What are the SOLID principles and what are they for?

SOLID principles are a set of five fundamental object-oriented programming rules aimed at creating flexible, extendable, and maintainable code:

• S (Single Responsibility Principle) — a class should have only one reason to change, meaning it should be responsible for a single task.
• O (Open/Closed Principle) — software entities should be open for extension but closed for modification.
• L (Liskov Substitution Principle) — objects of subclasses should be interchangeable with objects of the base class without affecting the correctness of the program.
• I (Interface Segregation Principle) — it is better to have several specific interfaces than a single general-purpose interface.
• D (Dependency Inversion Principle) — high-level modules should not depend on low-level modules; both should depend on abstractions.

These principles help create code that is easier to test, maintain, and extend without risking breaking existing functionality.