How can you design classes that are open for extension but closed for modification?

Short answer: To design classes that follow OCP: Use abstraction (interfaces or abstract classes). Rely on polymorphism and inheritance. Apply composition over inheritance when suitable. Follow design principles like Strategy, Decorator, or Template Method patterns. ✅ Extend behavior through new classes rather than modifying existing code.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.