Re: When do I need to override hash?
Re: When do I need to override hash?
- Subject: Re: When do I need to override hash?
- From: Roland King <email@hidden>
- Date: Thu, 20 Aug 2009 09:35:19 +0800
Seth Willits wrote:
On Aug 19, 2009, at 4:28 PM, Gideon King wrote:
So do I need to override hash too? If so, are there any
recommendations as to how to determine the hash easily?
I probably shouldn't admit this, but I've yet to override hash and have
yet to notice any problems. The docs say I should, so I realize I
likely should, but I would really like a concrete reason to do so. I've
never called hash, nor seen it ever called by anyone else's code. I
assume the frameworks call it from somewhere, but where?
When it sticks things in any kind of hashtable or wants for some reason
a way to partition objects roughly to cut down the number of comparisons
it has to do on them. So it's very important that if two objects are
isEqual: to each other, they must have the same hash. Else what could
happen is you insert one into any of the collections which use hash to
bucket the objects, then you put another one in which isEqual: to the
first. If it has a different hash (as it's not supposed to do), the code
will go look in the wrong bucket, won't find the object you put there
earlier and then you'll have two objects in the collection which are
isEqual: to each other.
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