Explain the SOLID principles and why they matter for maintainable software.

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SOLID is an acronym for five design principles that help create maintainable, flexible, and understandable object-oriented software.

The Single Responsibility Principle states that a class should have only one reason to change. Each class should focus on one concern. A UserService that handles authentication, sends emails, and generates reports violates this principle. Instead, separate these into AuthenticationService, EmailService, and ReportService.

The Open-Closed Principle states that software entities should be open for extension but closed for modification. You should add new functionality without changing existing code. A PaymentProcessor using a strategy pattern can support new payment methods by adding new strategy classes, not modifying the processor.

The Liskov Substitution Principle states that objects of a superclass should be replaceable with subclass objects without affecting correctness. If Square extends Rectangle but breaks when you set width and height independently, it violates this principle.

The Interface Segregation Principle states that clients should not depend on interfaces they do not use. Instead of one large interface, create smaller, focused interfaces. A Printer interface with print, scan, and fax methods forces simple printers to implement scanning.

The Dependency Inversion Principle states that high-level modules should depend on abstractions, not low-level modules. A UserService should depend on a UserRepository interface, not directly on MySQLUserRepository.

These principles lead to loosely coupled, cohesive code. Apply them pragmatically since over-engineering simple code adds unnecessary complexity.