A class that provides services according to a protocol defined in a protocol class.

参考协议类中定义的协议来提供服务的类。

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Any behavior of the class must be implemented as protocol methods.

类的任何行为都必须作为协议方法实现。

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Because this specification includes a behavioral protocol, we model it using an abstract class that defines the roles (properties) involved in the service protocol.

由于该规范包括行为协议，所以使用一个对服务协议中涉及到的角色(属性)进行定义的抽象类来进行建模。

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