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今天講Objective-C類名唯一的問題，這一點和java不一樣。也順帶講下方法名、屬性名唯一的問題。另外，會講下Objective-C的很多類為什麼以NS開頭。

Class Names Must Be Unique

It’s important to note that the name of each class must be unique within an app, even across included libraries or frameworks. If you attempt to create a new class with the same name as an existing class in a project, you’ll receive a compiler error.

For this reason, it’s advisable to prefix the names of any classes you define, using three or more letters. These letters might relate to the app you’re currently writing, or to the name of a framework of reusable code, or perhaps just your initials.

All examples given in the rest of this document use class name prefixes, like this:

@interface XYZPerson : NSObject
@property (readonly) NSString *firstName;
@property (readonly) NSString *lastName;
@end
           

Historical Note: If you’re wondering why so many of the classes you encounter have an NS prefix, it’s because of the past history of Cocoa and Cocoa Touch. Cocoa began life as the collected frameworks used to build apps for the NeXTStep operating system. When Apple purchased NeXT back in 1996, much of NeXTStep was incorporated into OS X, including the existing class names. Cocoa Touch was introduced as the iOS equivalent of Cocoa; some classes are available in both Cocoa and Cocoa Touch, though there are also a large number of classes unique to each platform.

Two-letter prefixes like NS and UI (for User Interface elements on iOS) are reserved for use by Apple.

Method and property names, by contrast, need only be unique within the class in which they are defined. Although every C function in an app must have a unique name, it’s perfectly acceptable (and often desirable) for multiple Objective-C classes to define methods with the same name. You can’t define a method more than once within the same class declaration, however, though if you wish to override a method inherited from a parent class, you must use the exact name used in the original declaration.

As with methods, an object’s properties and instance variables (described in Most Properties Are Backed by Instance Variables) need to be unique only within the class in which they are defined. If you make use of global variables, however, these must be named uniquely within an app or project.

Further naming conventions and suggestions are given in Conventions.