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From: "Lew" <lew@1:261/38.remove-pjg-this>
Subject: Re: hashCode
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  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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