Requirements:

The ATM system should support the following operations:

• Balance inquiry
• Cash withdrawal
• Cash deposit
• User authentication using card and PIN
• Interaction with a bank's backend system to validate user accounts and perform transactions
• Cash dispensing functionality
• Handling of concurrent access and ensuring data consistency
• User-friendly interface

Sequnece Diagram

Design Overview:

The low-level design for the ATM system involves several components that will interact with each other to handle various tasks such as authentication, balance inquiry, withdrawal, deposit, and cash dispensing. Below are the key components that need to be designed:

• User: A class to represent the user details.
• BankAccount: A class to represent the user's bank account and manage balance and transaction history.
• ATM: The central class managing the interaction between the user, the bank system, and the ATM hardware (cash dispenser, etc.).
• CashDispenser: The class to simulate dispensing cash.
• Transaction: A class to manage the transaction details such as withdrawal, deposit, etc.
• BackendSystem: Simulates the bank's backend that validates user information and performs actions.

Code Implementation:

// BankAccount Class - Represents the user's bank account
class BankAccount {
    private String accountNumber;
    private double balance;

    public BankAccount(String accountNumber, double initialBalance) {
        this.accountNumber = accountNumber;
        this.balance = initialBalance;
    }

    public double getBalance() {
        return balance;
    }

    public boolean withdraw(double amount) {
        if (balance >= amount) {
            balance -= amount;
            return true;
        }
        return false;
    }

    public void deposit(double amount) {
        balance += amount;
    }
}

// User Class - Represents an ATM user
class User {
    private String cardNumber;
    private String pin;
    private BankAccount bankAccount;

    public User(String cardNumber, String pin, BankAccount bankAccount) {
        this.cardNumber = cardNumber;
        this.pin = pin;
        this.bankAccount = bankAccount;
    }

    public boolean authenticate(String inputCardNumber, String inputPin) {
        return cardNumber.equals(inputCardNumber) && pin.equals(inputPin);
    }

    public BankAccount getBankAccount() {
        return bankAccount;
    }
}

// CashDispenser Class - Simulates the dispensing of cash
class CashDispenser {
    public void dispenseCash(double amount) {
        System.out.println("Dispensing cash: " + amount);
    }
}

// BackendSystem Class - Simulates the bank's backend system
class BackendSystem {
    public boolean validateAccount(String cardNumber, String pin) {
        // Simulate validating credentials (in real life, interact with the database or system)
        return cardNumber.equals("12345") && pin.equals("1234");
    }
}

// ATM Class - Represents the ATM system
class ATM {
    private CashDispenser cashDispenser;
    private BackendSystem backendSystem;
    private User currentUser;

    public ATM() {
        this.cashDispenser = new CashDispenser();
        this.backendSystem = new BackendSystem();
    }

    public void authenticateUser(String cardNumber, String pin) {
        if (backendSystem.validateAccount(cardNumber, pin)) {
            // Simulate finding the user from the system
            BankAccount account = new BankAccount(cardNumber, 1000); // Assume initial balance
            this.currentUser = new User(cardNumber, pin, account);
            System.out.println("Authentication successful.");
        } else {
            System.out.println("Authentication failed. Invalid card number or PIN.");
        }
    }

    public void checkBalance() {
        if (currentUser != null) {
            System.out.println("Your current balance is: " + currentUser.getBankAccount().getBalance());
        } else {
            System.out.println("Please authenticate first.");
        }
    }

    public void depositCash(double amount) {
        if (currentUser != null) {
            currentUser.getBankAccount().deposit(amount);
            System.out.println("Deposited: " + amount);
        } else {
            System.out.println("Please authenticate first.");
        }
    }

    public void withdrawCash(double amount) {
        if (currentUser != null) {
            boolean success = currentUser.getBankAccount().withdraw(amount);
            if (success) {
                cashDispenser.dispenseCash(amount);
            } else {
                System.out.println("Insufficient funds.");
            }
        } else {
            System.out.println("Please authenticate first.");
        }
    }
}

// Main class to simulate ATM operations
public class ATMSystemApp {
    public static void main(String[] args) {
        ATM atm = new ATM();

        // Authenticate user
        atm.authenticateUser("12345", "1234");

        // Perform operations
        atm.checkBalance();
        atm.depositCash(200);
        atm.checkBalance();
        atm.withdrawCash(300);
        atm.checkBalance();
        atm.withdrawCash(1000); // Insufficient funds case
    }
}

---

Sample Input:

    Authenticate user with card number "12345" and PIN "1234".
    Deposit 200 units.
    Withdraw 300 units.
    Withdraw 1000 units (which should fail due to insufficient balance).

Sample Output:

    Authentication successful.
    Your current balance is: 1000.0
    Deposited: 200
    Your current balance is: 1200.0
    Dispensing cash: 300
    Your current balance is: 900.0
    Insufficient funds.

Explanation of Output:

• The user is authenticated using card number "12345" and PIN "1234".
• The balance after successful authentication is 1000.0.
• The user deposits 200 units, increasing the balance to 1200.0.
• The user successfully withdraws 300 units, reducing the balance to 900.0.
• When attempting to withdraw 1000 units, the system responds with "Insufficient funds" since the available balance is only 900.0.

Design Considerations:

• Concurrency: In a real-world scenario, we need to ensure that the ATM system handles multiple users accessing the machine simultaneously. This can be achieved through synchronized methods or other concurrency management techniques.
• Security: Sensitive user data like PIN and account number should be securely stored and transmitted. Encryption and secure communication protocols should be used in a real-world implementation.
• Bank System Integration: In the actual system, the backend system would need to interact with a real bank's backend API or database for account validation, transaction history, and other tasks.
• Error Handling: The design assumes simple success/failure cases. In reality, more robust error handling (e.g., invalid input, transaction failures) should be included.

Extensions & Future Improvements:

• Implementing transaction history to keep track of deposits, withdrawals, and balance changes.
• Adding multi-currency support to allow users to choose their preferred currency.
• Adding a display interface for users to view transaction details and confirmation.
• Handling advanced features such as PIN change, card blocking, and multi-ATM system integration.