On the journey to becoming a seasoned developer, understanding and mastering S.O.L.I.D is an essential milestone. It reshapes how you think about code organization, boundary definition, and interactions among modules, classes, and methods—making your codebase readable, clean, and maintainable. It empowers you to write Testable Code for Unit Tests, forms the architectural foundation for IoC/DI, and significantly boosts your confidence during technical interviews :smile_cat:.
Introduction to S.O.L.I.D principles
S.O.L.I.D is an acronym for five foundational design principles in Object-Oriented Programming (OOP), introduced by Robert C. Martin (author of the developer classic Clean Code[1]) and Michael Feathers. The 5 principles are:
- Single responsibility principle
- Open/Closed principle
- Liskov substitution principle
- Interface segregation principle
- Dependency inversion principle
→ S.O.L.I.D is a clever mnemonic formed from the first letters of each principle ;)).
Single responsibility principle
A class should only have a single responsibility, that is, only changes to one part of the software’s specification should be able to affect the specification of the class.
— Wikipedia
This principle states that a class should have only one reason to change, encapsulated around a single responsibility. This is especially vital in large-scale systems where coupling multiple concerns into a single class makes even minor changes ripple through the entire codebase.
Open/Closed principle
Software entities … should be open for extension, but closed for modification.
— Wikipedia
This principle advocates that you should extend existing behavior by adding new classes or implementing interfaces rather than modifying battle-tested existing classes. This protects legacy systems from regressions when adding new functionality.
Liskov substitution principle
Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.
— Wikipedia
Subtypes must be substitutable for their base types without breaking program behavior or invariants. Violating LSP creates brittle inheritance trees where callers must know concrete implementation details to avoid runtime exceptions.
Interface segregation principle
Many client-specific interfaces are better than one general-purpose interface.
— Wikipedia
This principle advises breaking large, monolithic interfaces into smaller, client-specific interfaces. Clients should never be forced to depend on methods they do not use.
Dependency inversion principle
One should “depend upon abstractions, [not] concretions”.
— Wikipedia
A more descriptive definition:
High-level modules should not depend on low-level modules; both should depend on abstractions. Abstractions should not depend on details; details should depend on abstractions.
This principle is often the most abstract to grasp initially ;)).
Consider the analogy of riding a bicycle vs. flying an airplane. While their physical mechanisms are vastly different, if we abstract them into higher-level contracts such as steering, accelerating/decelerating, and braking, the high-level orchestration remains identical, while concrete implementations vary.
Check out the post on Inversion of Control and Dependency Injection to see how IoC/DI directly applies this principle in practice.
Conclusion
S.O.L.I.D principles may seem theoretical at first, but they truly shine when applied to real-world engineering. Once you embrace SOLID design, writing, testing, and maintaining code becomes significantly more intuitive. This article introduces the conceptual foundations; upcoming posts will dive into concrete implementation patterns.
Thanks for reading!