public class Withdrawal extends Transaction
{
    private int amount;     // amount to withdraw
    private Keypad keypad;  // references to keypad
    private CashDispenser cashDispenser;    // references to cash dispenser

    // constant corresponding to menu option to cancel
    private final static int CANCELED = 6;

    // Withdrawal constructor
    public Withdrawal(int userAccountNumber, Screen atmScreen, BankDatabase atmBankDatabase, Keypad atmKeypad, CashDispenser atmCashDispenser){
        // initializes superclass variables
        super(userAccountNumber, atmScreen, atmBankDatabase);

        // initializes references to keypad and cash dispenser
        keypad = atmKeypad;
        cashDispenser = atmCashDispenser;
    }   // end Withdrawal constructor

    // perform transaction
    @Override
    public void execute(){
        boolean cashDispensed = false;  // cash was not dispensed yet
        double availableBalance;        // amount available for withdrawal

        // get references to bank database and screen
        BankDatabase bankDatabase = getBankDatabase();
        Screen screen = getScreen();

        // loop until cash is dispensed or the user cancels
        do{
            // obtain a chosen withdrawal amount from the user
            amount = displayMenuOfAmounts();

            // check whether user choose a withdrawal amount or canceled
            if(amount != CANCELED){
                // get available balance of account involved
                availableBalance = bankDatabase.getAvailableBalance(getAccountNumber());

                // check whether the user has enough money in the account
                if(amount <= availableBalance){

                    // check whether the cash dispenser has enough money
                    if(cashDispenser.isSufficientCashAvailable(amount)){

                        // update the account involved to reflect the withdrawal
                        bankDatabase.debit(getAccountNumber(), amount);

                        cashDispenser.dispenseCash(amount);     // dispense cash
                        cashDispensed = true;   // cash was dispensed

                        // instructs user to take cash
                        screen.displayMessageLine("\nYour cash has been dispensed. Please take your cash now.");
                    }   // end if
                    else{
                        // cash dispenser does not have enough cash
                        screen.displayMessageLine("\nInsufficient cash available in the ATM.");
                        screen.displayMessageLine("\nPlease choose a smaller amount.");
                    }   // end if
                }   // end if
                else{
                    // not enough money available in user's account
                    screen.displayMessageLine("\nInsufficient funds in your account.");
                    screen.displayMessageLine("\nPlease choose a smaller amount.");
                }   // end if
            }   // end if
            else{
                // user choose cancel menu option
                screen.displayMessageLine("\nCancelling transactions...");
                return;     // return to main menu because user canceled
            }   // end if
        }   while(!cashDispensed);
    }   // end method execute

    // display a menu of withdrawal amounts and the options to cancel
    // return the chosen amount or 0 if the user chooses to cancel
    private int displayMenuOfAmounts(){
        int userChoice = 0;     // local variable to store return value

        Screen screen = getScreen();    // get screen references

        // array of amounts to correspond to menu numbers
        int[] amounts = {0, 20, 40, 60, 100, 200};

        // loop while no valid choice has been made
        while(userChoice == 0){
            // display the withdrawal menu
            screen.displayMessageLine("\nWithdrawal Menu : ");
            screen.displayMessageLine("1 - $20");
            screen.displayMessageLine("2 - $40");
            screen.displayMessageLine("3 - $60");
            screen.displayMessageLine("4 - $100");
            screen.displayMessageLine("5 - $200");
            screen.displayMessageLine("6 - Cancel Transaction");
            screen.displayMessage("\nChoose a withdrawal amount : ");

            int input = keypad.getInput();      // get user input through keypad

            // determine how to proceed based on the input value
            switch(input){
                // if the user choose a withdrawal amount
                // i.e choose option 1, 2, 3, 4 or 5
                // return the corresponding amount from amounts's array
                case 1 :
                case 2 :
                case 3 :
                case 4 :
                case 5 :
                    userChoice = amounts[input];        // save user's choice
                    break;

                case CANCELED :
                    // the user choose to cancel
                    userChoice = CANCELED;      // save user's choice
                    break;

                default :
                    // the user did not enter value between 1-6
                    screen.displayMessageLine("\nInvalid selection.");
                    screen.displayMessageLine("Try again.");
            }   // end switch
        }   // end while

        return userChoice;      // return withdrawal amount or CANCELED
    }   // end method displayMenuOfAmounts
}   // end class Withdrawal