Use the following files:

ATMTester.java

import java.io.IOException;

public class ATMTester
{
   public static void main(String[] args) throws IOException
   {
      Bank bank = new Bank();
      bank.readCustomers("customers.txt");
      ATM atm = new ATM(bank);
      atm.setCustomerNumber(1);
      atm.selectCustomer(1234);
      atm.selectAccount(ATM.SAVINGS);
      atm.deposit(4000);
      atm.transfer(2000);
      System.out.println("Savings: " + atm.getBalance());
      System.out.println("Expected: 2000");
      atm.back();
      atm.selectAccount(ATM.CHECKING);      
      atm.transfer(500);
      System.out.println("Checking: " + atm.getBalance());
      System.out.println("Expected: 1500");
      atm.back();
      atm.selectAccount(ATM.SAVINGS);      
      System.out.println("Savings: " + atm.getBalance());
      System.out.println("Expected: 2500");
   }
}

Bank.java

import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Scanner;

/**
   A bank contains customers with bank accounts.
*/
public class Bank
{  
   /**
      Constructs a bank with no customers.
   */
   public Bank()
   {  
      customers = new ArrayList<Customer>();
   }
   
   /**
      Reads the customer numbers and pins 
      and initializes the bank accounts.
      @param filename the name of the customer file
   */
   public void readCustomers(String filename) 
         throws IOException
   {  
      Scanner in = new Scanner(new FileReader(filename));
      while (in.hasNext())
      {  
         int number = in.nextInt();
         int pin = in.nextInt();
         Customer c = new Customer(number, pin);
         addCustomer(c);
      }
      in.close();
   }
   
   /**
      Adds a customer to the bank.
      @param c the customer to add
   */
   public void addCustomer(Customer c)
   {  
      customers.add(c);
   }
   
   /** 
      Finds a customer in the bank.
      @param aNumber a customer number
      @param aPin a personal identification number
      @return the matching customer, or null if no customer 
      matches
   */
   public Customer findCustomer(int aNumber, int aPin)
   {  
      for (Customer c : customers)
      {  
         if (c.match(aNumber, aPin))
            return c;
      }
      return null;
   }

   private ArrayList<Customer> customers;
}

BankAccount.java

/**
   A bank account has a balance that can be changed by 
   deposits and withdrawals.
*/
public class BankAccount
{  
   /**
      Constructs a bank account with a zero balance.
   */
   public BankAccount()
   {  
      balance = 0;
   }

   /**
      Constructs a bank account with a given balance.
      @param initialBalance the initial balance
   */
   public BankAccount(double initialBalance)
   {  
      balance = initialBalance;
   }
 
   /** 
      Deposits money into the account.
      @param amount the amount of money to withdraw
   */
   public void deposit(double amount) 
   {  
      balance = balance + amount;
   }

   /** 
      Withdraws money from the account.
      @param amount the amount of money to deposit
   */
   public void withdraw(double amount) 
   {  
      balance = balance - amount;
   }

   /** 
      Gets the account balance.
      @return the account balance
   */
   public double getBalance()
   {  
      return balance; 
   }

   private double balance; 
}

Customer.java

/**
   A bank customer with a checking and a savings account.
*/
public class Customer
{  
   /**
      Constructs a customer with a given number and PIN.
      @param aNumber the customer number
      @param aPin the personal identification number
   */
   public Customer(int aNumber, int aPin)
   {  
      customerNumber = aNumber;
      pin = aPin;
      checkingAccount = new BankAccount();
      savingsAccount = new BankAccount();
   }
   
   /** 
      Tests if this customer matches a customer number 
      and PIN.
      @param aNumber a customer number
      @param aPin a personal identification number
      @return true if the customer number and PIN match
   */
   public boolean match(int aNumber, int aPin)
   {  
      return customerNumber == aNumber && pin == aPin;
   }
   
   /** 
      Gets the checking account of this customer.
      @return the checking account
   */
   public BankAccount getCheckingAccount()
   {  
      return checkingAccount;
   }
   
   /** 
      Gets the savings account of this customer.
      @return the checking account
   */
   public BankAccount getSavingsAccount()
   {  
      return savingsAccount;
   }

   private int customerNumber;
   private int pin;
   private BankAccount checkingAccount;
   private BankAccount savingsAccount;
}