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

import java.util.Random;


/**
 * A given number of threads, contenders, generate a random
 * number. The contender who submitts the largest value wins.
 *
 * In order for a conteder to know if they won, they must be
 * able to wait untill all contenders are done.
 */
public class CountDownExample {

    private static final int NUM_THREADS = 10;

    public static void main(String[] args) {
        // Make barrier, set to wait for NUM_THREADS call to countDown().

        CountDownLatch allDoneBarrier = new CountDownLatch(...);
        // Make and start threads. No need to store threads for waiting,
        // we can use the barrier for this.
        KeepLargestMonitor monitor = new KeepLargestMonitor();
        for (int i = 0; i < NUM_THREADS; i++) {
            ...
        }

        // wait for all contenders to be done
        try {
            ...
        } catch (InterruptedException e) {}

        System.out.println("The largest number: " + monitor.getLargest());

    }

}

/* Monitor to save the largest recieved value.  */
class KeepLargestMonitor {

    private final Lock lock = new ReentrantLock();
    private int largest;

    public int getLargest() {
        return largest;
    }

    public void giveNumber(int number) {
        ...
        try {
            largest = Math.max(largest, number);
        } finally {
            lock.unlock();
        }
    }

}


class Contender implements Runnable {
    private final KeepLargestMonitor monitor;
    private final CountDownLatch allDoneBarrier;
    private final int id;

    public Contender(KeepLargestMonitor monitor, CountDownLatch allDoneBarrier, int id) {
        this.id = id;
        this.monitor = monitor;
        this.allDoneBarrier = allDoneBarrier;
    }

    public void ...() {
        Random random = new Random();
        int number = random.nextInt(100); // max of 100
        System.out.printf("Thread #%d generated number: %d\n", id, number);
        monitor.giveNumber(number);

        // Report that done, wait for the rest
        allDoneBarrier. ...();
        try {
            allDoneBarrier. ...();
        } catch (InterruptedException e) {}

        if (number == monitor.getLargest()) {
            System.out.printf("Thread #%d won!\n", id);
        }
    }
}