@startuml
left to right direction
package com.iluwatar.specialcase {
  class App {
    - LOGGER : Logger {static}
    + App()
    + main(args : String[]) {static}
  }
  interface ApplicationServices {
    + loggedInUserPurchase(String, String) : ReceiptViewModel {abstract}
  }
  class ApplicationServicesImpl {
    - domain : DomainServicesImpl
    + ApplicationServicesImpl()
    - isDownForMaintenance() : boolean
    + loggedInUserPurchase(userName : String, itemName : String) : ReceiptViewModel
  }
  class Db {
    - instance : Db {static}
    - itemName2Product : Map<String, Product>
    - user2Account : Map<User, Account>
    - userName2User : Map<String, User>
    + Db()
    + findAccountByUser(user : User) : Account
    + findProductByItemName(itemName : String) : Product
    + findUserByUserName(userName : String) : User
    + getInstance() : Db {static}
    + seedItem(itemName : String, price : Double)
    + seedUser(userName : String, amount : Double)
  }
  class Account {
    - amount : Double
    + Account(this$0 : Double)
    + getAmount() : Double
    + withdraw(price : Double) : MoneyTransaction
  }
  class Product {
    - price : Double
    + Product(this$0 : Double)
    + getPrice() : Double
  }
  class User {
    - userName : String
    + User(this$0 : String)
    + getUserName() : String
    + purchase(item : Product) : ReceiptDto
  }
  interface DomainServices {
  }
  class DomainServicesImpl {
    + DomainServicesImpl()
    - purchase(user : User, account : Account, itemName : String) : ReceiptViewModel
    + purchase(userName : String, itemName : String) : ReceiptViewModel
  }
  class DownForMaintenance {
    - LOGGER : Logger {static}
    + DownForMaintenance()
    + show()
  }
  class InsufficientFunds {
    - LOGGER : Logger {static}
    - amount : Double
    - itemName : String
    - userName : String
    + InsufficientFunds(userName : String, amount : Double, itemName : String)
    + show()
  }
  class InvalidUser {
    - LOGGER : Logger {static}
    - userName : String
    + InvalidUser(userName : String)
    + show()
  }
  class MaintenanceLock {
    - LOGGER : Logger {static}
    - instance : MaintenanceLock {static}
    - lock : boolean
    + MaintenanceLock()
    + getInstance() : MaintenanceLock {static}
    + isLock() : boolean
    + setLock(lock : boolean)
  }
  class MoneyTransaction {
    - amount : Double
    - price : Double
    + MoneyTransaction(amount : Double, price : Double)
  }
  class OutOfStock {
    - LOGGER : Logger {static}
    - itemName : String
    - userName : String
    + OutOfStock(userName : String, itemName : String)
    + show()
  }
  class ReceiptDto {
    - LOGGER : Logger {static}
    - price : Double
    + ReceiptDto(price : Double)
    + getPrice() : Double
    + show()
  }
  interface ReceiptViewModel {
    + show() {abstract}
  }
}
User --+ Db
Product --+ Db
MaintenanceLock -->  "-instance" MaintenanceLock
Db -->  "-instance" Db
ApplicationServicesImpl -->  "-domain" DomainServicesImpl
Account --+ Db
ApplicationServicesImpl ..|> ApplicationServices 
DomainServicesImpl ..|> DomainServices 
DownForMaintenance ..|> ReceiptViewModel 
InsufficientFunds ..|> ReceiptViewModel 
InvalidUser ..|> ReceiptViewModel 
OutOfStock ..|> ReceiptViewModel 
ReceiptDto ..|> ReceiptViewModel 
@enduml