Build an ATM

• States such as idle, card inserted, authenticated and operating, with only valid transitions allowed (for example withdraw before PIN verification is rejected)
• insertCard(cardNumber), enterPin(pin), checkBalance(), withdraw(amount), deposit(amount) (optional) and ejectCard(); a wrong PIN decrements remaining attempts, and the card is blocked after three failures
• withdraw verifies the account balance, the ATM's cash and an optional daily limit, then dispenses notes using the available denominations (for example 2000, 500, 100) with a greedy strategy that fails cleanly when the amount cannot be made
• Dispensing and debiting happen as one unit: if the dispenser cannot produce the cash, the account is not debited
• Every operation is recorded as a transaction (type, amount, balance after, time) available as a mini-statement
• The bank is a separate interface (verify PIN, debit, credit, get balance), so the ATM works against a fake bank in tests

Only after every requirement works.

• Receipt printing and an out-of-cash state
• Session timeout that ejects the card after inactivity
• Chain of Responsibility for the cash dispensers per denomination