BankTest.java

import java.util.ArrayList;

// Account class remains unchanged
class Account {
    private double bal; // The current balance
    private int accnum; // The account number

    public Account(int a) {
        bal = 0.0;
        accnum = a;
    }

    public void deposit(double sum) {
        if (sum > 0)
            bal += sum;
        else
            System.err.println("Account.deposit(...): "
                    + "cannot deposit negative amount.");
    }

    public void withdraw(double sum) {
        if (sum > 0)
            bal -= sum;
        else
            System.err.println("Account.withdraw(...): "
                    + "cannot withdraw negative amount.");
    }

    public double getBalance() {
        return bal;
    }

    public double getAccountNumber() {
        return accnum;
    }

    public String toString() {
        return "Acc " + accnum + ": " + "balance = " + bal;
    }

    public final void print() {
        // Don't override this,
        // override the toString method
        System.out.println(toString());
    }
}

// Derived SavingsAccount class
class SavingsAccount extends Account {
    private double interestRate;

    public SavingsAccount(int a, double rate) {
        super(a);
        interestRate = rate;
    }

    public void addInterest() {
        super.deposit(super.getBalance() * interestRate);
    }
}

// Derived CurrentAccount class
class CurrentAccount extends Account {
    private double overdraftLimit;

    public CurrentAccount(int a, double limit) {
        super(a);
        overdraftLimit = limit;
    }

    @Override
    public void withdraw(double sum) {
        if (super.getBalance() - sum >= -overdraftLimit)
            super.withdraw(sum);
        else
            System.err.println("CurrentAccount.withdraw(...): "
                    + "exceeds overdraft limit.");
    }
}

// Bank class
class Bank {
    private ArrayList<Account> accounts;

    public Bank() {
        accounts = new ArrayList<>();
    }

    public void openAccount(Account acc) {
        accounts.add(acc);
    }

    public void closeAccount(Account acc) {
        accounts.remove(acc);
    }

    public void payDividend(double dividend) {
        for (Account acc : accounts) {
            acc.deposit(dividend);
        }
    }

    public void update() {
        for (Account acc : accounts) {
            if (acc instanceof SavingsAccount) {
                ((SavingsAccount) acc).addInterest();
            } else if (acc instanceof CurrentAccount && acc.getBalance() < 0) {
                System.out.println("Sending letter to account number " + acc.getAccountNumber() + " for overdraft.");
            }
        }
    }
}

public class BankTest {
    public static void main(String[] args) {
        // Testing classes
        SavingsAccount savings = new SavingsAccount(1001, 0.05);
        CurrentAccount current = new CurrentAccount(2001, 500);
        Bank bank = new Bank();

        bank.openAccount(savings);
        bank.openAccount(current);

        savings.deposit(1000);
        current.deposit(2000);

        bank.update();

        System.out.println("Savings Account Balance: " + savings.getBalance());
        System.out.println("Current Account Balance: " + current.getBalance());

        bank.payDividend(100);
        bank.update();

        System.out.println("After paying dividend...");
        System.out.println("Savings Account Balance: " + savings.getBalance());
        System.out.println("Current Account Balance: " + current.getBalance());
    }
}