🧱 SOLID Principles
The SOLID principles are five foundational guidelines for designing maintainable, flexible, and robust object‑oriented software. They help you avoid tightly coupled, fragile code and instead build systems that are easy to extend and modify over time.
SOLID principles — definition
Section titled “SOLID principles — definition”SOLID is an acronym for five design principles that guide how classes, modules, and objects should be structured and how responsibilities should be distributed.
They answer:
- How do we keep classes focused?
- How do we avoid breaking existing code when adding new features?
- How do we reduce coupling and increase flexibility?
The SOLID principles
Section titled “The SOLID principles”1. Single Responsibility Principle
Section titled “1. Single Responsibility Principle”A class should have one and only one reason to change.
It should do one thing well.
This improves cohesion and reduces complexity.
2. Open/Closed Principle
Section titled “2. Open/Closed Principle”Software entities should be open for extension but closed for modification.
You add new behavior by extending existing code, not by changing it.
This prevents regressions and supports plug‑in architectures.
3. Liskov Substitution Principle
Section titled “3. Liskov Substitution Principle”Subtypes must be substitutable for their base types without breaking behavior.
If B extends A, then objects of type B should behave like A.
This ensures reliable inheritance and polymorphism.
4. Interface Segregation Principle
Section titled “4. Interface Segregation Principle”Clients should not be forced to depend on interfaces they do not use.
Prefer small, focused interfaces over large, general ones.
This reduces unnecessary coupling.
5. Dependency Inversion Principle
Section titled “5. Dependency Inversion Principle”High‑level modules should not depend on low‑level modules.
Both should depend on abstractions.
This enables loose coupling, testability, and flexibility.
Why SOLID matters
Section titled “Why SOLID matters”SOLID helps you:
- Reduce coupling
- Increase cohesion
- Improve testability
- Make code easier to extend
- Avoid breaking existing functionality
- Enable clean architecture and modular design
- Support long‑term maintainability
SOLID is especially important in large systems where many developers collaborate.
How SOLID relates to architecture
Section titled “How SOLID relates to architecture”SOLID operates at the code level, while architecture operates at the system level.
- Architecture defines boundaries, modules, services.
- SOLID defines how classes inside those modules should behave.
SOLID is the foundation for implementing clean architecture, hexagonal architecture, and domain‑driven design.
Examples
Section titled “Examples”If you apply SOLID to a Daily Scrum application:
- SRP → separate classes for summary generation, PDF export, notifications
- OCP → add new summary formats without modifying existing logic
- LSP → ensure different summary generators behave consistently
- ISP → separate interfaces for PDF export, summary generation, notifications
- DIP → inject abstractions (interfaces) into services for testability
SOLID keeps your codebase clean and evolvable.
Takeaway
Section titled “Takeaway”SOLID principles are five essential guidelines for writing flexible, maintainable, and robust object‑oriented code. They reduce coupling, increase cohesion, and make your system easier to evolve without breaking existing functionality.