What Is SOLID Principles?
SOLID Design Principles
SOLID is five OOP design principles that make software maintainable, flexible, and scalable. Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. They guide how to structure classes and modules to minimize coupling and maximize cohesion.
How SOLID Principles Works
Single Responsibility: a UserService handles user logic, not email sending. Open/Closed: add new payment methods by implementing a PaymentProcessor interface, not modifying existing code. Dependency Inversion: depend on abstractions (interfaces), not concrete implementations — makes testing and swapping implementations easy.
Key Concepts
- Single Responsibility — A class should have one reason to change — do one thing well
- Open/Closed — Open for extension, closed for modification — add behavior without changing existing code
- Liskov Substitution — Subtypes must be substitutable for their base types without breaking the program
- Interface Segregation — Many specific interfaces are better than one general-purpose interface
- Dependency Inversion — Depend on abstractions, not concrete implementations — enables testability and flexibility
Frequently Asked Questions
Do I need to follow SOLID religiously?
No. SOLID principles are guidelines, not rules. Apply them where they add clarity and flexibility. Over-applying SOLID creates unnecessary abstraction layers.