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）Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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1.11.1 抽象
就像继承一样，组合也是一种构建对象的机制。事实上，我想说只有两种方式来使用其他类构建新类，它们就是继承和组合。之前已经看到了，继承允许一个类继承另一个类。我们可以把属性和行为提取到通用类中。例如，狗和猫都是哺乳动物，因为狗是（is-a）哺乳动物，猫也是（is-a）哺乳动物。而使用组合，我们也可以把类嵌入到其他类中来构造新类。
我们可以拿车和引擎之间的关系来举例。把引擎从车中分离出来是明显可行的。我们可以单独构建引擎，把它用到各种车上，更别提还有其他优势。但我们不能说引擎是（is-a）一辆车。顺口说出来我们就觉得不对（因为我们在为真实世界的系统建模，这正是我们想要的效果）。我们最好用has-a术语来描述组合关系。车有（has-a）引擎。
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