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Design Principles Interview Questions — What Senior Engineers Need to Know

Design principles questions reveal whether you write code that other engineers want to work with. Interviewers aren’t looking for you to recite SOLID — they want to see that you can apply these principles pragmatically, knowing when to follow them and when to bend them.

These 6 questions cover the design principles that matter most in production codebases: SOLID principles with real trade-offs, composition vs inheritance decisions, and when DRY and KISS actually conflict. The kind of judgment that separates engineers who write clean code from those who over-engineer it.

All 6 Questions

SingletonWhat is the Singleton pattern and what are its drawbacks?FactoryWhat is the Factory pattern and when would you use it?StrategyWhat is the Strategy pattern and when would you use it?ObserverWhat is the Observer pattern and when would you use it?IoC / Dependency InjectionWhat is Inversion of Control and Dependency Injection?High Cohesion / Loose CouplingWhat is high cohesion and loose coupling and why do they matter?

How to Prepare

Focus on understanding concepts deeply enough to explain them in your own words. For each topic, practice articulating the trade-offs and real-world applications — interviewers care about practical judgment, not textbook definitions.

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