Could you explain the main principles underlying SOLID and how they are applied in designing software systems?

SOLID is a set of five principles of object-oriented design that help create flexible and maintainable systems:

• S (Single Responsibility Principle) — a class should have only one responsibility.
• 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 breaking the logic.
• I (Interface Segregation Principle) — it's better to have several specialized interfaces than one general interface.
• D (Dependency Inversion Principle) — high-level modules should not depend on low-level modules; both should depend on abstractions.

Applying these principles allows creating code that is easier to test, extend, and maintain. For example, separation of responsibilities helps avoid bulky classes, and using interfaces makes component replacement easier without changing the rest of the code.