Objective-C supports extensions of classes using categories. Class categories are defined similar as the class itself. The example defines a class MyClass. The header file declares variables myString and myInt, and methods setMyString and setMyInt.
As you all know an NSDictionary object stores a mutable set of entries. An NSDictionary is immutable. If you need a dictionary which you can change, then you should use NSMutableDictionary. NSMutableDictionary is a subclass of NSDictionary, so that you can do with an NSMutableDictionary everything which you can do with a simple NSDictionary plus, you can modify it.