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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1.9.3 is-a关系
在Shape（形状）例子中，Circle（圆形）、Square（矩形）和Star（星形）都直接继承自Shape。这种关系通常被称为is-a关系，因为圆是一个形状，而矩形也是形状。当子类继承自父类时，任何父类能做的事情子类都可以做。即Circle、Square和Star都是Shape的扩展。
在图1.17中，每个对象上的名字分别代表了Circle、Start和Square对象的draw方法。当我们设计Shape系统时，这种方式对各种各样的形状进行标准化非常有帮助。如果我们约定绘制形状时只需调用draw方法，而不用管具体是什么形状。那么只要遵守这一约定，无论什么时候绘制形状，只需调用draw方法即可。这正好展示了多态性的基本概念，即绘制Circle、Star或Square是各自对象自身的职责。在如今很多软件应用（比如在绘图和文字处理应用中）中都会使用多态性概念。
图1.17 Shape继承体系图
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