SOLID - Object-Oriented Design Principles

SOLID is a mnemonic acronym for five fundamental principles of object-oriented programming, popularized by Robert C. Martin (Uncle Bob). These principles guide the design of clean, modular code that resists change. When properly applied, they reduce technical debt and facilitate future evolution.

The Five SOLID Principles

• **S**ingle Responsibility Principle (SRP): A class should have only one reason to change
• **O**pen/Closed Principle (OCP): Open for extension, closed for modification
• **L**iskov Substitution Principle (LSP): Subtypes must be substitutable for their base types
• **I**nterface Segregation Principle (ISP): Prefer multiple specific interfaces over a general one
• **D**ependency Inversion Principle (DIP): Depend on abstractions, not concrete implementations

Benefits of Applying SOLID

• Significant improvement in code maintainability and readability
• Reduced coupling between components, facilitating unit testing
• Easier refactoring and addition of new features
• Fewer bugs related to side effects during modifications
• Better architectural scalability to support project growth

Practical Example: SRP Violation and Application

// ❌ SRP Violation: class with multiple responsibilities
class UserService {
  createUser(data: UserData) {
    // Validation
    if (!data.email.includes('@')) {
      throw new Error('Invalid email');
    }
    
    // Persistence
    database.save(data);
    
    // Notification
    emailService.send(data.email, 'Welcome!');
    
    // Logging
    logger.log(`User ${data.email} created`);
  }
}

// ✅ SRP Application: separation of concerns
class UserValidator {
  validate(data: UserData): boolean {
    return data.email.includes('@');
  }
}

class UserRepository {
  save(user: User): void {
    database.save(user);
  }
}

class UserNotifier {
  notifyCreation(email: string): void {
    emailService.send(email, 'Welcome!');
  }
}

class UserService {
  constructor(
    private validator: UserValidator,
    private repository: UserRepository,
    private notifier: UserNotifier
  ) {}
  
  createUser(data: UserData): void {
    if (!this.validator.validate(data)) {
      throw new Error('Invalid user data');
    }
    
    const user = new User(data);
    this.repository.save(user);
    this.notifier.notifyCreation(user.email);
  }
}

Progressive Implementation

1. Start with Single Responsibility Principle in new development
2. Identify variation points and apply Open/Closed to anticipate them
3. Use dependency injection to respect Dependency Inversion Principle
4. Review inheritance hierarchies to ensure Liskov Substitution
5. Break down large interfaces according to Interface Segregation
6. Integrate SOLID principles into code reviews and team standards

Related Tools and Practices

• Dependency injection frameworks (InversifyJS, TSyringe, NestJS)
• Static analyzers to detect violations (SonarQube, ESLint with architectural rules)
• Automated refactoring tools (IntelliJ IDEA, VS Code Refactoring tools)
• UML diagrams to visualize dependencies and abstractions
• Unit tests and mocks to validate responsibility isolation

SOLID principles represent a long-term investment that transforms a team's ability to deliver new features quickly. By reducing accidental complexity and promoting clear architecture, they enable maintaining high velocity even on mature projects. Mastering them differentiates senior developers capable of anticipating code evolution.