Which is better for API design: interface or abstract class?

Short answer: Interfaces are preferred for public APIs because they: Allow multiple inheritance Support loose coupling Avoid breaking changes when adding new implementations Abstract classes are better for internal APIs where shared code is required.

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.