Design A Vending Machine - Object-Oriented Design

Requirements

The Vending machine should be able to dispense the asked item for the user.
The Vending machine should be able to give an error if the item asked by the user is not available.
The vending machine should trigger warning if an item goes below the minimum count limit.
The Vending machine should be able to process payments by various methods such as coins or cards.
The Vending machine will trigger alerts for maintenance required on a quarterly basis.

Define Core Objects

VendingMachine: Manages the overall operation and acts as a controller.

Item: Represents an item in the machine.

Inventory: Manages the stock of items.

PaymentProcessor: Handles payments (coins, cash, card).

PaymentType (Enum): Represents different payment methods.

Slot: Represents a slot containing a type of item.

MaintenanceAlert: Handles alerts for low inventory or technical issues.

Analyze Relationships

VendingMachine will be a Singleton object which will be common.
VendingMachine will have a list of buttons and as buttons will not exist without the machine they will have a composition relationship.
Items will also have a composition relationship with VendingMachine as without the machine they have no meaning.
CoinCollector and CardReader will also have composition relationship with vending machine.
Payment will have an association relationship with the vending machine.

Establish Hierarchy

There will be an Item class which will be parent for chocolcates and candies and chips.
Button class will be a super class for QuantityButton, OrderButton, PaymentButton
Notification class will be a super class for ErrorNotification and MaintenanceNotification.

Design Patterns

Singleton: For the VendingMachine to ensure a single instance across the system.
Strategy: For handling different payment types.
Observer: For sending maintenance alerts when inventory is low.
Factory: For creating payment processors based on the payment type.

Define Class Members (write code)

Attributes: For each class, define the attributes (data) it will hold...

Methods: Define the methods (functions) that operate on the attributes. Ensure they align with the object's responsibilities and adhere to the principle of encapsulation.

public class Item {
    private String name;
    private double price;
public Item(String name, double price) {
    this.name = name;
    this.price = price;
}

public String getName() {
    return name;
}

public double getPrice() {
    return price;
}

}
public class Slot {
private Item item;
private int quantity;
public Slot(Item item, int quantity) {
    this.item = item;
    this.quantity = quantity;
}

public Item getItem() {
    return item;
}

public int getQuantity() {
    return quantity;
}

public void dispenseItem() {
    if (quantity &gt; 0) {
        quantity--;
    } else {
        throw new RuntimeException(&quot;Item out of stock!&quot;);
    }
}

}
import java.util.HashMap;
import java.util.Map;
public class Inventory {
private Map<String, Slot> slots = new HashMap<>();
public void addSlot(String slotId, Slot slot) {
    slots.put(slotId, slot);
}

public Slot getSlot(String slotId) {
    return slots.get(slotId);
}

public boolean isLowInventory(String slotId) {
    Slot slot = slots.get(slotId);
    return slot.getQuantity() &lt; 5; // Example threshold for low inventory
}

}
public enum PaymentType {
COIN, CASH, CARD
}
public interface PaymentProcessor {
boolean processPayment(double amount);
}
public class CoinPaymentProcessor implements PaymentProcessor {
@Override
public boolean processPayment(double amount) {
System.out.println("Processing coin payment of $" + amount);
return true; // Simulate success
}
}
public class CashPaymentProcessor implements PaymentProcessor {
@Override
public boolean processPayment(double amount) {
System.out.println("Processing cash payment of $" + amount);
return true; // Simulate success
}
}
public class CardPaymentProcessor implements PaymentProcessor {
@Override
public boolean processPayment(double amount) {
System.out.println("Processing card payment of $" + amount);
return true; // Simulate success
}
}
public class PaymentProcessorFactory {
public static PaymentProcessor getPaymentProcessor(PaymentType type) {
switch (type) {
case COIN:
return new CoinPaymentProcessor();
case CASH:
return new CashPaymentProcessor();
case CARD:
return new CardPaymentProcessor();
default:
throw new IllegalArgumentException("Unsupported payment type");
}
}
}
public class VendingMachine {
private Inventory inventory;
private MaintenanceAlert maintenanceAlert;
private static volatile VendingMachine instance;

private VendingMachine() {
    this.inventory = new Inventory();
    this.maintenanceAlert = new MaintenanceAlert();
}

public static VendingMachine getInstance() {
    if (instance == null) {
        synchronized (VendingMachine.class) {
            if (instance == null) {
                instance = new VendingMachine();
            }
        }
    }
    return instance;
}

public void addSlot(String slotId, Slot slot) {
    inventory.addSlot(slotId, slot);
}

public void selectItem(String slotId, PaymentType paymentType) {
    Slot slot = inventory.getSlot(slotId);
    if (slot == null) {
        System.out.println(&quot;Invalid slot selected.&quot;);
        return;
    }

    Item item = slot.getItem();
    PaymentProcessor paymentProcessor = PaymentProcessorFactory.getPaymentProcessor(paymentType);

    if (paymentProcessor.processPayment(item.getPrice())) {
        slot.dispenseItem();
        System.out.println(&quot;Dispensed: &quot; + item.getName());

        if (inventory.isLowInventory(slotId)) {
            maintenanceAlert.sendLowInventoryAlert(slotId);
        }
    } else {
        System.out.println(&quot;Payment failed.&quot;);
    }
}

}

Adhere to SOLID Guidelines

Single Responsibility: Each class has a single responsibility (e.g., Inventory manages items, PaymentProcessor handles payments).

Open/Closed: New payment types can be added without modifying existing classes (via the Strategy pattern).

Liskov Substitution: Payment processors implement the PaymentProcessor interface.

Interface Segregation: Interfaces are small and focused.

Dependency Inversion: High-level module Vending Machine depends on abstractions

Consider Scalability and Flexibility

Explain how your design can handle changes in scale and whether it would be easily to extend with new functionalities...

Create/Explain your diagram(s)

Try creating a class, flow, state and/or sequence diagram using the diagramming tool. Mermaid flow diagrams can be used to represent system use cases. You can ask the interviewer bot to create a starter diagram if unfamiliar with the tool. Briefly explain your diagrams if necessary...

Future improvements

Critically examine your design for any flaws or areas for future improvement...