2010/8/5 abhinav sarkar <abhinav.sar...@gmail.com>
> If two objects are equal, they must have same hash code. But it does not
> imply the other way too. Two objects having the same hash code may be not
> equal (see, hash collision). So this approach may give false positives.
>
No, you didn't read the thread carefully enough. The OP is suggesting a
fail-fast approach based on the comparison of the hash-code, not a
success-fast (which as you said, is not possible from hash codes)
>
> -
> Abhinav
>
>
> On Thu, Aug 5, 2010 at 1:45 PM, Nicolas Oury <nicolas.o...@gmail.com>wrote:
>
>> That's might be an interesting trade-of to try.
>> It is really good when big data structures are nearly equal. The
>> comparison is linear but the comparaison once you know the hash is
>> O(1) with a high probability.
>> It is bad on very different structure, if you force a hash.
>> Ideally, the equals method could hash as it goes. When 2 structures
>> are equal, their hashes would be known.
>> If not, by comparing the numbero fo elments hashed to the count, the
>> implementation could decide whether or not to continue hashing...
>>
>> On Wed, Aug 4, 2010 at 10:40 PM, sune.simonsen <sune.simon...@gmail.com>
>> wrote:
>> > I was looking around the Clojure source code and noticed that the
>> > persistent immutable collections caches their hash codes.
>> >
>> > The java doc says the following about the equals method: (http://
>> > download.oracle.com/javase/1.4.2/docs/api/java/lang/Object.html)
>> > "If two objects are equal according to the equals(Object) method, then
>> > calling the hashCode method on each of the two objects must produce
>> > the same integer result."
>> >
>> > So I was thinking, wouldn't it be possible to use the cached hash code
>> > to rule out cases of equals where equals returns false?
>> >
>> > public boolean equals(Object obj){
>> > ...
>> > if (_hash != -1 && hashCode() != obj.hashCode()) {
>> > return false;
>> > }
>> > ...
>> > }
>> >
>> > This might of cause result in an extra call to hashCode(), so I don't
>> > know if it is worth it.
>> > Maybe some internal functionality could be added, to ask the
>> > collections if their hash code has been cached.
>> >
>> > What are your thoughts?
>> >
>> > Kind regards Sune Simonsen
>> >
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