Introduction to SOLID
What is SOLID?
SOLID is an acronym for five design principles intended to make software designs more understandable, flexible, and maintainable. Coined by Robert C. Martin (Uncle Bob), these principles have become the gold standard for Object-Oriented Programming (OOP) and software architecture.
The 5 principles are:
- 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)
Why do we need SOLID?
When building small scripts or prototypes, architecture doesn’t matter much. But as a codebase grows to hundreds of thousands of lines of code with multiple developers contributing, code rot sets in:
- Rigidity: Changing one piece of code breaks other unrelated parts.
- Fragility: The code is impossible to test.
- Immobility: Code cannot be reused because it is highly entangled with its current context.
SOLID principles combat this by promoting loose coupling and high cohesion. Following them ensures that your application is modular, testable, and capable of adapting to new business requirements without rewriting existing logic.
Interview Questions
Q: Are SOLID principles only applicable to Object-Oriented Programming?
A: While historically associated with OOP (classes and interfaces), the core concepts of SOLID apply universally to any paradigm. For example, in Functional Programming or React Hooks, you still want functions that do one thing (SRP), take abstract callbacks instead of hardcoded dependencies (DIP), and can be extended without modifying their core logic (OCP).
Q: Is it possible to over-engineer a system using SOLID?
A: Yes. Applying SOLID principles everywhere prematurely can lead to excessive abstractions, too many tiny files, and unnecessary interfaces. You should apply them pragmatically when you anticipate the need for flexibility, rather than blindly implementing them from day one (YAGNI - You Aren’t Gonna Need It).