Design Vending Machine

// ========== Product ==========
class Product {
    private String id;
    private String name;
    private double price;
    private int quantity;
    
    public Product(String id, String name, double price, int qty) {
        this.id = id; this.name = name; 
        this.price = price; this.quantity = qty;
    }
    
    public double getPrice() { return price; }
    public int getQuantity() { return quantity; }
    public void decrementQuantity() { quantity--; }
    public String getName() { return name; }
    public String getId() { return id; }
}

// ========== State Interface ==========
interface VendingState {
    void insertCoin(VendingMachine vm, double amount);
    void selectProduct(VendingMachine vm, String productId);
    void dispense(VendingMachine vm);
    void refund(VendingMachine vm);
}

// ========== Idle State ==========
class IdleState implements VendingState {
    @Override
    public void insertCoin(VendingMachine vm, double amount) {
        vm.addBalance(amount);
        System.out.println("Inserted: $" + amount + ". Balance: $" + vm.getBalance());
        vm.setState(new HasMoneyState());
    }
    
    @Override
    public void selectProduct(VendingMachine vm, String id) {
        System.out.println("Please insert coins first.");
    }
    
    @Override
    public void dispense(VendingMachine vm) {
        System.out.println("Nothing to dispense.");
    }
    
    @Override
    public void refund(VendingMachine vm) {
        System.out.println("No balance to refund.");
    }
}

// ========== Has Money State ==========
class HasMoneyState implements VendingState {
    @Override
    public void insertCoin(VendingMachine vm, double amount) {
        vm.addBalance(amount);
        System.out.println("Added: $" + amount + ". Balance: $" + vm.getBalance());
    }
    
    @Override
    public void selectProduct(VendingMachine vm, String id) {
        Product product = vm.getProduct(id);
        if (product == null) {
            System.out.println("Invalid product.");
            return;
        }
        if (product.getQuantity() == 0) {
            System.out.println(product.getName() + " is out of stock.");
            return;
        }
        if (vm.getBalance() < product.getPrice()) {
            System.out.println("Insufficient balance. Need $" + 
                (product.getPrice() - vm.getBalance()) + " more.");
            return;
        }
        vm.setSelectedProduct(product);
        vm.setState(new DispensingState());
        vm.dispense();
    }
    
    @Override
    public void dispense(VendingMachine vm) {
        System.out.println("Please select a product first.");
    }
    
    @Override
    public void refund(VendingMachine vm) {
        System.out.println("Refunding: $" + vm.getBalance());
        vm.resetBalance();
        vm.setState(new IdleState());
    }
}

// ========== Dispensing State ==========
class DispensingState implements VendingState {
    @Override
    public void insertCoin(VendingMachine vm, double amount) {
        System.out.println("Please wait, dispensing...");
    }
    
    @Override
    public void selectProduct(VendingMachine vm, String id) {
        System.out.println("Please wait, dispensing...");
    }
    
    @Override
    public void dispense(VendingMachine vm) {
        Product product = vm.getSelectedProduct();
        double change = vm.getBalance() - product.getPrice();
        
        product.decrementQuantity();
        System.out.println("Dispensing: " + product.getName());
        
        if (change > 0) {
            System.out.println("Change: $" + change);
        }
        
        vm.resetBalance();
        vm.setSelectedProduct(null);
        vm.setState(new IdleState());
    }
    
    @Override
    public void refund(VendingMachine vm) {
        System.out.println("Cannot refund during dispensing.");
    }
}

// ========== Vending Machine ==========
class VendingMachine {
    private VendingState state;
    private Map<String, Product> inventory;
    private double balance;
    private Product selectedProduct;
    
    public VendingMachine() {
        this.state = new IdleState();
        this.inventory = new HashMap<>();
        this.balance = 0;
    }
    
    public void addProduct(Product product) {
        inventory.put(product.getId(), product);
    }
    
    // Delegated to current state
    public void insertCoin(double amount) { state.insertCoin(this, amount); }
    public void selectProduct(String id) { state.selectProduct(this, id); }
    public void dispense() { state.dispense(this); }
    public void refund() { state.refund(this); }
    
    // Helpers for states
    public void setState(VendingState s) { this.state = s; }
    public void addBalance(double amt) { this.balance += amt; }
    public void resetBalance() { this.balance = 0; }
    public double getBalance() { return balance; }
    public Product getProduct(String id) { return inventory.get(id); }
    public void setSelectedProduct(Product p) { this.selectedProduct = p; }
    public Product getSelectedProduct() { return selectedProduct; }
}

// ========== Usage ==========
public class Main {
    public static void main(String[] args) {
        VendingMachine vm = new VendingMachine();
        vm.addProduct(new Product("A1", "Coke", 1.50, 10));
        vm.addProduct(new Product("A2", "Chips", 2.00, 5));
        
        vm.selectProduct("A1");    // Error: insert coins first
        vm.insertCoin(1.00);       // Balance: $1.00
        vm.insertCoin(1.00);       // Balance: $2.00
        vm.selectProduct("A1");    // Dispenses Coke, $0.50 change
    }
}