Multiple access example

With this example we are going to demonstrate how to have multiple access over an object. The example describes the transfer of amounts of bank accounts in a bank. In short:

  • We have created a class named SwissBank. It has a double array called accounts, a Lock called bankLock and a Condition called sufficientFunds. It has a constructor in which for a specified int n and a specified double initBalance, it creates a new accounts array with capacity equal to n, and sets each field of the array to the specified initial balance. It also creates a new ReentrantLock and a new Condition bound to this lock, using newCondition() API method of Lock. In its transfer(int src, int target, double amount) method, it aqcuires the lock, using lock() API method of Lock and calls await() API method of Condition, that causes the current thread to wait until it is signalled or interrupted. After making the transfer of the specified double amount from a source account to the target account of the accounts array, it wakes up all waiting threads, calling signalAll() API method of Condition and releases the lock, with unlock() API method of Lock. In its getTotalBalance() method, it acquires the lock, using lock() API method of Lock gets the sum of all fields in the accounts array and then releases the lock, with unlock() API method of Lock. In its size() method it gets the length of the accounts array.
  • We also have a class named TransferThread that implements the Runnable. It has a SwissBank variable, an int srcAmmount, a double maxAmount and an int timeout. It overrides the run() method of the Runnable, where it calls the transfer() method of its SwissBank object forever, after sleeping for the specified timeout.
  • We create a new instance of the SwissBank with a specified account number and initial balance, and for each one of the accounts we create a new TransferThread and cause the thread’s execution with its start() API method.

Let’s take a look at the code snippet that follows:  

package com.javacodegeeks.snippets.core;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class MultipleAccess {

    public static final int ACCOUNTNUM = 100;
    public static final double INIT_BALANCE = 1000;

    public static void main(String[] args) {

  SwissBank b = new SwissBank(ACCOUNTNUM, INIT_BALANCE);

  int i;

  for (i = 0; i < ACCOUNTNUM; i++) {

TransferThread tThread = new TransferThread(b, i, INIT_BALANCE);

Thread thr = new Thread(tThread);



class TransferThread implements Runnable {

    private SwissBank bank;
    private int srcAccount;
    private double maxAmount;
    private int timeout = 10;

    public TransferThread(SwissBank b, int src, double amount) {

  bank = b;

  srcAccount = src;

  maxAmount = amount;

    public void run() {

  try {

while (true) {

    int targetAccount = (int) (bank.size() * Math.random());

    double cashAmount = maxAmount * Math.random();

    bank.transfer(srcAccount, targetAccount, cashAmount);

    Thread.sleep((int) (timeout * Math.random()));


  } catch (InterruptedException ex) {


class SwissBank {

    private final double[] accounts;
    private Lock bankLock;
    private Condition sufficientFunds;

    public SwissBank(int n, double initBalance) {

  accounts = new double[n];

  for (int i = 0; i < accounts.length; i++) {

accounts[i] = initBalance;


  bankLock = new ReentrantLock();

  sufficientFunds = bankLock.newCondition();

    public void transfer(int src, int target, double amount) throws InterruptedException {


  try {

while (accounts[src] < amount) {




accounts[src] -= amount;

System.out.printf(" %10.2f from %d to %d", amount, src, target);

accounts[target] += amount;

System.out.printf(" Total Balance: %10.2f%n", getTotalBalance());


  } finally {



    public double getTotalBalance() {


  try {

double am = 0;

for (double a : accounts) {

    am += a;


return am;

  } finally {



    public int size() {

  return accounts.length;

This was an example of how to have multiple access over an object in Java.

Byron Kiourtzoglou

Byron is a master software engineer working in the IT and Telecom domains. He is an applications developer in a wide variety of applications/services. He is currently acting as the team leader and technical architect for a proprietary service creation and integration platform for both the IT and Telecom industries in addition to a in-house big data real-time analytics solution. He is always fascinated by SOA, middleware services and mobile development. Byron is co-founder and Executive Editor at Java Code Geeks.
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