What are SOLID principles?

SOLID is an acronym for five basic principles of object-oriented design, developed by Robert Martin (Uncle Bob). They help create flexible, maintainable, and extendable software.

S - Single Responsibility Principle.
- A class should have only one reason to change. This means that each class or module should be responsible for one well-defined part of functionality.
O - Open/Closed Principle.
- Software entities (classes, modules, functions, etc.) should be open for extension but closed for modification. This is achieved through the use of polymorphism and abstractions.
L - Liskov Substitution Principle.
- Objects in a program can be replaced with their subtypes without affecting the correctness of the program. This means subclasses should adhere to the contracts (behavior) of their superclasses.
I - Interface Segregation Principle.
- Clients should not depend on interfaces they do not use. It is better to have many small, specific interfaces than one large general interface.
D - Dependency Inversion Principle.
- 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 is usually implemented through dependency injection (DI).

Applying SOLID principles leads to more structured and modular code, which is easier to understand, test, and maintain throughout the entire development lifecycle.