Such operations will appear in features of other classes, called the clients of class COUNTER.
这些操作可以出现在其它类的特性中,叫做类COUNTER的委托。
This interface defines the contract between the class and any of its clients.
此接口将在类与其所有使用者之间定义契约。
Design by contract avoids extra code and improves performance, as long as all clients of a class abide by the contract.
契约式设计避免了额外的编码并提高了性能,只要所有的客户都遵守契约。
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