Subclassing an Objective-C Class
Classes, like NSMutableArray, exist in a hierarchy, and every class has exactly one superclass – except for the root class of the entire hierarchy: NSObject (Figure 2.8). A class inherits the behavior of its superclass, which means, at a minimum, every class inherits the methods and instance variables defined in NSObject.
As the top superclass, NSObject’s role is to implement the basic behavior of every object in Cocoa Touch. Three of the methods NSObject implements are alloc, init, and description. (We sometimes say “description is a method on NSObject” and mean the same thing.)
Figure 2.8 Class hierarchy
A subclass ...