今天讲Objective-C的类的继承，以及通过看NSMutableString和NSString两个类，来深入学习Objective-C的类的继承。其实和java的类的继承是一样的。
Mutability Determines Whether a Represented Value Can Be Changed
Some classes define objects that are immutable. This means that the internal contents must be set when an object is created, and cannot subsequently be changed by other objects. In Objective-C, all basic NSString and NSNumber objects are immutable. If you need to represent a different number, you must use a new NSNumber instance.
Some immutable classes also offer a mutable version. If you specifically need to change the contents of a string at runtime, for example by appending characters as they are received over a network connection, you can use an instance of the NSMutableString class. Instances of this class behave just like NSString objects, except that they also offer functionality to change the characters that the object represents.
Although NSString and NSMutableString are different classes, they have many similarities. Rather than writing two completely separate classes from scratch that just happen to have some similar behavior, it makes sense to make use of inheritance.
Classes Inherit from Other Classes
In the natural world, taxonomy classifies animals into groups with terms like species, genus, and family. These groups are hierarchical, such that multiple species may belong to one genus, and multiple genera to one family.
Gorillas, humans, and orangutans, for example, have a number of obvious similarities. Although they each belong to different species, and even different genera, tribes, and subfamilies, they are taxonomically related since they all belong to the same family (called “Hominidae”), as shown in Figure 1-1.
Figure 1-1 Taxonomic relationships between species
In the world of object-oriented programming, objects are also categorized into hierarchical groups. Rather than using distinct terms for the different hierarchical levels such as genus or species, objects are simply organized into classes. In the same way that humans inherit certain characteristics as members of the Hominidae family, a class can be set to inherit functionality from a parent class.
When one class inherits from another, the child inherits all the behavior and properties defined by the parent. It also has the opportunity either to define its own additional behavior and properties, or override the behavior of the parent.
In the case of Objective-C string classes, the class description for NSMutableString specifies that the class inherits from NSString, as shown in Figure 1-2. All of the functionality provided by NSString is available in NSMutableString, such as querying specific characters or requesting new uppercase strings, but NSMutableString adds methods that allow you to append, insert, replace or delete substrings and individual characters.
Figure 1-2 NSMutableString class inheritance
