NSObject 其实 实现了 NSObject协议，并不像java里的 Object 类一样。此协议定义了如下方法：
Identifying Classes
- required method
- required method
Identifying and Comparing Objects
- required method
- required method
- required method
Managing Reference Counts
- required method
- required method
- required method
- required method
Testing Object Inheritance, Behavior, and Conformance
- required method
- required method
- required method
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Describing Objects
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Sending Messages
- required method
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- required method
Determining Allocation Zones
- required method