What is the ThreadGroup.activeGroupCount() function in Java?

import java.lang.Thread;
import java.lang.ThreadGroup;
class thread_new extends Thread   
{  
    thread_new(String thread_name, ThreadGroup Tg)  
    {  
        super(Tg, thread_name);  
    }  
    public void run()  
    {  
        System.out.println(Thread.currentThread().getName() +   
             " EXECUTION COMPLETED");  
    }  
}
public class EdPresso
{  
    public static void main(String arg[])  
    {  
        //create the thread group  
        ThreadGroup thg1 = new ThreadGroup("thread group-Parent");  
        ThreadGroup thg2 = new ThreadGroup(thg1, "threadGroup-Child");  
        ThreadGroup thg3 = new ThreadGroup(thg1, "threadGroup-Child");  
     
        // create the threads  
        thread_new th1 = new thread_new("Thread#1", thg1);  
        System.out.println(th1.getName() + " begins");  
        // calling the run() method  
        th1.start();  
  
        // check the total number of active thread  
        System.out.println("# of active threads: "  
                           + thg1.activeGroupCount());  
    }  
}