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@@ -1168,6 +1168,8 @@ SadActor
 
 纯粹的替代意味着派生类可以完美地替代基类，当使用它们时，完全不需要知道这些子类的信息。也就是说，基类可以接收任意发送给派生类的消息，因为它们具有完全相同的接口。只需将派生类向上转型，不要关注对象的具体类型。所有一切都可以通过多态处理。
 
+![](../images/1562409366638.png)
+
 按这种方式思考，似乎只有纯粹的“is - a”关系才是唯一明智的做法，其他任何设计只会导致混乱且注定失败。这其实也是个陷阱。一旦按这种方式开始思考，就会转而发现继承扩展接口（遗憾的是，extends 关键字似乎怂恿我们这么做）才是解决特定问题的完美方案。这可以称为“is - like - a” 关系，因为派生类就像是基类——它有着相同的基本接口，但还具有需要额外方法实现的其他特性：
 
 ![](../images/1562409366637.png)