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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第9章 创建对象及面向对象设计
前面两个章节讲述了继承和组合这两个主题。第7章中我们学到了继承和组合是创建对象的两种主要方式。而第8章中我们学习了不同程度的继承，以及继承、接口、抽象类和组合是如何在一起使用的。
本章会讨论在整体设计中对象之间是如何相互关联的。你可能认为之前已经讲过该问题，的确如此。继承和组合代表了对象交互的方式。然而继承和组合在构建对象时有一处明显的不同。当使用继承时（至少从概念上来说）最终结果是子类包括了整个继承体系中的所有行为和属性。而当使用组合时，会用一些类构建其他类。
虽然继承确实表示了两个类之间的关系，但真正的情况是创建了一个父类，它包含了子类的属性和方法。我们再来看Person（人）和Employee（雇员）类的例子（见图9.1）。
尽管这两个类是相互独立的类，但它们之间的关系并不简单——它们之间是继承关系。通常来说一名雇员是一个人。Employee对象不会向Person对象发送消息，而且Employee对象不需要Person对象的服务。这是因为Employee对象就是Person对象。
然而组合是不同的情形。组合代表了不同对象之间的交互。第8章主要讲述了继承的多样性，本章会深入了解组合的多样性，以及对象之间如何进行交互。
图9.1 继承体系关系示例
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