Liskov Substitution Principle

1. Introduction

The Liskov Substitution Principle (LSP) is one of the five SOLID principles of object-oriented design. It focuses on ensuring that a subclass can stand in for its parent class without altering the desirable properties of the program.

2. Definition

The Liskov Substitution Principle states that:

“If S is a subtype of T, then objects of type T may be replaced with objects of type S without altering any of the desirable properties of the program.”

3. Key Concepts

• **Substitutability**: Subtypes should be substitutable for their base types.
• **Behavior Preservation**: The behavior of the program should remain consistent when a subclass is used in place of a superclass.
• **Contract Compliance**: Subclasses must adhere to the contract defined by the parent class.

4. Code Examples

class Bird {
    fly() {
        console.log("I can fly!");
    }
}

class Sparrow extends Bird {
    fly() {
        console.log("Sparrow flying!");
    }
}

function letBirdFly(bird: Bird) {
    bird.fly();
}

let mySparrow = new Sparrow();
letBirdFly(mySparrow); // Output: Sparrow flying!
            

In this example, the Sparrow class correctly extends Bird, maintaining the behavior expected by the letBirdFly function.

5. Best Practices

• Always ensure that subclasses adhere strictly to the expectations of their base class.
• Use interfaces to define contracts that subclasses must follow.
• Test subclasses to ensure they can replace their superclasses without issues.
• Avoid overriding methods in a way that changes expected behavior.

6. FAQ