Home Depot Object-Oriented Design: Tool Rental System

Design a tool rental platform where customers create accounts, browse inventory, rent tools, pay deposits and fees, return items, and receive receipts. You must model core entities (User, Tool, Rental, Payment, Receipt), enforce validation for availability and payments, and support overtime charging (double daily rate per late day). The system should integrate with a payment gateway and produce a clear receipt after successful payment.

High-level flow: authenticate the user; search or browse tools; check availability and validation rules; calculate deposit + rental cost (plus potential discounts or promotions); process payment (deposit + rental fee) via a payment service; create a Rental record and decrement inventory; when the tool is returned, calculate overtime charges, charge the user if needed, update inventory, and process any deposit refunds.

Design considerations and skill signals: you should demonstrate object-oriented modeling (entities and value objects), clean separation of concerns (InventoryService, RentalService, PaymentService), and use of Repository Pattern for persistence. Show how Strategy Pattern can encapsulate pricing/overtime policies and how validation logic prevents overlapping rentals and inconsistent state. Explain transaction boundaries and idempotency for payments, and how you’d design for scalability, reliability, security (encrypted payment data, secure auth), and maintainability (extensible tool types, pluggable payment methods). Include common edge cases (partial returns, damaged items, concurrent reservations) and brief ideas for testing and monitoring.