Wait until child threads completed : Java

Question

Problem description : -

Step 1: Take input FILE_NAME from user at main thread.

Step 2: Perform 10 operations on

Answer 1

You may want to choose CountDownLatch from java.util.concurrent. From JavaDocs:

A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

Sample code:

import java.util.concurrent.CountDownLatch;

public class Test {
    private final ChildThread[] children;
    private final CountDownLatch latch;

    public Test() {
        this.children = new ChildThread[4];
        this.latch = new CountDownLatch(children.length);
        children[0] = new ChildThread(latch, "Task 1");
        children[1] = new ChildThread(latch, "Task 2");
        children[2] = new ChildThread(latch, "Task 3");
        children[3] = new ChildThread(latch, "Task 4");
    }

    public void run() {
        startChildThreads();
        waitForChildThreadsToComplete();
    }

    private void startChildThreads() {
        Thread[] threads = new Thread[children.length];

        for (int i = 0; i < threads.length; i++) {
            ChildThread child = children[i];
            threads[i] = new Thread(child);
            threads[i].start();
        }
    }

    private void waitForChildThreadsToComplete() {
        try {
            latch.await();
            System.out.println("All child threads have completed.");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private class ChildThread implements Runnable {
        private final String name;
        private final CountDownLatch latch;

        protected ChildThread(CountDownLatch latch, String name) {
            this.latch = latch;
            this.name = name;
        }

        @Override
        public void run() {
            try {
                // Implementation
                System.out.println(name + " has completed.");
            } finally {
                latch.countDown();
            }
        }
    }

    public static void main(String[] args) {
        Test test = new Test();
        test.run();
    }
}

Output:

Task 1 has completed. Task 4 has completed. Task 3 has completed. Task 2 has completed. All child threads have completed.