ThreadGroup in Java:

It represents combination of threads within a group. A thread group can also include other thread groups .With the help of thread group we can easily suspend, resume or interrupt group of threads by a single method call.

It is implemented by java.lang.ThreadGroup class.

Example:

    public class ThreadDemo implements Runnable{  
        public void run() {  
              System.out.println(Thread.currentThread().getName());  
        }  
       public static void main(String[] args) {  
          ThreadDemo runnable = new ThreadDemo();  
              ThreadGroup tg1 = new ThreadGroup("Parent ThreadGroup"); //creating new group named Parent ThreadGroup 
                
              Thread t1 = new Thread(tg1, runnable,"one");  
              t1.start();  
              Thread t2 = new Thread(tg1, runnable,"two");  
              t2.start();  
              Thread t3 = new Thread(tg1, runnable,"three");  
              t3.start();  
                   
              System.out.println("Thread Group Name: "+tg1.getName());  
             tg1.list();  
      
        }  
       }  

Output:

one
two
three
Thread Group Name: Parent ThreadGroup
java.lang.ThreadGroup[name=Parent ThreadGroup,maxpri=10]
    Thread[one,5,Parent ThreadGroup]
    Thread[two,5,Parent ThreadGroup]
    Thread[three,5,Parent ThreadGroup]

In the above program we are creating a new thread group named Parent ThreadGroup which contains three threads (One,Two and Three) with priority 5.

By default  Java runtime system creates a ThreadGroup named main. Unless specified , all new threads that are created become members of the main thread group.