that was Object's implementation?

>>
>> It has a different behavior that may not be valid if you override equals.

>
> I think at this point we are doing Bob's homework for him.


Could be.

But I think the question whether returning a constant from hashCode is a valid
Java question for understanding the contract for that method.

And I am pretty sure that I have seen other similar examples (just with 42 as
constant).

Arne

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Roedy Green 08-11-2012 06:17 PM

Re: hashCode
 
To: bob smith
From: Roedy Green <see_website@mindprod.com.invalid>

On Fri, 10 Aug 2012 08:47:12 -0700 (PDT), bob smith
<bob@coolfone.comze.com> wrote, quoted or indirectly quoted someone
who said :

> @Override
> public int hashCode() {
> return 1;
> }


that's about the worst possible hashCode function.

See http://mindprod.com/jgloss/hashcode.html for tips on how to write good
ones.
--
Roedy Green Canadian Mind Products http://mindprod.com A new scientific truth
does not triumph by convincing its opponents and making them see the light,
but rather because its opponents eventually die, and a new generation grows up
that is familiar with it.
~ Max Planck 1858-04-23 1947-10-04

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Lew 08-11-2012 06:17 PM

Re: hashCode
 
To: Roedy Green
From: Lew <lewbloch@gmail.com>

Roedy Green wrote:
> bob smith wrote, quoted or indirectly quoted someone
> who said :
>
>> @Override
> > public int hashCode() {
> > return 1;
> > }

>
> that's about the worst possible hashCode function.


Normally that's correct, but it's conceivable that one might do it for some
hackish reason. In most situations where one might do such an override as this,
one would do better not to override hashCode().

> See http://mindprod.com/jgloss/hashcode.html for tips on how to write
> good ones.


The default of assembling it via the mix-in of attribute hash codes using the
Knuth constants is usually good enough.

h(object) = Sum(i rnn 0.. n-1) of ( attribute[i] * pow(31, n - 1 - i) );

or

public static int calculateHash(Foo arg) {
int h = 0;

for ( each attribute of Foo that contributes to 'equals()' )
{
h = 31 * h + attribute.hashCode();
}
return h;
}

http://en.wikipedia.org/wiki/Hash_function
http://en.wikipedia.org/wiki/Java_hashCode()
http://en.wikipedia.org/wiki/Java_hashCode()#Java

--
Lew

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bob smith 08-11-2012 06:17 PM

Re: hashCode
 
To: Eric Sosman
From: bob smith <bob@coolfone.comze.com>

On Friday, August 10, 2012 11:34:28 AM UTC-5, Eric Sosman wrote:
> On 8/10/2012 11:47 AM, bob smith wrote:
>
> > Is it always technically correct to override the hashCode function like so:

>
> >

>
> > @Override

>
> > public int hashCode() {

>
> > return 1;

>
> > }

>
> >

>
> > Would it be potentially better if that was Object's implementation?

>
>
>
> Define "better."
>
>
>
> --
>
> Eric Sosman
>
> esosman@ieee-dot-org.invalid


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.

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Lew 08-11-2012 06:17 PM

Re: hashCode
 
To: bob smith
From: Lew <lewbloch@gmail.com>

bob smith wrote:
> Eric Sosman wrote:
>> bob smith wrote:
>>> Is it always technically correct to override the hashCode function like so:


It complies with the contract for 'hashCode()'. Is that all it takes to be
correct?

>>> Would it be potentially better if that was Object's implementation?


Would what be better if what were Object's implementation of what?

>> Define "better."


> 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.

No.

No matter what 'Object''s 'hashCode()' implementation were, it would need to be
overridden when you want value equality instead of object identity for
'equals()'.

See Joshua Bloch's seminal work /Effective Java/, which has items that pertain
to this.

Bottom line: 'hashCode()', 'equals()', and when present, 'compareTo()' must be
consistent.

'toString()' should be consistent with those.

As long as 'hashCode()' fulfills the contract, your code will work -
functionally. But a bad
'hashCode()' could and likely will noticeably affect performance. There is more
to correctness
than mere functional conformance.

--
Lew

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Arne Vajh°j 08-11-2012 06:17 PM

Re: hashCode
 
To: Roedy Green
From: Arne Vajhoj <arne@vajhoej.dk>

On 8/10/2012 3:17 PM, Roedy Green wrote:
> On Fri, 10 Aug 2012 08:47:12 -0700 (PDT), bob smith
> <bob@coolfone.comze.com> wrote, quoted or indirectly quoted someone
> who said :
>
>> @Override
>> public int hashCode() {
>> return 1;
>> }

>
> that's about the worst possible hashCode function.


Yes, but the posted asked "Is it always technically correct to ..." not whether
is was "best possible".

Arne

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