// Java Program to demonstrate how  // to use CountDownLatch Its used  // when a thread needs to wait for other  // threads before starting its work. import java.util.concurrent.CountDownLatch; public class CountDownLatchDemo {  public static void main(String args[])   throws InterruptedException  {  // Let us create task that is going to   // wait for four threads before it starts  CountDownLatch latch = new CountDownLatch(4);  // Let us create four worker   // threads and start them.  Worker first = new Worker(1000 latch   'WORKER-1');  Worker second = new Worker(2000 latch   'WORKER-2');  Worker third = new Worker(3000 latch   'WORKER-3');  Worker fourth = new Worker(4000 latch   'WORKER-4');  first.start();  second.start();  third.start();  fourth.start();  // The main task waits for four threads  latch.await();  // Main thread has started  System.out.println(Thread.currentThread().getName() +  ' has finished');  } } // A class to represent threads for which // the main thread waits. class Worker extends Thread {  private int delay;  private CountDownLatch latch;  public Worker(int delay CountDownLatch latch  String name)  {  super(name);  this.delay = delay;  this.latch = latch;  }  @Override  public void run()  {  try  {  Thread.sleep(delay);  latch.countDown();  System.out.println(Thread.currentThread().getName()  + ' finished');  }  catch (InterruptedException e)  {  e.printStackTrace();  }  } }