Liskov Substitution Principle (LSP) – Complete Guide

What is LSP?

The Liskov Substitution Principle (LSP) is a core concept in SOLID programming. It states that objects of a superclass should be replaceable with objects of a subclass without affecting program correctness. LSP ensures predictable behavior in inheritance hierarchies.

Why LSP is Important

• Enables safe polymorphism.
• Prevents unexpected behavior when using subclasses.
• Ensures maintainable and extendable code.
• Supports reliable inheritance hierarchies.

Stepwise Implementation of LSP

1. Step 1 – Identify Base Class Contracts: Understand what behavior the base class promises.
2. Step 2 – Create Subclasses Correctly: Ensure subclasses fulfill all base class contracts.
3. Step 3 – Avoid Breaking Overrides: Do not override methods in a way that changes expected behavior.
4. Step 4 – Test Polymorphism: Replace base class objects with subclass objects and verify correctness.
5. Step 5 – Refactor Violations: Split or redesign classes to maintain substitutability.

Example: LSP in Action

Suppose we have a payment system:

• Base class: PaymentMethod
• Subclass 1: CreditCardPayment
• Subclass 2: PayPalPayment

Each subclass can replace PaymentMethod without breaking the system:

class PaymentMethod {
    public void pay(double amount) {
        // base payment logic
    }
}

class CreditCardPayment extends PaymentMethod {
    @Override
    public void pay(double amount) {
        // credit card payment logic
    }
}

class PayPalPayment extends PaymentMethod {
    @Override
    public void pay(double amount) {
        // PayPal payment logic
    }
}
      

This way, any subclass can substitute the base class without breaking functionality, following LSP.