Question

So i’m overriding isEquals and hash to compare custom objects to be able to remove duplicates from a NSArray. The problem is that i’m missing some values in the list which contains no duplicated items, and it seems that my hash or isEquals implementation is wrong. The custom object is a Course object which has some variables like: id and name I’ll put the code here:

- (BOOL)isEqual:(id)object {
    if ([object isKindOfClass:[Course self]]) {
        return YES;
    } 
    if(self == object){
        return YES;
    }
    else {
        return NO;
    }
}

- (unsigned)hash {

    NSString *idHash = [NSString stringWithFormat: @"%d", self._id];
    return [idHash hash];
}

Then, after querying the database i put the values in an array and then in a set which should remove the duplicated items like this:

NSMutableSet *noDuplicates = [[NSMutableSet alloc] initWithArray:tempResults];

Can you see what i’m doing wrong in the isEquals or hash implementation?

Thanks a lot.

Answer 1

this is how you implement hash and isEqual (at-least the one which is working for me for purpose of identifying duplicates)

Hash Function

The Apple Doc says that the hash of two objects should be same for those which are considered equal( logically). hence I would implement the hash as below

-(unsigned int)hash  
{  
    return 234;//some random constant  
} 

isEqual: method implemenation would be something like

-(BOOL)isEqual:(id)otherObject  
{  
  MyClass *thisClassObj = (MyClass*)otherObject;

  *// this could be replaced by any condition statement which proves logically that the two object are same even if they are two different instances* 

return ([thisClassObj primaryKey] == [self primaryKey]);

}

More reference here : Techniques for implementing -hash on mutable Cocoa objects

Implementing -hash / -isEqual: / -isEqualTo…: for Objective-C collections