Design a Vending Machine

Requirements

• The machine holds multiple products, each with a price and a stock count.
• A user inserts money, selects a product, and receives the product plus change (or an error if underfunded or out of stock).
• The machine moves through distinct states: idle, has-money, dispensing.

Key classes to identify before you look at the reference

What's different about a vending machine compared to most LLD problems: its behavior genuinely depends on what state it's currently in — inserting a coin means something different when idle vs. when a product is already selected. What pattern exists specifically for "behavior changes based on internal state, and state transitions should be explicit rather than a tangle of booleans and if-checks"?