我正在學(xué)習(xí)多線程的概念，我試圖查找數(shù)組中的活動線程數(shù)，但ThreadGroup.activeCount()方法僅返回零值。這是代碼：線程對象類：-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");    }}主要課程：-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());        }    }}