SOLID Principles
SOLID is an acronym for five object-oriented design principles — Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion — intended to produce more maintainable, flexible, and testable…
Definition
SOLID is an acronym for five object-oriented design principles — Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion — intended to produce more maintainable, flexible, and testable software.
Overview
The SOLID principles, popularized by Robert C. Martin ("Uncle Bob"), give developers a checklist for evaluating object-oriented programming (OOP) design decisions. Single Responsibility says a class should have one reason to change; Open/Closed says code should be open for extension but closed for modification; Liskov Substitution says subtypes must be usable anywhere their base type is expected without breaking correctness; Interface Segregation favors many small, specific interfaces over broad general-purpose ones; and Dependency Inversion says high-level code should depend on abstractions, not concrete implementations. Taken together, these principles push toward code with low coupling and high cohesion — components that are easy to change independently and easy to test in isolation — and they underpin much of the reasoning behind common design patterns like Strategy, Factory, and Dependency Injection. Dependency Inversion in particular is closely tied to the widespread practice of coding against interfaces and injecting dependencies, which makes unit testing far more practical by allowing collaborators to be mocked or substituted. SOLID principles are guidelines, not laws: applying them rigidly to every class, regardless of actual need, can produce excessive abstraction and indirection, which experienced engineers often push back against as over-engineering. Used with judgment, though, SOLID remains one of the most widely taught and referenced frameworks for reasoning about maintainable object-oriented software design.
Key Concepts
- Single Responsibility: a class should have one reason to change
- Open/Closed: open for extension, closed for modification
- Liskov Substitution: subtypes must be safely substitutable for their base type
- Interface Segregation: prefer small, focused interfaces
- Dependency Inversion: depend on abstractions, not concrete implementations
- Popularized by Robert C. Martin ("Uncle Bob")
- Aims for low coupling and high cohesion in OOP design
Use Cases
Frequently Asked Questions
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