Design a Vending Machine

Beginner Object-oriented design

The problem

Design the software for a vending machine that sells snacks and drinks. Customers insert money, choose a product and receive the item and change.

This is an object-oriented design interview. The focus is classes, state, patterns and edge cases.

You can assume the hardware (coin slot, motor, display) exposes simple methods.

Think about what happens when something goes wrong halfway.

The machine has up to 40 slots, each holding one product with a price and quantity.
It accepts coins and notes and returns change.
It must keep working with no network.
An admin restocks and collects cash.

Drive the conversation. Ask questions before drawing anything. State your assumptions out loud so they can be corrected early.
Use numbers. Turn the givens into requests per second, storage and bandwidth. Every later choice should point back to one of them.
Start simple, then evolve. Get a working version on the board first. Add caches, sharding and queues only when a number or a requirement asks for them.
Go deep on the hard parts. For this problem the hard parts are modelling behaviour by state, making change correctly, and failure cases like jams and power loss.
Name the trade-offs. Say what you chose, what you gave up, and what would make you change your mind.
Think about failure. Point out single points of failure and say what happens when a node, a cache or a whole zone goes down.

State pattern

An object changes its behaviour when its internal state changes; each state is a class.

State transition

A move from one state to another caused by an event, such as Idle to HasMoney when a coin is inserted.

Greedy change

Return the largest coin that fits, repeatedly. Works for common currencies.

Lost update

Two writers read the same value and one overwrites the other's change.

Offline-first

The device works fully without a connection and syncs later.

Decisions you confirm will collect here as you go.

Your design so far