Design a vending machine

Easy30 minFree, no account

A state machine with money in it, which is the version that has to be right.

The question

Design a vending machine: select a product, insert coins, dispense, give change.

Handle cancellation at any point, and the case where the machine cannot make change.

Functional

Select a product, insert coins incrementally, dispense when paid.
Refund everything on cancel.
Give correct change, or refuse the sale before taking the money.
Restock and collect cash as an operator.

Non-functional

Money must never be created or destroyed by any sequence of actions.
A power cut mid-sale must not lose the customer's coins.

30:00Commit to an answer before you open the solution. Reading it first teaches you to recognise good answers, which is not the skill being tested.

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The worked solution

written by a person · not a grade

Score yourself

0 of 5 marked

A state machine written before the classes, with illegal transitions named30
Stated the money invariant and used escrow to preserve it30
Change computed correctly, with greedy's limits acknowledged20
Change checked before taking payment, not after15
Addressed persistence across a power cut5

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