Understanding SOLID Principles in Software Development

Introduction

As software engineers, writing maintainable and scalable code is crucial. The SOLID principles provide a strong foundation for designing robust, flexible, and extensible object-oriented systems. These principles help prevent tightly coupled code and make applications easier to maintain and test.

In this post, we’ll explore what SOLID is and how each principle improves software design.

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What Are SOLID Principles?

SOLID is an acronym representing five key principles of object-oriented design (OOD) introduced by Robert C. Martin (Uncle Bob).

• S – Single Responsibility Principle (SRP)
• O – Open/Closed Principle (OCP)
• L – Liskov Substitution Principle (LSP)
• I – Interface Segregation Principle (ISP)
• D – Dependency Inversion Principle (DIP)

Let’s break down each principle with examples.

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1. Single Responsibility Principle (SRP)

“A class should have only one reason to change.”

A class should focus on only one functionality and not handle multiple concerns.

Example: Instead of having a class that handles both user authentication and report generation, split them into separate classes.

Bad Example:

jsCopyEditclass User {
    login() { /* logic */ }
    logout() { /* logic */ }
    generateReport() { /* logic */ } // ❌ Violates SRP
}

Good Example:

jsCopyEditclass AuthService {
    login() { /* logic */ }
    logout() { /* logic */ }
}

class ReportService {
    generateReport() { /* logic */ }
}

Now, each class has one responsibility, making it easier to modify and test.

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2. Open/Closed Principle (OCP)

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

A class should allow new functionality without altering existing code.

Bad Example:

jsCopyEditclass PaymentProcessor {
    processPayment(type) {
        if (type === "PayPal") { /* PayPal logic */ }
        else if (type === "Stripe") { /* Stripe logic */ }
    }
}

Good Example (Using Polymorphism):

jsCopyEditclass Payment {
    process() {}
}

class PayPal extends Payment {
    process() { /* PayPal logic */ }
}

class Stripe extends Payment {
    process() { /* Stripe logic */ }
}

// No need to modify existing classes to add new payment methods

This allows us to extend functionality (add new payment methods) without modifying existing code.

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3. Liskov Substitution Principle (LSP)

“Derived classes should be substitutable for their base classes.”

A subclass should extend the behavior of the parent class without breaking existing functionality.

Bad Example: (Subclass changes behavior)

jsCopyEditclass Bird {
    fly() { /* flying logic */ }
}

class Penguin extends Bird {
    fly() { throw new Error("Penguins can't fly!"); } // ❌ Violates LSP
}

Good Example: (Using composition instead of inheritance)

jsCopyEditclass Bird {}

class FlyingBird extends Bird {
    fly() { /* flying logic */ }
}

class Penguin extends Bird {
    swim() { /* swimming logic */ }
}

Now, Penguin does not incorrectly inherit behavior it cannot support.

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4. Interface Segregation Principle (ISP)

“A class should not be forced to implement interfaces it does not use.”

Avoid creating large, bloated interfaces that force classes to implement unnecessary methods.

Bad Example:

jsCopyEditclass MultiFunctionPrinter {
    print() { /* logic */ }
    scan() { /* logic */ }
    fax() { /* logic */ }
}

class SimplePrinter extends MultiFunctionPrinter {
    fax() { throw new Error("Fax not supported!"); } // ❌ Violates ISP
}

Good Example:

jsCopyEditclass Printer {
    print() { /* logic */ }
}

class Scanner {
    scan() { /* logic */ }
}

class Fax {
    fax() { /* logic */ }
}

class MultiFunctionPrinter extends Printer, Scanner, Fax {} // Flexible
class SimplePrinter extends Printer {} // Only prints

Now, each class only implements the methods it actually needs.

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5. Dependency Inversion Principle (DIP)

“Depend on abstractions, not on concretions.”

High-level modules should not depend on low-level modules directly. Instead, both should depend on abstractions (interfaces).

Bad Example: (Tightly coupled)

jsCopyEditclass MySQLDatabase {
    connect() { /* connection logic */ }
}

class UserService {
    constructor() {
        this.db = new MySQLDatabase(); // ❌ Direct dependency
    }
}

Good Example: (Loosely coupled using dependency injection)

jsCopyEditclass Database {
    connect() {}
}

class MySQLDatabase extends Database {
    connect() { /* MySQL logic */ }
}

class UserService {
    constructor(db) {
        this.db = db; // ✅ Injecting dependency
    }
}

Now, UserService works with any database implementation (MySQL, MongoDB, etc.).

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Conclusion

The SOLID principles help us write maintainable, scalable, and flexible code. By following them, we avoid code smells, reduce technical debt, and make our applications more robust.