What are the SOLID principles?

SOLID is a group of five object-oriented design principles intended to improve maintainability, flexibility, and testability.

S — Single Responsibility Principle: a class should have one primary reason to change.

O — Open/Closed Principle: software should be open for extension but closed for unnecessary modification.

L — Liskov Substitution Principle: derived types should be usable in place of their base types without breaking expected behavior.

I — Interface Segregation Principle: clients should not be forced to depend on members they do not need.

D — Dependency Inversion Principle: high-level modules should depend on abstractions rather than concrete low-level implementations.

SOLID is not a requirement to create an interface or class for every small operation. The principles should be applied pragmatically to reduce coupling and clarify responsibilities.