Why the first Thread.activeCount() return 1 but another return 2 in my code?

Question

Here is my application:

public class NamedThread extends Thread {
    /* This will store name of the thread */

    String name;

    @Override
    public void r         


        

Answer 1

If we print all the threads then we can observe that default thread group is System and inside the system, there are different threads and groups like :

Group[system:class java.lang.ThreadGroup]
 Reference
  Finalizer
  Signal Dispatcher
  Group[main:class java.lang.ThreadGroup]
     main
     Monitor
  Group[InnocuousThreadGroup:class java.lang.ThreadGroup]
    Common-Cleaner

In java code default thread group is main. When we print active thread count then it will give an answer as 2 because the main thread group has two threads main and monitor.

Answer 2

public static void main(String[] args) {
    System.out.println(Thread.activeCount());

    //doSomething()
}

The code above output 2

GC occupies a thread, so you get 2, instead of 1

This function is designed for debugging

If you run on debug mode, you would get 1

Answer 3

InteliJ generates another thread that is called Monitor Ctrl-Break thread.

The following code (took it from https://www.tutorialspoint.com/java/lang/thread_activecount.htm):

public static void main(String[] args) {

            Thread t = Thread.currentThread();
            t.setName("Admin Thread");

            // set thread priority to 1
            t.setPriority(1);

            // prints the current thread
            System.out.println("Thread = " + t);

            int count = Thread.activeCount();
            System.out.println("currently active threads = " + count);

            Thread th[] = new Thread[count];
            // returns the number of threads put into the array
            Thread.enumerate(th);

            // prints active threads
        for (int i = 0; i < count; i++) {
            System.out.println(i + ": " + th[i]);
        }
    }

will generate the following output in inteliJ:

Thread = Thread[Admin Thread,1,main]
currently active threads = 2
0: Thread[Admin Thread,1,main]
1: Thread[Monitor Ctrl-Break,5,main]

You can run it in debug and notice that this thread isnt created. Therefore, this thread is only added during normal run.