Walkthrough: vending machine

1 // State Interface
2 class VendingState {
3   insertCoin(machine, amount) { throw new Error("Action not permitted in this state"); }
4   selectItem(machine, code) { throw new Error("Action not permitted in this state"); }
5   dispense(machine) { throw new Error("Action not permitted in this state"); }
6   refund(machine) { throw new Error("Action not permitted in this state"); }
7 }
8 
9 // 1. Idle State
10 class IdleState extends VendingState {
11   insertCoin(machine, amount) {
12     machine.addBalance(amount);
13     machine.setState(new HasMoneyState());
14     return `Accepted $${amount}. Total balance: $${machine.balance}`;
15   }
16 }
17 
18 // 2. HasMoney State
19 class HasMoneyState extends VendingState {
20   insertCoin(machine, amount) {
21     machine.addBalance(amount);
22     return `Added $${amount}. Total: $${machine.balance}`;
23   }
24 
25   selectItem(machine, code) {
26     const item = machine.getItem(code);
27     if (!item || item.quantity <= 0) {
28       return "Item out of stock";
29     }
30     if (machine.balance < item.price) {
31       return `Insufficient funds. Price is $${item.price}, balance is $${machine.balance}`;
32     }
33     machine.setSelectedItem(item);
34     machine.setState(new DispensingState());
35     return machine.dispense();
36   }
37 
38   refund(machine) {
39     const refunded = machine.balance;
40     machine.resetBalance();
41     machine.setState(new IdleState());
42     return `Refunded $${refunded}`;
43   }
44 }
45 
46 // 3. Dispensing State
47 class DispensingState extends VendingState {
48   dispense(machine) {
49     const item = machine.selectedItem;
50     item.quantity--;
51     const change = machine.balance - item.price;
52     machine.resetBalance();
53     machine.setState(new IdleState());
54     return `Dispensed ${item.name}! Change returned: $${change.toFixed(2)}`;
55   }
56 }
57 
58 // Machine Context
59 class VendingMachine {
60   #inventory = new Map();
61   #balance = 0;
62   #state = new IdleState();
63   selectedItem = null;
64 
65   addItem(code, name, price, quantity) {
66     this.#inventory.set(code, { name, price, quantity });
67   }
68 
69   getItem(code) { return this.#inventory.get(code); }
70   setState(state) { this.#state = state; }
71   addBalance(amount) { this.#balance += amount; }
72   resetBalance() { this.#balance = 0; this.selectedItem = null; }
73   get balance() { return this.#balance; }
74   get stateName() { return this.#state.constructor.name; }
75 
76   setSelectedItem(item) { this.selectedItem = item; }
77 
78   insertCoin(amount) { return this.#state.insertCoin(this, amount); }
79   selectItem(code) { return this.#state.selectItem(this, code); }
80   dispense() { return this.#state.dispense(this); }
81   refund() { return this.#state.refund(this); }
82 }
83 
84 // Verification Test
85 const vm = new VendingMachine();
86 vm.addItem("A1", "Soda", 1.50, 3);
87 
88 // 1. Insert coin: Idle -> HasMoney
89 vm.insertCoin(1.00);
90 vm.insertCoin(1.00);
91 console.assert(vm.balance === 2.00);
92 console.assert(vm.stateName === "HasMoneyState");
93 
94 // 2. Select item: HasMoney -> Dispensing -> Idle
95 const receipt = vm.selectItem("A1");
96 console.assert(receipt.includes("Dispensed Soda"));
97 console.assert(receipt.includes("Change returned: $0.50"));
98 console.assert(vm.stateName === "IdleState");
99 console.assert(vm.balance === 0);
100 console.log(receipt);

1 public class Example {
2   static final class Item {
3     final String name;
4     final double price;
5     int quantity;
6     Item(String name, double price, int qty) { this.name = name; this.price = price; this.quantity = qty; }
7   }
8