Clean coding principles aren’t just buzzwords—they’re what separate junior developers from professionals who write maintainable, scalable, and reliable software. In interviews, you’ll often be asked about software best practices to test your ability to write code that’s easy to understand, extend, and maintain.
This article covers the most common interview questions and answers around coding principles like SOLID, KISS, DRY, and YAGNI, plus general development best practices.
⚙️ General Best Practices
Q: Why are coding standards and conventions important?
- They make code consistent, easier to read, and easier for teams to maintain.
- Reduce bugs and onboarding time for new developers.
Q: What is code refactoring and why is it necessary?
- Refactoring = restructuring existing code without changing behavior.
- Improves readability, reduces duplication, and simplifies design for long-term maintainability.
Q: Why is writing tests important?
- Ensures correctness, prevents regressions, supports refactoring with confidence.
- Unit tests validate small pieces, integration tests ensure components work together, end-to-end tests validate the full system.
🧱 SOLID Principles
Q: What is the SOLID principle in OOP?
- Acronym for five principles that make code more maintainable and flexible.
S – Single Responsibility Principle (SRP)
- A class/module should have only one reason to change.
- Example: Don’t mix database logic with UI logic.
O – Open/Closed Principle (OCP)
- Entities should be open for extension but closed for modification.
- Example: Add new functionality via inheritance or interfaces, not by editing core code.
L – Liskov Substitution Principle (LSP)
- Subtypes should be substitutable for their base types without breaking functionality.
- Example: If Square inherits Rectangle, it must behave like a Rectangle in all cases.
I – Interface Segregation Principle (ISP)
- Don’t force clients to depend on methods they don’t use.
- Example: Instead of a giant interface IVehicle, split into smaller ones like IDrive, IFly.
D – Dependency Inversion Principle (DIP)
- Depend on abstractions, not concrete implementations.
- Example: Code against interfaces, not specific classes.
✨ Other Clean Code Principles
Q: What is KISS?
- Keep It Simple, Stupid → Simplicity is key; avoid unnecessary complexity.
- Example: Prefer a simple loop over a complex chain of functional calls if it’s more readable.
Q: What is DRY?
- Don’t Repeat Yourself → Avoid duplicating logic; extract common functionality.
- Example: Instead of repeating validation in multiple places, centralize it in one method.
Q: What is YAGNI?
- You Aren’t Gonna Need It → Don’t build features until they are actually required.
- Example: Don’t implement multi-language support unless the product roadmap calls for it.
🧩 Scenario Questions
Q: Your codebase is becoming hard to maintain. What steps do you take?
- Apply refactoring.
- Check for violations of DRY/KISS.
- Revisit architecture to apply SOLID principles.
- Improve unit/integration test coverage.
Q: You notice duplicate logic across multiple modules. What would you do?
- Refactor to follow DRY: extract into a shared function, service, or utility.
Q: A colleague added a very complex design pattern for a simple feature. How do you respond?
- Remind them of KISS & YAGNI → complexity should only exist if needed.
🧩 Expanded Scenario Questions
Q: You’re reviewing a teammate’s code and notice a single class with 1,000+ lines handling database queries, business logic, and UI rendering. What principle is being violated and how do you fix it?
- Violation: Single Responsibility Principle (SRP).
- Fix: Break the class into smaller classes/modules (e.g., UserRepository, UserService, UserController) so each has one clear responsibility.
*Q: The product manager asks you to add a “dark mode” feature, but your teammate hard-coded colors throughout the UI. How do you handle this?
*
- Violation: DRY (duplicate logic all over the code).
- Fix: Centralize styling into a config/theme system so colors can be reused and updated in one place.
Q: Your API client library has methods that 80% of consumers never use, but they are still forced to implement them. What principle is broken?
- Violation: Interface Segregation Principle (ISP).
- Fix: Split the large interface into smaller, more specific interfaces (e.g., IReadableClient, IWritableClient).
Q: A teammate wants to add a caching layer “just in case” traffic increases in the future. How do you respond?
- Violation: YAGNI.
- Fix: Don’t add complexity until the problem actually arises. Wait until metrics justify caching.
Q: You’re debugging an issue and find that a subclass overrides a method but changes its behavior in a way that breaks the parent class contract. What principle is violated?
- Violation: Liskov Substitution Principle (LSP).
- Fix: Redesign the subclass so it behaves consistently with the parent, or create a separate hierarchy.
Q: You find several places in the code where database queries are written inline as string concatenations. What are the risks, and how would you improve it?
- Risk: SQL injection vulnerability, hard-to-maintain queries.
- Fix: Use parameterized queries/ORM, extract common queries into repository functions.
Q: Your team has multiple services tightly coupled to a specific third-party payment provider. The company is considering switching providers. What principle was ignored?
- Violation: Dependency Inversion Principle (DIP).
- Fix: Introduce an abstraction/interface (PaymentGateway) and have services depend on that, so you can swap implementations easily.
Q: A new developer finds the code hard to follow because of clever tricks, deeply nested ternaries, and over-engineered patterns. How do you explain what’s wrong?
- Violation: KISS (Keep It Simple, Stupid).
- Fix: Refactor into simple, readable methods. Emphasize readability > “cleverness.”
Q: Your microservice logs show identical error handling code repeated across multiple endpoints. What principle is not being followed?
- Violation: DRY.
- Fix: Extract common error handling into middleware or a reusable utility.
Q: A feature works fine, but performance is slightly slow. A teammate wants to rewrite it using a complex caching system, new data structures, and parallelism. What do you do?
- Violation: YAGNI + Premature Optimization.
- Fix: Optimize only after profiling shows a bottleneck. Start with simple improvements first.
✅ Conclusion
Best practices and clean coding principles ensure your software is not just working today, but maintainable tomorrow. By understanding SOLID, KISS, DRY, and YAGNI, and applying them in real-world code, you’ll not only ace interview questions but also grow as a professional developer.
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