This is one of those questions that feels manageable at first and then quietly expands on you.
Start by clarifying requirements and scope, then present a high-level class diagram with core abstractions like Item, Cart, PricingRule, and PaymentMethod. Walk through each component, explaining how they interact and how you'd apply design patterns to handle pricing, taxes, and extensibility.
Pro tip: Emphasize separation of concerns and extensibility: use the Strategy pattern for pricing and taxes, and the Factory pattern for payment methods. This shows you design for change, which is crucial in real-world systems.
Ask questions to understand the system's boundaries: single store or chain? Multiple currencies? Discounts, coupons, loyalty programs? This ensures you design the right thing.
Define key classes: Item (with barcode, price), Cart (holds items), PricingRule (interface for pricing strategies), TaxCalculator, PaymentMethod (interface), Receipt. Explain their responsibilities.
Draw or describe the class diagram: Cart contains Items, uses PricingRule and TaxCalculator, PaymentMethod is used by CheckoutService, Receipt is generated after payment. Show inheritance and composition.
Use Strategy for pricing rules (e.g., buy-one-get-one, bulk discount) and taxes (e.g., different rates per region). Use Factory for creating payment methods. Use Observer for updating totals when cart changes.
Trace a typical flow: add items to cart, apply pricing rules, calculate taxes, select payment method, process payment, generate receipt. Highlight how each component contributes.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Got asked this as a follow-up and I think I gave a decent answer.
Start by clarifying the functional requirements and constraints, then propose a rule-based architecture that separates pricing logic from the core application. Focus on extensibility patterns like strategy, rules engine, or plugin systems, and discuss trade-offs such as complexity, performance, and maintainability.
Pro tip: Emphasize that the goal is to make the system open for extension but closed for modification (Open/Closed Principle), and mention that you would use a configuration-driven approach so that new promotions can be added via data, not code, reducing deployment risks.
Ask about expected promotion types, frequency of addition, performance needs, and existing system architecture to tailor the solution.
Propose an abstraction (e.g., Promotion interface with methods like isApplicable and apply) that all promotion types implement, ensuring uniformity.
Use a rules engine or strategy pattern to evaluate and apply promotions dynamically, allowing new types to be plugged in without modifying existing code.
Store promotion definitions in a database or configuration files, so new promotions can be added via data changes rather than code changes.
Address trade-offs like increased complexity, potential performance overhead, and testing strategies, and highlight how the design supports future growth.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Talked through a PaymentMethod interface with validate and capture as the two core methods, then a factory that returns the right implementation.
Start by explaining that you would design the new payment method as a pluggable module behind a stable abstraction, ensuring existing code depends only on interfaces. Then describe how you would introduce it incrementally with feature flags, thorough testing, and monitoring to avoid regressions.
Pro tip: Emphasize that you would first write characterization tests for the existing payment flow to lock in current behavior before making any changes. This demonstrates a mature, risk-aware approach that prevents accidental breakage.
Map out how payments are currently processed, identifying key components, interfaces, and dependencies. Look for tight coupling and areas that would need to be abstracted.
Create or refine an abstraction (e.g., PaymentProcessor interface) that all payment methods must implement. Ensure the existing code interacts only with this interface, not concrete implementations.
Develop the new payment method in isolation, adhering to the interface. Keep it decoupled from existing code to minimize risk.
Add the new method behind a feature flag, enabling it for internal testing or a small subset of users first. Monitor for errors and performance issues.
Write unit, integration, and end-to-end tests for the new method, and ensure existing tests still pass. Set up logging and alerts to catch any regressions in production.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.