What is the difference between an interface and a class?

Short answer: Feature Interface Class Implementatio No implementation (except default methods) Can have full implementation Fields Cannot have fields Can have fields Instantiation Cannot instantiate Can instantiate Inheritance Can inherit multiple interfaces Single class inheritance only

Real-world example (ShopNest)

Checkout calls IPaymentGateway.Charge(). Razorpay and Stripe adapters implement the same interface, so ShopNest can switch gateways without rewriting the order service.

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.