public class ATM
{
    private boolean userAuthenticated;  // whether user is authenticated
    private int currentAccountNumber;   // current user's account number
    private Screen screen;  // ATM's screen
    private Keypad keypad;  // ATM's keypad
    private CashDispenser cashDispenser;    // ATM's cash dispenser
    private DepositSlot depositSlot;        // ATM's deposit slot
    private BankDatabase bankDatabase;      // account information database

    // constants corresponding to main menu options
    private static final int BALANCE_INQUIRY = 1;
    private static final int WITHDRAWAL = 2;
    private static final int DEPOSIT = 3;
    private static final int EXIT = 4;

    // no-argument ATM constructor initializes instance variables
    public ATM(){
        userAuthenticated = false;  // user is not authenticated to start
        currentAccountNumber = 0;   // no current account number to start
        screen = new Screen();      // create screen
        keypad = new Keypad();      // create keypad
        cashDispenser = new CashDispenser();    // create cash dispenser
        depositSlot = new DepositSlot();        // create deposit slot
        bankDatabase = new BankDatabase();      // create account info database
    }   // end no argument ATM constructor

    // start ATM
    public void run(){
        // welcome and authenticate user; perform transactions
        while(true){
            // loop while user is not yet authenticated
            while(!userAuthenticated){
                screen.displayMessageLine("\nWelcome!");
                authenticateUser();     // authenticate user
            }   // end while

            performTransactions();      // user is now authenticated
            userAuthenticated = false;  // reset before next ATM session
            currentAccountNumber = 0;   // reset before next ATM session
            screen.displayMessageLine("\nThank You!\nGoodbye!");
        }   // end while
    }   // end method run

    // attempts to authenticate user against database
    private void authenticateUser(){
        screen.displayMessage("\nPlease enter your account number : ");
        int accountNumber = keypad.getInput();  // input account number
        screen.displayMessage("\nEnter your PIN : ");   // prompt for PIN
        int pin = keypad.getInput();    // input PIN

        // set userAuthenticated to boolean value returned by database
        userAuthenticated = bankDatabase.authenticateUser(accountNumber, pin);

        // check whether authentication succeeded
        if(userAuthenticated){
            currentAccountNumber = accountNumber;   // save user's account
        }   // end if
        else{
            screen.displayMessageLine("Invalid Account Number or PIN.");
            screen.displayMessageLine("Please Try Again.");
        }
    }   // end method authenticateUser

    // display the main menu and perform transactions
    private void performTransactions(){
        // local variable to store transaction currently being processed
        Transaction currentTransaction = null;
        boolean userExited = false;     // user has not chosen to exit

        // loop while user has not chosen option to exit system
        while(!userExited){
            // show main menu and get user selection
            int mainMenuSelection = displayMainMenu();

            // decide how to proceed based on user's menu selection
            switch(mainMenuSelection){
                // user choose to perform one of three transaction types
                case BALANCE_INQUIRY :
                case WITHDRAWAL :
                case DEPOSIT :

                    // initialize as new object choosen type
                    currentTransaction = createTransaction(mainMenuSelection);

                    currentTransaction.execute();   // execute transaction
                    break;

                case EXIT :
                // user choose to terminate session
                    screen.displayMessageLine("\nExiting the system...");
                    userExited = true;      // this ATM session should end
                    break;

                default :
                // user did not enter an integer between 1-4
                    screen.displayMessageLine("\nYou did not enter a valid selection.");
                    screen.displayMessageLine("Please try again.");
                    break;
            }   // end switch
        }   // end while
    }   // end method performTransactions

    // display the main menu and return an input selection
    private int displayMainMenu(){
        screen.displayMessageLine("\nMain Menu :");
        screen.displayMessageLine("1 - View my balance");
        screen.displayMessageLine("2 - Withdraw cash");
        screen.displayMessageLine("3 - Deposit funds");
        screen.displayMessageLine("4 - Exit\n");
        screen.displayMessage("Enter a choice : ");
        return keypad.getInput();   // return user's selection
    }   // end method of displayMainMenu

    // return object of specified Transaction subclass
    private Transaction createTransaction(int type){
        Transaction temp = null;        // temporary Transaction variable

        // determine which type of Transaction to create
        switch(type){
            case BALANCE_INQUIRY :
            // create new BalanceInquiry transaction
                temp = new BalanceInquiry(currentAccountNumber, screen, bankDatabase);
                break;

            case WITHDRAWAL :
            // create new Withdrawal transaction
                temp = new Withdrawal(currentAccountNumber, screen, bankDatabase, keypad, cashDispenser);
                break;

            case DEPOSIT :
            // create new Deposit transaction
                temp = new Deposit(currentAccountNumber, screen, bankDatabase, keypad, depositSlot);
                break;
        }   // end switch

        return temp;    // return newly created object
    }   // end method createTransaction
}   // end class ATM