How do you refactor legacy code to follow SOLID principles?

Short answer: Identify classes violating SRP and break them down. Introduce abstractions and interfaces to decouple components (DIP). Replace conditional logic with polymorphism to respect OCP. Split large interfaces (ISP). Check inheritance hierarchies to maintain LSP. Inject dependencies instead of direct instantiation. Incrementally refactor with unit tests to ensure behavior remains consistent.

Real-world example (ShopNest)

Open/Closed in ShopNest: add a new payment method by adding a class, not by editing a giant switch in CheckoutService.

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.