• ThreadGroup creates the group of the threads.
• Each thread group has its name.
• Each thread uses the thread group in its object creation

public class Threadgroup {

	public static void main(String[] args) {

		ThreadGroup grp = new ThreadGroup("group1");
		mythread m1 = new mythread(grp, "thread1");
		mythread m2 = new mythread(grp, "thread2");
		mythread m3 = new mythread(grp, "thread3");
		m1.start();
		m2.start();
		m3.start();
	}
}

class mythread extends Thread {
	public mythread(ThreadGroup g, String s) {
		super(g, s);
	}

	@Override
	public void run() {
		System.out.println("Thread group, prority, thread name "
				+ Thread.currentThread());
	}
}

Output

Thread group, prority, thread name Thread[thread2,5,group1]
Thread group, prority, thread name Thread[thread3,5,group1]
Thread group, prority, thread name Thread[thread1,5,group1]