Vending Machine

Ownership and transitions

A purchase is one state transition over stock and credit. Lookup and validate stock/payment before applying either decrement, so insufficient credit and sold-out selections preserve both. Refund captures then clears credit, making repeated refunds return zero. Selection is expected O(1); inventory snapshots copy O(products) entries.

Java boundaries

Use the supplied nested records as immutable value carriers. A record is only shallowly immutable: copy every retained mutable input and every outward list/map, including lists inside records. Absence is null; validation failures use IllegalArgumentException, missing-key purchase errors use NoSuchElementException, and invalid transaction boundaries use IllegalStateException. Java primitive types already exclude booleans from numeric inputs. Numeric values use long where signatures say long and all computations must fit signed 64-bit values. Calls in these service exercises are serialized.

Credit is retained on a failed sale. Plan the prospective charge only after both stock and affordability are known:

import java.util.*;

class Example {
    public static void main(String[] args) {
        long credit = 150, price = 100, stock = 0;
        boolean canPurchase = stock > 0 && credit >= price;
        assert !canPurchase;
        assert credit == 150;
        assert stock == 0;
    }
}

Computing a false eligibility result has not changed any money or inventory. Only the owning purchase transition may apply both changes after all conditions pass.

Design boundary

One machine instance models one serialized credit session. A physical dispenser can fail after authorization, so production hardware would require a separate dispensing outcome and reconciliation. The exercise returns a change amount and does not model coin availability.