ThreadGroup.activeCount() method is not working in java

I am learning the concept of multi-threading, where i was trying to find the number of active threads in an array, but the method of ThreadGroup.activeCount() is only returning zero value for it.

Here is the code:

Thread object class :-

class th1 extends Thread
{
    public th1(String threadName, ThreadGroup tg1)
    {
        super(tg1, threadName);
    }

    @Override
    public void run() 
    {
        try {
            Thread.sleep(5000);
        } 
        catch (InterruptedException e) 
        {
            e.printStackTrace();
        }

        System.out.println(Thread.currentThread().getName() + " is running");
    }
}

main class :-

public class enumerate_demo 
{
    public static void main(String[] args) 
    {
        ThreadGroup tg1 = new ThreadGroup("group 1");

        Thread t1 = new Thread(new th1("t-1", tg1));
        t1.start();

        Thread t2 = new Thread(new th1("t-2", tg1));
        t2.start();

        Thread t3 = new Thread(new th1("t-3", tg1));
        t3.start();

        System.out.println("Number of active count :- " + tg1.activeCount());

        Thread[] group = new Thread[tg1.activeCount()];

        int count = tg1.enumerate(group);

        for (int i = 0; i < count; i++)
        {
            System.out.println("Thread " + group[i].getName());
        }
    }
}

The issue is that while creating instances th1 class you are using them as Runnable as oppose to Thread. And those wrapper threads aren't associated with any ThreadGroup . Declare variables as follows.

        Thread t1 = new th1("t-1", tg1);
        t1.start();