> For the complete documentation index, see [llms.txt](https://hanfak.gitbook.io/workspace/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hanfak.gitbook.io/workspace/general-paradigms/principles/solid/3-lsp.md).

# 3. Liskov Substitution Principle

* calling code should be able to use an instance of a base class or an instance of a derived class without knowing it, or having to do anything special.
* It says that if the X class inherits from the Y class, then Y's entire black-box test suit should also pass for X.
* Example
  * A square is not a rectangle
* As long as an interface doesn't change, the clients don't care if you change the implementation
* if code starts to look like if (shape is Square) ... else if (shape is Rectangle) ..., it can be a “code smell”
* Why?
  * reduce complexity in calling code so that it can work generically on any class or subclass without additional conditional code.
  *

## Testing

* <https://blog.caplin.com/2010/09/30/a-junit-trick-for-ensuring-solid-design/>

## Issues

* Sometimes you do want to have different behaviour to the super class
* It was original stated by
* Even the author says, the idea of substitutability was informal and was not evening in formal definition (<https://dl.acm.org/doi/10.1145/197320.197383>, <https://www.youtube.com/watch?v=-Z-17h3jG0A>)

## Links

* <https://stackify.com/solid-design-liskov-substitution-principle/>
* <https://dzone.com/articles/solid-principles-liskov-substitution-principle>
* <https://medium.com/hackernoon/the-liskov-substitution-principle-and-why-you-might-want-to-enforce-it-6f5bbb05c06d>
* <https://reflectoring.io/lsp-explained/>