Bank.java

import java.io.Serializable;
import java.util.HashMap;
import java.util.Map;

public class Bank implements Serializable {

    private Map<String, Customer> customers = new HashMap<>();
    private int customerCounter = 1000;

    // Generate unique Customer ID
    public String generateCustomerId() {
        customerCounter++;
        return "CUST" + customerCounter;
    }

    // Add new customer
    public void addCustomer(Customer customer) {

        if (customer == null)
            return;

        if (customers.containsKey(customer.getCustomerId())) {
            System.out.println("Customer ID already exists.");
            return;
        }

        customers.put(customer.getCustomerId(), customer);
    }

    // Used during login (optional)
    public Customer authenticate(String id, String pin) {

        Customer customer = customers.get(id);

        if (customer != null && customer.authenticate(pin))
            return customer;

        return null;
    }

    // Used in updated BankService login
    public Customer findCustomer(String id) {
        return customers.get(id);
    }

    // Used for transfer functionality
    public Account findAccount(String accNo) {

        if (accNo == null)
            return null;

        for (Customer customer : customers.values()) {

            Account account = customer.getAccount(accNo);

            if (account != null)
                return account;
        }

        return null;
    }
}