why am I getting nullPointerException

Question

I have the following code; however, it seems that I'm accessing an index in the arraylist that doesn't exist... here's the code. Any help appreciated.

import java.util.*;

public class Main {


    public static void main(String[] args) {

        ArrayList<BankAccount> allAccounts = new ArrayList<BankAccount>();

        Customer john = new Customer();
        john.firstName = "John";
        john.lastName = "Doe";

        BankAccount johnBa = new BankAccount();
        johnBa.accNumber = "111-222-333";
        johnBa.balance = 200;
        johnBa.myCustomer = john;

        Customer nick = new Customer();
        nick.firstName = "Nick";
        nick.lastName = "James";

        BankAccount nickBa = new BankAccount();
        nickBa.accNumber = "222-333-444";
        nickBa.balance = 100;

        allAccounts.add(johnBa);
        allAccounts.add(nickBa);

        ArrayList<Customer> allCust = new ArrayList<Customer>();
        allCust = extractCustomers(allAccounts);

        for(Customer c : allCust) {
            System.out.println(c.firstName+" "+c.lastName);
        }       


    }

    static ArrayList<Customer> extractCustomers(ArrayList<BankAccount> ba) {
        ArrayList<Customer> cu = new ArrayList<Customer>();

        for(BankAccount b: ba) {
            cu.add(b.myCustomer);
        }

        return cu;
    }

}


public class BankAccount {

    String accNumber;
    double balance; 

    Customer myCustomer;

}

public class Customer {

    String firstName;
    String lastName;

}

Answer 1

 BankAccount nickBa = new BankAccount();
        nickBa.accNumber = "222-333-444";
        nickBa.balance = 100;

No customer assigned here for second BankAccount .

But, you are trying to print customer details. For second BankAccount c would be null . Any operation on null reference results in NullPointerException .

 for(Customer c : allCust) {
            System.out.println(c.firstName+" "+c.lastName);
        }   

Make sure c is not null before making any calls on c to avoid NullPointerException .

  for(Customer c : allCust) {
           if(c != null){
            System.out.println(c.firstName+" "+c.lastName);
               }
        } 

Answer 2

You need to initialisze your Customer obj in BankAccount class as below.

Customer myCustomer = new Customer();

Also its not recommended to access class variables like you're doing. Create get and set methods and access your Customer obj by invoking getCustomer()