Solution: Direction of navigation

 1 import java.util.ArrayList;
 2 
 3 public class Customer {
 4 
 5     //fields
 6     private String name;
 7     private int customerID;
 8     private ArrayList<Account> accounts = new ArrayList<Account>();
 9     private static int cntCustomerID = 1000000;
10 
11     //constructor
12     public Customer() {
13         customerID = cntCustomerID++;
14     }
15 
16     //methods
17     public int getCustomerID() {
18         return customerID;
19     }
20     public void setName(String name) {
21         this.name = name;
22     }
23     public String getName() {
24         return name;
25     }
26     public void addAccount(Account account) {
27         accounts.add(account);
28     }
29     public ArrayList<Account> getAccounts() {
30         return accounts;
31     }
32 }

 1 import java.util.ArrayList;
 2 
 3 public class Account {
 4     private int accountNumber;
 5     private float balance;
 6     private ArrayList<Customer> customers = new ArrayList<>(); // an account holds more than one authorized customers
 7 
 8     private static int cntAccountNumber = 10001000;
 9 
10     public Account() {
11         accountNumber = cntAccountNumber++;
12     }
13     public int getAccountNumber() {
14         return accountNumber;
15     }
16     public void transaction(float sum) {
17         balance += sum;
18     }
19     public float getBalance() {
20         return balance;
21     }
22 
23     public ArrayList<Customer> getCustomers() {
24         return customers;
25     }
26     public void addCustomer(Customer customer) {
27         customers.add(customer);
28     }
29 }

 1 import java.util.ArrayList;
 2 
 3 public class Main {
 4 
 5     public static void main(String[] args) {
 6         Customer customer1 = new Customer();
 7         customer1.setName("Mueller");
 8         Customer customer2 = new Customer();
 9         customer2.setName("Meier");
10 
11         Account account1 = new Account();
12         Account account2 = new Account();
13         customer1.addAccount(account1);
14         customer1.addAccount(account2);
15         account1.addCustomer(customer1);
16         account1.addCustomer(customer2);
17 
18         account2.addCustomer(customer1);
19         
20         account1.transaction(100);
21         account2.transaction(-20);
22 
23         System.out.println("customer id="+customer1.getCustomerID()+"\tName="+customer1.getName()+":");
24         float sum = 0;
25         ArrayList<Account> accounts =  customer1.getAccounts();  // navigates from customer to account
26         for (Account account: accounts) {
27             System.out.println("\taccount number="+account.getAccountNumber()+"\tbalance="+account.getBalance()+":");
28             ArrayList<Customer> customers = account.getCustomers();  // navigates from account to customer
29             for (Customer customer: customers) {
30                 System.out.println("\tauthorized customerID="+customer.getCustomerID()+"\tname="+customer.getName()+":");
31             }
32             sum+=account.getBalance();
33         }
34         System.out.print("\n\ttotal sum="+sum);
35     }
36 }