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Design a Vending Machine Controller
Design a vending machine that manages inventory, accepts money, supports product selection and cancellation, returns change, and enforces a clear transaction state machine.
1. Problem Statement
Design an ATM: it authenticates a card and PIN, then supports balance inquiry, cash withdrawal with denomination dispensing, and deposits, serving one customer session at a time. Model the card reader, cash dispenser, and bank backend as distinct collaborators.
2. Live Coding Format
Work in the provided coding workspace while explaining your decisions to the live interviewer. Validation may use inline tests, examples, comments, or a verbal walkthrough.
3. Key Focus Areas
- 1Session state machine (idle/card-inserted/authenticated)
- 2BankingService boundary/interface design
- 3Transaction modeling (withdraw/deposit/balance inquiry)
- 4CashDispenser denomination algorithm
- 5Failure-mode handling (card retention, network failure after dispense)
4. What Strong Candidates Should Demonstrate
- ✓Model a session lifecycle explicitly (State pattern) instead of a flag-heavy conditional mess.
- ✓Separate hardware/backend collaborators (dispenser, banking service) from orchestration logic.
- ✓Design a concrete denomination-dispensing strategy after the core object model is coherent, and know its limits.
- ✓Reason about failure modes: wrong PIN limits, network drop mid-transaction, insufficient denominations.
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Practice this problem in the live coding workspace while the interviewer probes your clarification, implementation, trade-offs, and validation.
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