SOLID Principles in Software Engineering: Write Clean, Maintainable Code

Writing code that works is easy. Writing code that is maintainable, extensible, and easy to refactor when requirements change is what defines a senior software engineer. The SOLID principles, introduced by Robert C. Martin (Uncle Bob), provide the blueprint for clean Object-Oriented design.

---

1. Single Responsibility Principle (SRP)

"A class should have one, and only one, reason to change."

• ❌ Violation: A UserService class that handles user registration, sends emails, and formats PDF reports.
• ✅ Refactored: Break into UserRepository, EmailNotificationService, and UserPdfReportGenerator.

---

2. Open/Closed Principle (OCP)

"Software entities should be open for extension, but closed for modification."

You should be able to add new functionality without altering existing, tested code.

// Using Strategy Pattern for Payment Processors
interface PaymentProcessor {
    processPayment(amount: number): boolean;
}

class StripeProcessor implements PaymentProcessor {
    processPayment(amount: number): boolean {
        // Stripe integration logic
        return true;
    }
}

class RazorpayProcessor implements PaymentProcessor {
    processPayment(amount: number): boolean {
        // Razorpay integration logic
        return true;
    }
}

---

3. Liskov Substitution Principle (LSP)

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

If class B is a subclass of A, any function expecting A should work seamlessly with B without throwing unexpected exceptions.

---

4. Interface Segregation Principle (ISP)

"Clients should not be forced to depend on methods they do not use."

Instead of one monolithic interface with 20 methods, create smaller, cohesive interfaces.

---

5. Dependency Inversion Principle (DIP)

"High-level modules should not depend on low-level modules. Both should depend on abstractions."

Always inject dependencies via constructors or interfaces rather than instantiating concrete classes directly inside business logic.