Requirements:

• The vending machine should support multiple products with different prices and quantities.
• The machine should accept coins and notes of different denominations.
• The machine should dispense the selected product and return change if necessary.
• The machine should keep track of the available products and their quantities.
• The machine should handle multiple transactions concurrently and ensure data consistency.
• The machine should provide an interface for restocking products and collecting money.
• The machine should handle exceptional scenarios, such as insufficient funds or out-of-stock products.

Sequnece Diagram

Components:

The system will consist of the following classes:

• Product: Class to represent the products with their price and quantity.
• Coin/Note: Class to represent different denominations of coins and notes.
• VendingMachine: The core class responsible for handling transactions, accepting coins/notes, dispensing products, and maintaining inventory.
• Transaction: Class to represent a single transaction, tracking the inserted money and the product selected.
• InventoryManager: A class responsible for managing the products in the vending machine (restocking, checking availability).
• CashManager: A class responsible for managing the coins and notes in the machine (for storing and returning change).

Code Implementation:

package com.lld;

import java.util.HashMap;
import java.util.Map;

class Coin {
    private int value;
    public Coin(int value) {
        this.value = value;
    }
    public int getValue() {
        return value;
    }
}

class Note {
    private int value;
    public Note(int value) {
        this.value = value;
    }
    public int getValue() {
        return value;
    }
}

class Product {
    private String name;
    private int price;
    private int quantity;
    public Product(String name, int price, int quantity) {
        this.name = name;
        this.price = price;
        this.quantity = quantity;
    }
    public String getName() {
        return name;
    }
    public int getPrice() {
        return price;
    }
    public int getQuantity() {
        return quantity;
    }
    public void decreaseQuantity() {
        if (quantity > 0) {
            quantity--;
        }
    }
    public void increaseQuantity(int amount) {
        this.quantity += amount;
    }
    public boolean isAvailable() {
        return quantity > 0;
    }
}

class CashManager {
    private int balance = 0;
    public void insertCoin(Coin coin) {
        balance += coin.getValue();
    }
    public void insertNote(Note note) {
        balance += note.getValue();
    }
    public int getBalance() {
        return balance;
    }
    public void deductAmount(int amount) {
        balance -= amount;
    }
    public int returnChange(int price) {
        int change = balance - price;
        deductAmount(price);
        return change;
    }
}

class InventoryManager {
    private Map<String, Product> productMap = new HashMap<>();
    public void addProduct(Product product) {
        productMap.put(product.getName(), product);
    }
    public Product getProduct(String productName) {
        return productMap.get(productName);
    }
    public boolean isProductAvailable(String productName) {
        return productMap.containsKey(productName) && productMap.get(productName).isAvailable();
    }
    public void restockProduct(String productName, int quantity) {
        Product product = productMap.get(productName);
        if (product != null) {
            product.increaseQuantity(quantity); // Fix: Increase quantity instead of decreasing
        } else {
            System.out.println("Product not found in inventory.");
        }
    }
}

class VendingMachine {
    private CashManager cashManager;
    private InventoryManager inventoryManager;
    public VendingMachine() {
        this.cashManager = new CashManager();
        this.inventoryManager = new InventoryManager();
    }
    public void addProduct(Product product) {
        inventoryManager.addProduct(product);
    }
    public void insertCoin(Coin coin) {
        cashManager.insertCoin(coin);
    }
    public void insertNote(Note note) {
        cashManager.insertNote(note);
    }
    public void selectProduct(String productName) {
        if (inventoryManager.isProductAvailable(productName)) {
            Product product = inventoryManager.getProduct(productName);
            if (cashManager.getBalance() >= product.getPrice()) {
                int change = cashManager.returnChange(product.getPrice());
                product.decreaseQuantity();
                System.out.println("Dispensing: " + productName);
                System.out.println("Returning change: " + change + " cents.");
            } else {
                System.out.println("Insufficient funds. Please insert more money.");
            }
        } else {
            System.out.println("Product is out of stock.");
        }
    }
    public void restockProduct(String productName, int quantity) {
        inventoryManager.restockProduct(productName, quantity);
    }
    public void collectMoney() {
        System.out.println("Collecting money: " + cashManager.getBalance() + " cents.");
    }
}

public class VendingMachineApp {
    public static void main(String[] args) {
        // Creating product objects
        Product soda = new Product("Soda", 50, 10);
        Product chips = new Product("Chips", 30, 5);
        Product candy = new Product("Candy", 20, 7);

        // Creating VendingMachine instance
        VendingMachine vendingMachine = new VendingMachine();
        vendingMachine.addProduct(soda);
        vendingMachine.addProduct(chips);
        vendingMachine.addProduct(candy);

        // Insert coins and select products
        vendingMachine.insertCoin(new Coin(25));
        vendingMachine.insertCoin(new Coin(25));
        vendingMachine.selectProduct("Soda");

        vendingMachine.insertCoin(new Coin(25));
        vendingMachine.insertCoin(new Coin(10));
        vendingMachine.selectProduct("Chips");

        // Restocking
        vendingMachine.restockProduct("Chips", 5);

        // Collecting money
        vendingMachine.collectMoney();
    }
}

Output:
Dispensing: Soda
Returning change: 0 cents.
Dispensing: Chips
Returning change: 5 cents.
Collecting money: 5 cents.
        

Design Considerations:

• Concurrency: To handle concurrent transactions, we can make use of synchronization in key methods like inserting money or updating the product inventory to ensure data consistency.
• Error Handling: The design checks for product availability and sufficient funds before dispensing products and returning change. More error handling can be added for cases like invalid coin insertion or invalid inputs.
• Restocking & Money Collection: Separate methods to restock products and collect money allow for easy maintenance and operational control.