Vending Machine

Requirements

• Accept coins one at a time and track the running balance
• Let a customer select a product only once enough money is inserted
• Dispense the product and return the correct change
• Refuse selection when the product is out of stock
• Allow the customer to cancel and get their money back at any point

Assumptions you may make

• A fixed set of coin denominations; you may assume the machine always has change
• One transaction at a time — no concurrency required
• Restocking is out of scope

Things the brief expects to exist

Candidates, not a design. How they relate is yours to decide.

VendingMachine

Holds the current state and delegates every action to it

MachineState

Decides what each action means right now, and what happens next

Product

Knows its price and identity

Inventory

Knows how many of each product remain

CoinBox

Accumulates inserted coins and makes change

What this exercise is teaching

• Model a machine whose legal actions depend on what has happened so far
• Avoid a boolean field per condition and a conditional at every entry point
• Keep the rule about what may happen next in one place

Once it works, make it survive these

• Add card payment alongside coins
• Add a maintenance mode only an operator can enter
• Handle the machine running out of change mid-transaction