You've actually seen one instance of another class hierarchy that also serves as a good illustration of the idea of extension by subclassing.

你已经看到了另一个类层次的实例，它很好的阐明了子类化的概念。

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A class that implements Runnable can run without subclassing Thread by instantiating a Thread instance and passing itself in as the target.

实现Runnable的类可以不继承Thread而实例化为Thread实例，将自身作为运行目标。

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