SOLID Principles
Five design principles for writing maintainable, flexible, and scalable object-oriented software, introduced by Robert C. Martin.
Detailed Explanation
SOLID stands for: Single Responsibility (one class, one job), Open/Closed (open for extension, closed for modification), Liskov Substitution (subtypes must be substitutable), Interface Segregation (many specific interfaces over one general), and Dependency Inversion (depend on abstractions, not concretions).
SOLID principles guide object-oriented design toward code that is easy to maintain, extend, and test. They promote loose coupling, high cohesion, and separation of concerns. While originally for OOP, the principles apply to any programming paradigm.
Why It Matters
SOLID principles are the foundation of good software design. They prevent common design problems and make codebases maintainable over time.
Real-World Example
A payment processing class that handles payment logic, email notifications, and database storage violates SRP. SOLID refactoring separates these into: PaymentProcessor (logic), EmailService (notifications), and PaymentRepository (storage).
When to Use
When designing classes, modules, and systems. Apply SOLID principles during design and code review. Refactor code that violates SOLID principles.
Advantages
- Easier to maintain and extend
- Promotes loose coupling
- Improves testability
- Reduces ripple effects from changes
- Provides clear design guidelines
Disadvantages
- Can lead to over-engineering
- Learning curve for beginners
- Too many classes/interfaces for simple problems
- May not map directly to all paradigms
- Requires discipline to apply consistently
Related Terms
Frequently Asked Questions
What is the Single Responsibility Principle?
A class should have only one reason to change—meaning it has only one job. If a class handles payment processing AND email notifications, it violates SRP. Split it into separate classes with distinct responsibilities.
How do SOLID principles help with testing?
SOLID code is easier to test: small, focused classes with clear interfaces are easier to mock and verify. Dependency Inversion lets you inject mocks instead of real dependencies.
Are SOLID principles applicable to functional programming?
Many principles translate: SRP (small functions), OCP (pure functions are easy to extend), LSP (polymorphism), ISP (specific function interfaces), DI (function composition). SOLID's intent transcends OOP.
Can I apply SOLID to a monolith?
Yes. SOLID applies at every level: classes, modules, and services. A well-structured monolith with SOLID principles is easier to maintain and can be decomposed into microservices later.
What is Dependency Inversion?
High-level modules should not depend on low-level modules—both should depend on abstractions. In practice: define interfaces, implement them separately, and inject implementations. This decouples components and improves testability.