从GraphNode派生BinaryTreeNode违反了Liskov的替代原理 [英] Is deriving BinaryTreeNode from GraphNode a violation of Liskov's Substitution Princple

问题描述

讨论在这里出现:

更改继承类中方法的可见性

问题是:"BTNode扩展GraphNode"设计真的违反了Liskov的Substitution Princeple吗?作为一个类似"的例子，表明了这种情况: 从矩形派生正方形违反了Liskov的替换原理?

但是我真的看不出为什么这是相似的.我对设计非常陌生，有人可以向我解释为什么(如果这样)吗?

解决方案

在 Wikipedia

中的Liskov替换原则

子类型要求:令ϕ(x)是关于类型T的对象x的可证明性质. ϕ ( ÿ )对于对象应该是正确的 类型S的y，其中S是T的子类型.

简单来说，

如果您可以对T类型执行操作X，那么您也应该可以对T的任何子类执行操作X.

The discussion comes up here:

Changing visibility of method in inherited class

question is: is really "BTNode extends GraphNode" design a violation of Liskov's Substitution Princeple? As an "similar" example it was shown that case: Is deriving square from rectangle a violation of Liskov's Substitution Principle?

but I cannot really see why that is similar. I am very new to design, could someone explain me why (if) that's the case?

解决方案

In Is deriving square from rectangle a violation of Liskov's Substitution Principle?, it basically says that Square cannot inherit from Rectangle because there are things that you can do with Rectangles but not with Squares - setting its width to a different number from its height.

You can't inherit BTNode from GraphNode because according to the original post you linked, GraphNode has a method called addChild. BTNode on the other hand, can only have two children. Inheriting from GraphNode will also inherit the addChild method. This allows you to add multiple children to BTNode, which a BTNode cannot handle.

Therefore, BTNode cannot inherit from GraphNode because there are things that you can do with GraphNodes but not with BTNodes - adding multiple children.

For completeness, here is the Liskov's Substitution principle from Wikipedia

Subtype Requirement: Let ϕ(x) be a property provable about objects x of type T. Then ϕ ( y ) should be true for objects y of type S where S is a subtype of T.

In simple terms,

If you can do action X on type T, you should also be able to do action X on any subclasses of T.

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