A vending machine has clear states: Idle (waiting for selection), HasMoney (coins inserted), Dispensing (releasing product), and OutOfStock. Each state responds differently to the same actions (insert coin, select product, dispense).

Code

interface VendingState {
  insertCoin(amount: number): void
  selectProduct(code: string): void
  dispense(): void
  cancelTransaction(): void
}

class IdleState implements VendingState {
  constructor(private machine: VendingMachine) {}

  insertCoin(amount: number): void {
    this.machine.addBalance(amount)
    this.machine.setState(new HasMoneyState(this.machine))
  }

  selectProduct(code: string): void {
    this.machine.displayMessage("Insert money first")
  }

  dispense(): void {
    this.machine.displayMessage("Insert money and select product")
  }

  cancelTransaction(): void {
    this.machine.displayMessage("No transaction to cancel")
  }
}

Question

How should you implement the vending machine states?

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LLD|❓ LLD Interview Questions| 25 min

Design Vending Machine

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Code

interface VendingState {
  insertCoin(amount: number): void
  selectProduct(code: string): void
  dispense(): void
  cancelTransaction(): void
}

class IdleState implements VendingState {
  constructor(private machine: VendingMachine) {}

  insertCoin(amount: number): void {
    this.machine.addBalance(amount)
    this.machine.setState(new HasMoneyState(this.machine))
  }

  selectProduct(code: string): void {
    this.machine.displayMessage("Insert money first")
  }

  dispense(): void {
    this.machine.displayMessage("Insert money and select product")
  }

  cancelTransaction(): void {
    this.machine.displayMessage("No transaction to cancel")
  }
}

Question

How should you implement the vending machine states?