SOLID is a set of five principles for writing object-oriented code that is easier to change, test, and reuse. Each letter stands for one idea.
S – Single Responsibility
One class (or module) should do one main job. If you can say “this class does X and Y,” try to split it so that one part does X and another does Y. That way, when you need to change X, you only touch one place.
O – Open/Closed
Code should be open for extension (you can add new behavior) but closed for modification (you avoid changing existing code to do it). You extend with new classes or functions instead of editing old ones, which reduces the risk of breaking what already works.
L – Liskov Substitution
If you have a base type (e.g. “Animal”) and a subtype (e.g. “Dog”), any code that uses “Animal” should still work correctly when you pass a “Dog.” Subtypes must not break the contract of the base type; they can add behavior but not take away or contradict it.
I – Interface Segregation
Don’t force a class to depend on methods it doesn’t use. Prefer many small, focused interfaces over one big “do everything” interface. That way each class only knows about the behavior it actually needs.
D – Dependency Inversion
Depend on abstractions (interfaces, contracts) rather than concrete implementations. High-level logic should not depend on low-level details; both should depend on the same abstraction. That makes it easy to swap implementations (e.g. different databases or services) without rewriting the rest of the app.
Together, SOLID helps you keep code modular, understandable, and ready for future changes.