I have heard that the Liskov Substitution Principle (LSP) is a fundamental principle of object oriented design. What is it and what are some examples of its use?
A great example illustrating LSP (given by Uncle Bob in a podcast I heard recently) was how sometimes something that sounds right in natural language doesn't quite work in code.
In mathematics, a Square
is a Rectangle
. Indeed it is a specialization of a rectangle. The "is a" makes you want to model this with inheritance. However if in code you made Square
derive from Rectangle
, then a Square
should be usable anywhere you expect a Rectangle
. This makes for some strange behavior.
Imagine you had SetWidth
and SetHeight
methods on your Rectangle
base class; this seems perfectly logical. However if your Rectangle
reference pointed to a Square
, then SetWidth
and SetHeight
doesn't make sense because setting one would change the other to match it. In this case Square
fails the Liskov Substitution Test with Rectangle
and the abstraction of having Square
inherit from Rectangle
is a bad one.
Y'all should check out the other priceless SOLID Principles Motivational Posters.