Explain the SOLID principles of object-oriented design.

SOLID: S - Single Responsibility: class should have one reason to change. O - Open/Closed: open for extension, closed for modification. L - Liskov Substitution: subtypes must be substitutable for base types without breaking behavior. I - Interface Segregation: many specific interfaces better than one general interface. D - Dependency Inversion: depend on abstractions, not concretions. Following SOLID leads to maintainable, flexible, testable code. Example violations: God classes (SRP), switch statements on types (OCP), square inheriting rectangle (LSP).