Liskov Substitution Principle

This is part three of a five part series about SOLID class design principles by Robert C. Martin. The SOLID principles focus on achieving code that is maintainable, robust, and reusable. In this post, I will discuss the Liskov Substitution Principle.

The Liskov Substitution Principle (LSP): functions that use pointers to base classes must be able to use objects of derived classes without knowing it.

Subtype Requirement:

Let ${\displaystyle \phi (x)}$ be a property provable about objects ${\displaystyle x}$ of type T. Then ${\displaystyle \phi (y)}$ should be true for objects ${\displaystyle y}$ of type S where S is a subtype of T.  

When first learning about object oriented programming, inheritance is usually described as an "is a" relationship. If a penguin "is a" bird, then the Penguin class should inherit from the Bird class. The "is a" technique of determining inheritance relationships is simple and useful, but occasionally results in bad use of inheritance.

The Liskov Substitution Principle is a way of ensuring that inheritance is used correctly.