Java - ThreadGroup list() Method with Examples

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Description

The Java ThreadGroup list() method prints information about this thread group to the standard output. This is useful only for debugging.

Declaration

Following is the declaration for java.lang.ThreadGroup.list() method

public void list()

Parameters

NA

Return Value

This method does not return any value.

Exception

NA

Example 1

The following example shows the usage of ThreadGroup list() method in case of a single ThreadGroup object. We've created a ThreadGroup object and assigned it a name. Then we've created two threads using the threadgroup object created earlier. Using list() method, we're getting the information of this thread group object.

package com.tutorialspoint;

public class ThreadGroupDemo implements Runnable {
   public static void main(String[] args) {
      ThreadGroupDemo tg = new ThreadGroupDemo();
      tg.start();
   }

   public void start() {
      try {     
         // create a ThreadGroup
         ThreadGroup threadGroup = new ThreadGroup("ThreadGroup");

         // create a thread
         Thread t1 = new Thread(threadGroup, this);
         System.out.println("Starting " + t1.getName() + "...");
         t1.start();
            
         // create another thread
         Thread t2 = new Thread(threadGroup, this);
         System.out.println("Starting " + t2.getName() + "...");
         t2.start();

         // print the details of the thread group
         threadGroup.list();

         // block until the other threads finish
         t1.join();
         t2.join();        


      } catch (InterruptedException ex) {
         System.out.println(ex.toString());
      }
   }

   // implements run()
   public void run() {

      for(int i = 0; i < 4;i++) {
         i++;
         try {
			Thread.sleep(50);
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
      }
      System.out.println(Thread.currentThread().getName() + " finished executing.");
   }
} 

Output

Let us compile and run the above program, this will produce the following result −

Starting Thread-0...
Starting Thread-1...
java.lang.ThreadGroup[name=ThreadGroup,maxpri=10]
    Thread[Thread-0,5,ThreadGroup]
    Thread[Thread-1,5,ThreadGroup]
Thread-1 finished executing.
Thread-0 finished executing.

Example 2

The following example shows the usage of ThreadGroup list() method in case of multiple ThreadGroup objects. We've created a ThreadGroup object and assigned it a name. As next, we've created a child ThreadGroup object. Then we've created two threads using the threadgroup objects created earlier. Using list() method, we're printing details of each threadgroup object.

package com.tutorialspoint;

public class ThreadGroupDemo implements Runnable {
   public static void main(String[] args) {
      ThreadGroupDemo tg = new ThreadGroupDemo();
      tg.start();
   }

   public void start() {
      try {     
         // create a parent ThreadGroup
         ThreadGroup pThreadGroup = new ThreadGroup("parent ThreadGroup");
		 
         // create a child ThreadGroup for parent ThreadGroup
         ThreadGroup cThreadGroup = new ThreadGroup(pThreadGroup, "child ThreadGroup");

         // create a thread
         Thread t1 = new Thread(pThreadGroup, this);
         System.out.println("Starting " + t1.getName() + "...");
         t1.start();
            
         // create another thread
         Thread t2 = new Thread(cThreadGroup, this);
         System.out.println("Starting " + t2.getName() + "...");
         t2.start();

         // print the details of the thread groups
         pThreadGroup.list();
         cThreadGroup.list();
         
         // block until the other threads finish
         t1.join();
         t2.join();

      } catch (InterruptedException ex) {
         System.out.println(ex.toString());
      }
   }

   // implements run()
   public void run() {

      for(int i = 0; i < 4;i++) {
         i++;
         try {
			Thread.sleep(50);
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
      }
      System.out.println(Thread.currentThread().getName() + " finished executing.");
   }
} 

Output

Let us compile and run the above program, this will produce the following result −

Starting Thread-0...
Starting Thread-1...
java.lang.ThreadGroup[name=parent ThreadGroup,maxpri=10]
    Thread[Thread-0,5,parent ThreadGroup]
    java.lang.ThreadGroup[name=child ThreadGroup,maxpri=10]
        Thread[Thread-1,5,child ThreadGroup]
java.lang.ThreadGroup[name=child ThreadGroup,maxpri=10]
    Thread[Thread-1,5,child ThreadGroup]
Thread-0 finished executing.
Thread-1 finished executing.

Example 3

The following example shows the usage of ThreadGroup list() method in case of child and grandchild ThreadGroup objects. We've created a ThreadGroup object and assigned it a name. As next, we've created a child ThreadGroup object. Then we've created two threads using the child and grandchild threadgroup objects created earlier. Using list() method, we're printing name of the parent of each threadgroup object.

package com.tutorialspoint;

public class ThreadGroupDemo implements Runnable {
   public static void main(String[] args) {
      ThreadGroupDemo tg = new ThreadGroupDemo();
      tg.start();
   }

   public void start() {
      try {     
         // create a parent ThreadGroup
         ThreadGroup pThreadGroup = new ThreadGroup("Parent ThreadGroup");
		 
         // create a child ThreadGroup for parent ThreadGroup
         ThreadGroup cThreadGroup = new ThreadGroup(pThreadGroup, "Child ThreadGroup");
		 
         // create a grandchild ThreadGroup for parent ThreadGroup
         ThreadGroup gThreadGroup = new ThreadGroup(cThreadGroup, "GrandChild ThreadGroup");

         // create a thread
         Thread t1 = new Thread(cThreadGroup, this);
         System.out.println("Starting " + t1.getName() + "...");
         t1.start();
            
         // create another thread
         Thread t2 = new Thread(gThreadGroup, this);
         System.out.println("Starting " + t2.getName() + "...");
         t2.start();
            
         // print the details of the thread groups
         pThreadGroup.list();
         cThreadGroup.list();
         gThreadGroup.list();

         // block until the other threads finish
         t1.join();
         t2.join();
      } catch (InterruptedException ex) {
         System.out.println(ex.toString());
      }
   }

   // implements run()
   public void run() {

      for(int i = 0; i < 4;i++) {
         i++;
         try {
			Thread.sleep(50);
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
      }
      System.out.println(Thread.currentThread().getName() + " finished executing.");
   }
} 

Output

Let us compile and run the above program, this will produce the following result −

Starting Thread-0...
Starting Thread-1...
java.lang.ThreadGroup[name=Parent ThreadGroup,maxpri=10]
    java.lang.ThreadGroup[name=Child ThreadGroup,maxpri=10]
        Thread[Thread-0,5,Child ThreadGroup]
        java.lang.ThreadGroup[name=GrandChild ThreadGroup,maxpri=10]
            Thread[Thread-1,5,GrandChild ThreadGroup]
java.lang.ThreadGroup[name=Child ThreadGroup,maxpri=10]
    Thread[Thread-0,5,Child ThreadGroup]
    java.lang.ThreadGroup[name=GrandChild ThreadGroup,maxpri=10]
        Thread[Thread-1,5,GrandChild ThreadGroup]
java.lang.ThreadGroup[name=GrandChild ThreadGroup,maxpri=10]
    Thread[Thread-1,5,GrandChild ThreadGroup]
Thread-1 finished executing.
Thread-0 finished executing.