1. Product Management:
   • Ability to add, update, or remove items from the vending machine's inventory.
   • Each product should have attributes such as ID, name, price, description, and quantity in stock.
   • Mechanisms for handling out-of-order products and a system to notify customers when an item is unavailable.
2. Payment Handling:
   • Support for multiple payment types: coins, cash, and card payments.
   • Mechanisms for validating and processing payments securely.
   • Provide change for cash payments and maintain a record of transactions.
   • Address edge cases such as network failures during card processing and clearly defined procedures for refund processing, including different scenarios (e.g., customer cancellation, failed payment).
3. User Interface:
   • A responsive and user-friendly display that shows a list of available products, prices, and any current promotions.
   • An interface for customers to make selections and confirm their choices before finalizing the purchase.
   • Accessibility features, such as voice commands, touch-friendly interfaces, and screen readers, to accommodate users with disabilities.
   • Error messages and prompts for invalid selections or payment issues should be clear and informative.
4. Inventory Management:
   • Real-time tracking of product quantities within the vending machine.
   • Notifications to maintenance when stock runs low, including a list of items that need to be restocked.
   • Implement a notification service to alert maintenance personnel of low stock and mechanical failures, incorporating event-driven architecture for scalability.
5. Maintenance and Alerts:
   • Self-monitoring capabilities to alert the maintenance personnel about any jams, malfunctioning mechanisms, or low power.
   • A reporting system to log maintenance activities, issues that have been resolved, and also handle error states during transactions (e.g., failed payments, mechanical failures).
6. Transaction History:
   • Maintain a comprehensive log of transactions that includes details like item purchased, time, payment method, error states (e.g., failed payments, refunds due to dispensing issues), and any resolutions processed.
   • Detailed error handling and recovery processes to ensure a smooth user experience during failures in payment processing or dispensing.
7. User Roles and Access:
   • Different types of users (customers, maintenance personnel, and administrative staff) should have customized access and functionalities.
   • Admins should have access to sales reports and inventory levels for effective decision-making.
8. State Management:
   • Define distinct states for the vending machine operation (e.g., "idle," "waiting for payment," "item dispensing") and describe how the machine transitions between these states based on user interactions and internal processes.
9. Security Measures:
   • Implement security protocols for handling card transactions to ensure compliance with standards such as PCI-DSS.
   • Ensure sensitive data, such as user payment information, is stored securely.

Define Core Objects

1. Product
   • Attributes:
     • productId: string (Unique identifier for the product)
     • name: string (Name of the product)
     • price: float (Price of the product)
     • description: string (Description of the product)
     • quantity: int (Quantity in stock)
     • isAvailable: boolean (Indicates if the product is available)
   • Methods:
     • updateQuantity(int amount): Update the quantity based on restocking or sales.
     • setAvailability(boolean status): Set the availability status based on stock.
2. Payment
   • Attributes:
     • paymentId: string (Unique identifier for the payment)
     • amount: float (Amount to be processed)
     • method: enum (Payment method: coins, cash, card)
     • status: enum (Payment status: success, failed, pending)
     • timestamp: datetime (Time of the transaction)
   • Methods:
     • validate(): Validate the payment method and amount.
     • process(): Process the payment.
     • refund(): Handle refund logic.
3. Transaction
   • Attributes:
     • transactionId: string (Unique identifier for the transaction)
     • productId: string (References the purchased product)
     • paymentId: string (References the payment made)
     • timestamp: datetime (Time when the transaction took place)
     • errorState: string (Details about any errors during the transaction)
   • Methods:
     • logTransaction(): Log the transaction details.
     • handleError(string errorMessage): Detailed error handling logic.
4. User
   • Attributes:
     • userId: string (Unique identifier for the user)
     • role: enum (User role: customer, maintenance, admin)
     • accessPrivileges: string (Details about permissions for admin roles)
   • Methods:
     • login(): Authenticates the user.
     • logout(): Logs the user out.
5. Inventory
   • Attributes:
     • products: List (List of products in the inventory)
     • lowStockThreshold: int (Threshold for low stock)
   • Methods:
     • addProduct(Product product): Add a new product to the inventory.
     • removeProduct(string productId): Remove a product from the inventory.
     • checkStock(): Check the stock levels and notify maintenance if low.
6. MaintenanceAlert
   • Attributes:
     • alertId: string (Unique identifier for the alert)
     • message: string (Details of the alert)
     • timestamp: datetime (Time of the alert)
   • Methods:
     • sendAlert(): Notify maintenance personnel about issues.
     • logAlert(): Keep a record of maintenance alerts.

Analyze Relationships

Determine how these objects will interact with each other to fulfill the use cases...

Establish Hierarchy

Design inheritance trees where applicable to promote code reuse and polymorphism. This step involves identifying common attributes and behaviors that can be abstracted into parent classes...

Design Patterns

Consider using design patterns (e.g., Factory, Singleton, Observer, Strategy) that fit the problem...

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.

Adhere to SOLID Guidelines

Check and explain whether your design adheres to solid principles (Ask interviewer what SOLID principle is if you can not recall it.)...

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...