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MyRunnable.java

public class MyRunnable implements Runnable
{
    public void run()
    {
        try
        {
            Thread.sleep(1000);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        System.out.println("Run by = "+Thread.currentThread().getName());
    }
}

/*
 * public int activeCount()
 * 
 * Returns an estimate of the number of active threads
 * in this thread group and its subgroups.
 */
public class ThreadGroupDemo
{
    public static void main(String[] args) throws InterruptedException
    {
        MyRunnable myRunnable = new MyRunnable();

        ThreadGroup parentTg = new ThreadGroup("Parent ThreadGroup");

        Thread t1 = new Thread(parentTg, myRunnable, "thread1");
        Thread t2 = new Thread(parentTg, myRunnable, "thread2");

        t1.start();
        t2.start();

        ThreadGroup childTg = new ThreadGroup(parentTg, "Child ThreadGroup");

        Thread t3 = new Thread(childTg, myRunnable, "thread3");
        t3.start();
        Thread t4 = new Thread(childTg, myRunnable, "thread4");
        t4.start();
        Thread t5 = new Thread(childTg, myRunnable, "thread5");
        t5.start();

        // display the number of active threads
        System.out.println("Active threads in \"" + childTg.getName() + "\" = "
                + childTg.activeCount());

        // display the number of active threads
        System.out.println("Active threads in \"" + parentTg.getName()
                + "\" = " + parentTg.activeCount());

    }
}

Active threads in "Child ThreadGroup" = 3
Active threads in "Parent ThreadGroup" = 5
Run by = thread2
Run by = thread4
Run by = thread3
Run by = thread5
Run by = thread1