overriden isEqual method not getting called every time during comparisons

Question

I have a class called

Contact;

In Contact I have (simple version to test, no hash yet)

- (BOOL)isEqual:(id)other {
    if (other == self)
        return YES;
    if (!other || ![other isKindOfClass:[self class]])
        return NO;
    return NO;
}

I do:

NSMutableArray *arr = [[NSMutableArray alloc] init];
Contact *contact = [Contact new];
[arr addObject:contact]

// I expect my isEqual to be called here but it does not get called
[arr containsObject:contact] // this evaluates to true somehow!!!

However if I add a another object to type NSString, then it gets called for comparing String object but not for the contact object. Which means

[arr addObject:@""] // so now arr has two elements

// here I expect two calls to isEqual but only one gets there
// when comparing string object against Contact
[arr containsObject:contact]

Why is isEqual not getting called in cases I mentioned above??

Answer 1

Please read the discussion about isEqual: in the NSObject Protocol in the Reference Library.

You'll find that for objects which are inside a collection (such as an NSArray), hash might be used to determine whether two objects are actually the same. If two pointers are actually pointing to the same object, there is no need to check for equality - hence isEqual: never gets called.

The solution as suggested by the reference library is to implement hash in your subclass as well.