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What is the best way to get the Hash value for a String

Posted on 2004-09-03
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Last Modified: 2010-05-18
Here is the Sun's way

    public int hashCode() {
      int h = hash;
      if (h == 0) {
          int off = offset;
          char val[] = value;
          int len = count;

            for (int i = 0; i < len; i++) {
                h = 31*h + val[off++];
            }
            hash = h;
        }
        return h;
    }
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Question by:sudhakar_koundinya
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7 Comments
 
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Accepted Solution

by:
sciuriware earned 125 total points
ID: 11979178
In general a hashing algorithm should spread a collection to a flat model.
In other words, it depends. If your initial collection has a majority of similar strings it will be
hard to create any algorithm. The SUN approach will work most of the time,
but in many cases it's easier to look at your data.
For instance you might consider a ZIP code as part of the hash value.
;JOOP!
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LVL 86

Expert Comment

by:CEHJ
ID: 11979710
Hash algorithms are a trade-off between speed and the ability to prevent code collisions. Why are you questioning Sun's implementation btw?
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LVL 86

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by:CEHJ
ID: 11979726
Sorry - but i don't really understand that accepted answer - perhaps someone can explain it to me? ;-)
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Expert Comment

by:sciuriware
ID: 11980880
CEHJ, I agree that a split was as acceptable as an A would have been.
Didn't you see in many cases that the award goes to the expert who (accidentally)
   hits a preception? Now you can determine what the question would have been.
Sorry, next question please.
;JOOP!
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Expert Comment

by:CEHJ
ID: 11981928
>>CEHJ, I agree that a split was as acceptable ...

I think you misunderstood - i was actually saying i didn't understand your answer, not 'i don't understand why that answer has been accepted' (not that i'm ruling out a connection between the two ;-))
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Expert Comment

by:sciuriware
ID: 11983484
Sorry, well, the idea is that if some record seems to contain an unique piece of data, that piece is eligible
for being used as hash (or any other) key.
When the spread of such a key over the total collection is about flat (no accumulation in some spots)
it is ideal for hashing (few collisions).
So, before you apply a general algorithm, look an the nature of your data.
For instance a phone number might be an unique part of a record, but if most of those start with the same digits
you will take some substring from it that doesn't always start the same.
That was my message.

;JOOP!
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LVL 86

Expert Comment

by:CEHJ
ID: 11983649
I'm not sure it's an answer to the question asked, but .. OK ;-)
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