Problem Statement
Discuss how you would apply the SOLID principles when designing a medium-scale system in your chosen language.
Explanation
When designing a medium-scale system, you first identify the main modules or classes and assign them single responsibilities (SRP). For example you might separate data access, business logic and presentation into distinct classes. /n/n You then design classes to be open for extension but closed for modification (OCP): you use abstract interfaces or base classes so that new behaviours can be added via subclassing or composition rather than editing existing classes. /n/n You ensure that subclasses can substitute for their base classes (LSP): for instance if you have a base `PaymentProcessor` and subclass `CreditCardProcessor`, you must ensure any code referencing `PaymentProcessor` works with `CreditCardProcessor` without change. /n/n Next you apply the Interface Segregation Principle (ISP): you avoid large interfaces that force implementations to implement methods they don't need. Instead you define narrower interfaces for different client needs. /n/n Finally you apply Dependency Inversion Principle (DIP): high-level modules depend on abstractions (interfaces) rather than concrete classes, and you apply dependency injection to decouple modules and improve testability. /n/n By integrating these principles you build a system that is modular, flexible, and maintainable. High cohesion and low coupling further support this goal. Testing and refactoring become easier, new features can be added without changing core classes, and the design remains clean over time.