Count Down Latch

/* 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();
        }
    }
}