Why do i get a null pointer exception

Question

import javax.swing.*;
class Rental
     public static void main (String [] args)
        int custCounter = 0;                              // counts the customers created so far
            Customer[] customers = new Customer[100];          // creates an array of 100 customers
        DomesticAssistant[] assistants = new DomesticAssistant[3];          // creates an array of [3] domestic assistants
        assistants[0] = new DomesticAssistant("Lazy", 20, 100, 90, 300);     // creates the domestic assistant and stores it in the array at position [0]
        assistants[0].setReservedFrom(0, 0, 0);          // sets the reserved from date to zero
        assistants[0].setReservedTo(0, 0, 0);          // sets the reserved to date to zero
        assistants[1] = new DomesticAssistant("Average", 30, 150, 135, 400);     // creates the domestic assistant and stores it in the array at position [1]
        assistants[1].setReservedFrom(0, 0, 0);          // sets the reserved from date to zero
        assistants[1].setReservedTo(0, 0, 0);          // sets the reserved to date to zero
        assistants[2] = new DomesticAssistant("Excellent", 50, 250, 220, 750);     // creates the domestic assistant and stores it in the array at position [2]
        assistants[2].setReservedFrom(0, 0, 0);          // sets the reserved from date to zero
        assistants[2].setReservedTo(0, 0, 0);          // sets the reserved to date to zero
          String firstChoice = JOptionPane.showInputDialog("MAIN MENU\n--------------------\n\nPlease enter a number based on the following: \n[1] Add New Customer\n[2] Login\n[3] Exit");     // the main menu
        if(firstChoice.equals("1"))          // if the user enteres "1" in the main menu (add a new customer)
            String custName = JOptionPane.showInputDialog(null, "NEW CUSTOMER MENU\n--------------------\n\nName: ");     // asks for name and stores in variable custName
               String custAddress = JOptionPane.showInputDialog(null, "NEW CUSTOMER MENU\n--------------------\n\nAddress: ");     // asks for sddress and stores in variable custAddress
            String custPostCode = JOptionPane.showInputDialog(null, "NEW CUSTOMER MENU\n--------------------\n\nPost Code: ");          // asks for post code and stores in variable custPostCode
            String custPhoneNo = JOptionPane.showInputDialog(null, "NEW CUSTOMER MENU\n--------------------\n\nTelephone Number: ");     // asks for phone number and stores in variable custPhoneNo
            String custEmail = JOptionPane.showInputDialog(null, "NEW CUSTOMER MENU\n--------------------\n\nEmail: ");          // asks for email and stores in variable custEmail
            String startingBalance = JOptionPane.showInputDialog(null, "NEW CUSTOMER MENU\n--------------------\n\nStarting Credit: ");     // asks for credit to start account with and stores in variable startingBalance
            int sBalance = Integer.parseInt(startingBalance);          // converts startingBalance from a string into an integer
            customers[custCounter] = new Customer(custName, custAddress, custPostCode, custPhoneNo, custEmail, sBalance);     // creates a customer with the variables/details entered above
            JOptionPane.showMessageDialog(null, "NEW CUSTOMER MENU\n--------------------\n\nWelcome" + custName + ".\nYour details were sucessfully added to the system.\nYour ID number is: " + customers[custCounter].getIdNumber() + "\nYour password is: " + customers[custCounter].getPassword());     // greets the new customer as well as giving them their id number and password
            custCounter++;          // increments the customer sounter by 1
        else if(firstChoice.equals("2"))          // if the user enters "2" in the main menu (login)
            String checkIdNo = JOptionPane.showInputDialog(null, "LOGIN\n--------------------\n\nPlease enter your ID Number: ");
            int checkIdNum = Integer.parseInt(checkIdNo);// asks user to enter their id number
            int position = 0;          //variable to store where we are in the array of customers so we know which customer we are dealing with
            for(int i = 0; i < customers.length; i++)          // loops through the array of customers
----->      if(customers.getIdNumber() == checkIdNum)          // when the id number entered is found
                         position = i;                              // set the position variable to that customer (the position in the customers array)
else                                             // if the id number is not found
JOptionPane.showMessageDialog(null, "LOGIN\n--------------------\n\nThat ID Number does not exist. ");     // tell the user that the id number wasnt found
System.exit(0);                                                                           // then exit the system
[\code]
Can anyone help. when i run the program i get a null pointer exception
can you tell me why, and possibly tell me how i can search through the array of customers searching for an id number
am i doing it a way that will work (not the best way) but just so it works
thanks in advance
D_H

Answer 1

Nevertheless, when you get to the loop where your exception occurs, you may have created any number of customers, but probably not 100.
The loop tries to go through all 100 spaces in the array, and as soon as it gets to a space you haven't assigned a Customer too, you'll get the exception.
Either make sure the loop doesn't go higher than you cutomercounter - 1 (to compensate for 0-based index of array), or check if the customer[i] == null before you try to call a method on it.
Does that make sense?
/D