Listing -. Using a Countdown Latch to Trigger a Coordinated Start

import java.util.concurrent.CountDownLatch;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class CountDownLatchDemo

{

    final static int NTHREADS = ;

    public static void main(String[] args)

    {

        final CountDownLatch startSignal = new CountDownLatch();

        final CountDownLatch doneSignal = new CountDownLatch(NTHREADS);

        Runnable r = new Runnable()

        {

            @Override

            public void run()

            {

                try

                {

                    report("entered run()");

                    startSignal.await(); // wait until told to ...

                    report("doing work"); // ... proceed

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

                    doneSignal.countDown(); // reduce count on which

                    // main thread is ...

                } // waiting

                catch (InterruptedException ie)

                {

                System.err.println(ie);

                }

            }

            void report(String s)

            {

            System.out.println(System.currentTimeMillis() +

            ": " + Thread.currentThread() +            ": " + s);

            }

        };

        ExecutorService executor = Executors.newFixedThreadPool(NTHREADS);

        for (int i = ; i < NTHREADS; i++)

            executor.execute(r);

            try

            {

                System.out.println("main thread doing something");

                Thread.sleep(); // sleep for 1 second

                startSignal.countDown(); // let all threads proceed

                System.out.println("main thread doing something else");

                doneSignal.await(); // wait for all threads to finish

                executor.shutdownNow();

            }

            catch (InterruptedException ie)

            {

            System.err.println(ie);

            }

    }

}