Re: hashCode() for Custom classes

From:
Patricia Shanahan <pats@acm.org>
Newsgroups:
comp.lang.java.programmer
Date:
Sat, 19 Apr 2008 06:38:00 -0700
Message-ID:
<kKidneQlTpGua5TVnZ2dnUVZ_hWdnZ2d@earthlink.com>
Lew wrote:

Roedy Green wrote:

On Sat, 19 Apr 2008 00:02:29 -0400, Lew <lew@lewscanon.com> wrote,
quoted or indirectly quoted someone who said :

Another point is that hashes are explicitly ints, not bitmaps. It's
more natural and self-documenting to use integer operations on them.


On the other hand, hashCodes are just patterns, not quantities, so you
could also argue it makes sense to use the integer bit manipulator
operators on them.

I was wondering if you get any better spread with + or ^ under some
circumstances.

Assume your numbers were all multiples of 4. With +, the hashcode
would always end in 2 zeros. With xor it would end in either 2 zeros
or 2 ones, but in neither case would you get those lower 2 bits nicely
scrambled.


Bits nicely scrambled is not a requirement of hash codes, only a means
to the real requirement. If my numbers were all multiples of 4, that
means some other input probably is not all multiples of 4, so the
likelihood of collision for non-equal values would remain small. That's
what matters.


I don't think it is the only thing that matters. In order for hash
tables to work, hash codes need to lead to a reasonably uniform
distribution of objects among buckets.

Patricia

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