Design A Vending Machine - Object-Oriented Design

Requirements

Personas: customer looking to purchase a snack, manager responsible for refilling inventory and collecting payment information.

Manager:

A manager can determine the total number of slots available in the machine.
A manager can determine how many slots are available and how many are occupied.
A manager can fill snacks in the slots.
A manager can know how much each snack costs.
The manager can define coins, cash, and cards as payment types.
The manager can obtain a report of an itemised list of items sold and gross revenue made per item.
The manager is notified when a customer encounters an error.
The manager is notified when it is time for a scheduled maintenance check.
Only a manager can manage the machine's inventory.

Customer:

A customer can know how much each snack costs.
A customer can select a snack by using a slot code as an identifier.
The customer must select which payment option to use.
The customer must pay for the snack in order to receive it.
The customer is told if there is an error receiving payment.
The customer is told if there is an error receiving the snack.
A customer can only buy one item at a time.
Only one customer may buy an item at a time.
An alarm sounds if the customer attempts to open the machine.

Define Core Objects

UserRole

defines a user's role in the system: Customer, Manager (administrator).
A customer can purchase items with a given payment type.
A manager can manage items and payment types.

User

represents a user and their UserRole.

VendingMachine

defines the number of available slots for snacks.
makes use of a payment system to accept payments on behalf of customers.
makes use of a payment system to provide inventory reports to managers
makes use of an inventory system to track dispensed snacks to customers
makes use of an inventory system to list available snacks for customers

PaymentType

Can be Coin, Cash, Card.
Is responsible for accepting payment according to type.

PaymentService

Ensures only managers can manage available PaymentTypes.
Processes payments for the vending machine.
Relays available PaymentType options
Relays any errors related to customers making payments according to their chosen payment type.
Accepts payment according to PaymentType.
Notifies the VendingMachine on successful payment.

Item

represents a snack in the vending machine's inventory.
has a price
has an id

InventoryService

ensures only managers can create items.
allows managers to create items for sale in the vending machine
allows managers to obtain a report of sold items and gross revenue
lists all available items for sale for customers
tracks items that are dispensed.

Analyze Relationships

A VendingMachine is composed of a PaymentService and an InventoryService. A VendingMachine performs actions on behalf of Customer users.

A PaymentService encapsulates functionality for payments. It accepts payments from Customers in the form of a PaymentType. A PaymentType knows how to process its payment and relay success or failure messages.

An InventoryService encapsulates creating Items for sale, listing available items for sale, tracking items that are sold and producing gross revenue per item reports.

Establish Hierarchy

interface: PaymentType

methods: processPayment();

Implementations: Coin, Cash, Card

VendingMachine is composed of:

PaymentService
InventoryService

Design Patterns

Singleton pattern for VendingMachine, PaymentService and InventoryService.
Strategy pattern for VendingMachine to accept payment and update inventory when a customer purchases an item.

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 VendingMachine { 
  PaymentService paymentService;
  InventoryService inventoryService;
VendingMachine(PaymentService paymentService, InventoryService inventoryService) {
this.paymentService = paymentService;
this.inventoryService = inventoryService;
}
public List<Item> listItems() {
return inventoryService.getItems();
}
public String purchaseItem(String itemId, Double amount, PaymentType paymentType) {
Item item = inventoryService.get(itemId);
if (!validateAmount(amount, item)) {
throw new Error("Insufficent amount to purchase item {itemId}");
}
boolean isSuccess = paymentService.purchase(String itemId, paymentType);
if (isSuccess) {
inventorySystem.decrement(itemId);
}
}
}

Adhere to SOLID Guidelines

Single Responsiblity Principle:

PaymentService handles payments
InventoryService handles inventory

Interface Segregation:

VendingMachine is composed of functionality using PaymentService and InventoryService

Consider Scalability and Flexibility

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...