[9] Review request for 8140503: JavaFX "Pair" Hash Code Collisions

Vadim Pakhnushev vadim.pakhnushev at oracle.com
Tue Nov 3 20:07:35 UTC 2015

Hmm, yeah, the actual difference is in the prime number only (that is 
changing the algorithm only doesn't improve anything), so the only 
remaining reason to fix this is that Objects.hash guards against null 
values (and I forgot to mention it in the review).
The key in Pair could actually be null and in this case hashCode will 
throw NPE.


On 03.11.2015 23:01, Vadim Pakhnushev wrote:
> Well, not exactly... Previously it was 13*hash(a) + hash(b) and now 
> it's 31*(31 + hash(a)) + hash(b).
> And apparently it improves the quality somehow. I did a test with 
> 100^4 combinations and collision probability dropped by the factor of 
> 3 from 0.065% to 0.022%.
> Not really impressive, but still, and it uses well-defined utility 
> method.
> Yeah, I know it's not really a bug since you don't want to rely on the 
> hashCode at all...
> Thanks,
> Vadim
> On 03.11.2015 22:35, Jim Graham wrote:
>> All this does is change the prime constant used to produce the hash 
>> value.
>> Objects.hash(a, b) uses 31*hash(a) + hash(b) instead of the 
>> 13*hash(a) + hash(b) that the embedded implementation uses.
>> I don't really think this is a bug.  The fact that Integer objects 
>> make it easy to reverse engineer and compute collisions of any 
>> reasonable hash combination computation don't mean that the technique 
>> has a bug, it just means that the submitter can read the code and 
>> think of a counter-example.
>> If there are practical problems being caused for some particular and 
>> popular use case by the use of this particular constant "13", then we 
>> need to understand those issues and come up with a more comprehensive 
>> solution than to simply hand off to another mechanism which uses the 
>> same procedure with a different prime constant...
>>             ...jim
>> On 11/3/15 3:06 AM, Vadim Pakhnushev wrote:
>>> Hi Chien,
>>> Could you please review the fix:
>>> https://bugs.openjdk.java.net/browse/JDK-8140503
>>> http://cr.openjdk.java.net/~vadim/8140503/webrev.00/
>>> Thanks,
>>> Vadim

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