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今天开始讲Objective-C的集合，今天先大概看下Overview。

Most Collections Are Objects

Although it’s possible to use a C array to hold a collection of scalar values, or even object pointers, most collections in Objective-C code are instances of one of the Cocoa and Cocoa Touch collection classes, like NSArray, NSSet and NSDictionary.

These classes are used to manage groups of objects, which means any item you wish to add to a collection must be an instance of an Objective-C class. If you need to add a scalar value, you must first create a suitable NSNumber or NSValue instance to represent it.

Rather than somehow maintaining a separate copy of each collected object, the collection classes use strong references to keep track of their contents. This means that any object that you add to a collection will be kept alive at least as long as the collection is kept alive, as described in Manage the Object Graph through Ownership and Responsibility.

In addition to keeping track of their contents, each of the Cocoa and Cocoa Touch collection classes make it easy to perform certain tasks, such as enumeration, accessing specific items, or finding out whether a particular object is part of the collection.

The basic NSArray, NSSet and NSDictionary classes are immutable, which means their contents are set at creation. Each also has a mutable subclass to allow you to add or remove objects at will.

For more information on the different collection classes available in Cocoa and Cocoa Touch, see Collections Programming Topics.