August 2021
Intermediate to advanced
450 pages
4h 46m
Chinese
Content preview from 面向对象的思考过程（原书第5版）
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7.4.3 对象职责
我们来重新回顾第1章中的Shape例子，请见图7.11。
图7.11 Shape类继承体系图
多态性是对继承的最优雅的使用之一。请记住Shape类是不能被实例化的。由于它有个抽象方法getArea()，所以它是一个抽象类。第8章将解释抽象类的细节。
不过Rectangle和Circle可以被实例化，因为它们是实实在在的类。尽管矩形和圆都是形状，但它们有不同之处。它们作为形状，可以计算各自的面积。而各自计算面积的公式是不同的。因此，计算面积的公式不能放置在Shape类中。
这就是多态性的由来。多态性的前提是你可以给不同的对象发送消息，而对象则会根据各自的类型进行响应。例如，如果你向Circle类发送getArea()消息，调用的计算方式与向Rectangle类发送相同的getArea()消息的计算方式是不同的。这是因为Circle和Rectangle拥有各自的响应方式。如果你让Circle返回其面积，它知道怎么做。如果你让圆绘制自身，它也知道怎么做。即使Shape对象可以被实例化，它也做不到这些，因为它缺乏对自身足够的信息。注意图7.11中的UML图，Shape类中的getArea()方法是斜体的。这表示了该方法是抽象方法。
假设如下一个简单的示例，我们有四个类：抽象类Shape，实体类Circle、Rectangle和Star。以下是代码：
注意每个类中只存在一个方法：draw()。以下是对象自身职责与多态性相关的重要一点，实体类自身负责实现绘制功能。Shape类并不提供绘制的代码，Circle，Rectangle和Star类需要各自实现绘制代码。以下是证明这一点的一些代码： ...
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