What is the difference between objectForKey and valueForKey?

Q: What is the difference between objectForKey and valueForKey? I looked both up in the documentation and they seemed the same to me.

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A: "objectForKey:" is an NSDictionary method. An NSDictionary is a collection class similar to an NSArray, except instead of using indexes, it uses keys to differentiate between items. A key is an arbitrary string you provide. No 2 objects can have the same key (just as no 2 objects in an NSArray can have the same index).

"valueForKey: is KVC method. It works with ANY class. "valueForKey" allows you to access an instance variable using a string. So for instance, if I have an Account class with an instance variable accountNumber, I can do the following:

NSNumber *anAccountNumber = [NSNumber numberWithInt:12345];
Account *newAccount = [[Account alloc] init];
[newAccount setAccountNumber:anAccountNUmber];
NSNumber *anotherAccountNumber = [newAccount accountNumber];

Using KVC, I can also do it like this:

NSNumber *anAccountNumber = [NSNumber numberWithInt:12345];
Account *newAccount = [[Account alloc] init];
[newAccount setValue:anAccountNumber forKey:@"accountNumber"];
NSNumber *anotherAccountNumber = [newAccount valueForKey:@"accountNumber"];

Those are equivalent sets of statements.

I know you're thinking: wow, but sarcastically. KVC doesn't look all that useful. In fact, it looks "wordy". But when you want to change things at runtime, you can do lots of cool things that are much more difficult in other languages (but this is beyond the scope of your question).

If you want to learn more about KVC, there are many tutorials if you Google especially at Scott Stevenson's blog.

Hope that helps.