Liskov Substitution Principle (LSP)

Overview

Subtypes must be substitutable for their base types without altering program correctness.

If S is a subtype of T, objects of type T can be replaced with objects of type S without breaking the program. Subclasses must honor the contracts of their parent classes.

When to Use

• Creating a class that extends another class
• Overriding methods from a parent class
• Implementing an interface
• Feeling like you need to throw exceptions in overridden methods
• Inheritance hierarchy feels "forced"

The Iron Rule