Codemia | Master System Design Interviews Through Active Practice

My Solution for Design a Vending Machine with Score: 8/10

by alchemy_zephyr540

Requirements

The Machine needs to present a list of what items is selling and the price
When a customer come in, he/she can pick an item
- if still have item in stock, he needs to pay: coin, paper cash or card
  - if customer pay cash, the machine also needs to give correct change
- if no in stock, display an message
Also need an internal management system:
- to keep track if an inventory like daily, and send out alert if stock is low
- keep the cash storage, if it is full or to low cannot give out change also need to alert the management team

Core Objects & Relationships

VendingMachine:

Inventory

Payments

Transaction

ManagementService

CashStorage

Inventory

Map inventory

addItem(item, count)

checkStock(String id)

removeItem(id)

addItem(id, count)

Payments:

Use Strategy pattern, create a PaymentStrategy Interface:

CashPaymentStrategy
CoinPaymentStrategy
CardPaymentStrategy

Use a factory class to create encapsulate the creation of the correct PaymentStrategy

Also need a Transaction Class to book keeping

item,
time
payment type
total cash get
change give out

ManagementService

check inventory, if the stock is less then MIN_ITEM_COUNT, send out an alert code ITEM_LOW_STOCK
check cash storage, if cash full send out CASH_FULL alert
check the coin storage, if coin count less than MIN_COINT_COUNT snedout an LOW_COIN alert

APIs & Class Members (write code)

public class VendingMachine{
  private Inventory inventory;
  private PaymentProcessor paymentProcessor;
  private ManagementService managementService;
  private TransactionCollector transaction;
  private CashStorage cashStorage;

  public boolean vendItem(String itemId, payType, amount);
}
public class Inventory { // singleten
  private List<String, Item> items;
  private List<String, Integer> stock;
  
  Inventory getInstance();
  void addNewItem(Item item, int stock);
  boolean refill(String code, int stock);
  int getStock(String code)
  int sellItem(String item)
}

public class Item{
  String code;
  String name;
  double price;
}

public PaymentProcessor {
  PaymentStrategy paymentStrategy;
  
  boolean pay(double amount);
}

public class CashPaymentStrategy implements PaymentProcessor 
public class  CoinPaymentStrategy implements PaymentProcessor 
public class  CardPaymentStrategy implements PaymentProcessor 

public class PaymentFactory{
  PaymentFactory createPaymentStrategy(PaymentType);
}

public enum PaymentType{
  CASH,
  COIN,
  CARD
}

public class TransactionCollector {
  List<Transaction> transactions;
  addTransaction(Transaction);
  getAll();
}

public class Transaction{
  Datetime createdAt;
  String ItemCode;
  String ItemName;
  String paymentType;
  String amountPaid;
}


public class Alert{
  String message;
  Type ErroType;
  Datetime createdAt;
}

Tradeoffs, SOLID & Scalability

Supports markdown