Thanks, I understand now :) On 26/05/15 21:06, Carlo wrote:
Hi
Any two objects that are = must answer the same #hash value. The hash is used in any of the HashedCollection's for storage and then to find the exact element the #= is used. If 2 objects are #= but their hash values are different the HashedCollection will not find the correct storage slot and you will have undefined behaviour when looking up objects.
Out of interest, Andres Valloud wrote a whole book on 'Hashing in Smalltalk' (http://www.lulu.com/content/1455536)
Also the Javadoc comments are pretty good in explaining the usage:
The general contract of hashCode is: Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application. 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. It is not required that if two objects are unequal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables. As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. Cheers Carlo
On 26 May 2015, at 8:45 PM, Julien Delplanque <julien@tamere.eu> wrote:
The subject of this mail is exactly my question.
I came to this question by looking at Object>>#= message implementation.
""" = anObject "Answer whether the receiver and the argument represent the same object. If = is redefined in any subclass, consider also redefining the message hash."
^self == anObject """
When do we need to redefine #hash message?
Is it the right way to implement equality between two objects or is there another message that I should override?
Regards, Julien