How does the Singleton pattern affect unit testing?

Short answer: Singletons can hinder unit testing because they introduce global state, making tests dependent on a shared instance. This can cause tests to be flaky or order-dependent. To mitigate this, use interfaces and dependency injection, or design the Singleton to allow resetting its state for tests.

Real-world example (ShopNest)

ShopNest payment fees use Strategy: IFeeCalculator with UPI/Card implementations chosen at runtime.

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.