Java.lang.ThreadGroup class in Java

ThreadGroup creates a group of threads. It offers a convenient way to manage groups of threads as a unit. This is particularly valuable in situation in which you want to suspend and resume a number of related threads.

  • The thread group form a tree in which every thread group except the initial thread group has a parent.
  • A thread is allowed to access information about its own thread group but not to access information about its thread group’s parent thread group or any other thread group.

Constructors:

  1. public ThreadGroup(String name): Constructs a new thread group. The parent of this new group is the thread group of the currently running thread.

    Throws: SecurityException - if the current thread cannot
     create a thread in the specified thread group.
    
  2. public ThreadGroup(ThreadGroup parent, String name): Creates a new thread group. The parent of this new group is the specified thread group.

    Throws: 
    NullPointerException - if the thread group argument is null.
    SecurityException - if the current thread cannot create a thread in the 
    specified thread group.
    

Methods

  1. int activeCount(): This method returns the number of threads in the group plus any group for which this thread is parent.

    Syntax: public int activeCount()
    Returns: This method returns an estimate of the number of 
    active threads in this thread group and in any other thread group 
    that has this thread group as an ancestor.
    Exception: NA
    
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    // Java code illustrating activeCount() method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 1000; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                    System.out.println("Exception encounterted");
                }
            }
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[])
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("parent thread group");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting one");
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting two");
      
            // checking the number of active thread
            System.out.println("number of active thread: "
                               + gfg.activeCount());
        }
    }

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    Output:

    Starting one
    Starting two
    number of active thread: 2
    
  2. int activeGroupCount(): This method returns an estimate of the number of active groups in this thread group.

    Syntax: public int activeGroupCount().
    Returns: Returns the number of groups for which the invoking thread is parent.
    Exception: NA.
    
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    // Java code illustrating activeGroupCount() method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 1000; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                    System.out.println("Exception encounterted");
                }
            }
            System.out.println(Thread.currentThread().getName() + 
                " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("gfg");
      
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting one");
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting two");
      
            // checking the number of active thread
            System.out.println("number of active thread group: "
                               + gfg.activeGroupCount());
        }
    }

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    Output:

    Starting one
    Starting two
    number of active thread group: 2
    one finished executing
    two finished executing
    
  3. void checkAccess(): Causes the security manager to verify that the invoking thread may access and/ or change the group on which checkAccess() is called.

    Syntax: final void checkAccess().
    Returns: NA.
    Exception: NA.
    
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    // Java code illustrating checkAccess() method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 1000; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                    System.out.println("Exception encounterted");
                }
            }
            System.out.println(Thread.currentThread().getName() +
                  " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
      
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting one");
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting two");
            gfg.checkAccess();
            System.out.println(gfg.getName() + " has access");
            gfg_child.checkAccess();
            System.out.println(gfg_child.getName() + " has access");
        }
    }

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    Output:

    Starting one
    Starting two
    Parent thread has access
    child thread has access
    one finished executing 
    two finished executing
    
  4. void destroy(): Destroys the thread group and any child groups on which it is called.

    Syntax: public void destroy().
    Returns: NA.
    Exception: 
    IllegalThreadStateException - if the thread group is not 
    empty or if the thread group has already been destroyed.
    SecurityException - if the current thread cannot modify this thread group.
    
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    // Java code illustrating destroy() method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                    System.out.println("Exception encounterted");
                }
            }
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
      
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting one");
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting two");
      
            // block until other thread is finished
            t1.join();
            t2.join();
      
            // destroying child thread
            gfg_child.destroy();
            System.out.println(gfg_child.getName() + " destroyed");
      
            // destroying parent thread
            gfg.destroy();
            System.out.println(gfg.getName() + " destroyed");
        }
    }

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    Output:

    Starting one
    Starting two
    child thread destroyed
    Parent thread destroyed
    
  5. int enumerate(Thread group[]): The thread that comprise the invoking thread group are put into the group array.

    Syntax: public int enumerate(Thread group[]).
    Returns: the number of threads put into the array.
    Exception: SecurityException - if the current thread 
    does not have permission to enumerate this thread group.
    
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    // Java code illustrating enumerate() method.
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) {
                    System.out.println("Exception encounterted");
                }
            }
            System.out.println(Thread.currentThread().getName() +
                 " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
      
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting one");
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting two");
      
            // returns the number of threads put into the array
            Thread[] group = new Thread[gfg.activeCount()];
            int count = gfg.enumerate(group);
            for (int i = 0; i < count; i++) 
            {
                System.out.println("Thread " + group[i].getName() + " found");
            }
        }
    }

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    Output:

    Starting one
    Starting two
    Thread one found
    Thread two found
    one finished executing
    two finished executing
    
  6. int enumerate(Thread[] group, boolean recurse): The threads that comprise the invoking thread group are put into the group array. If all is true, then threads in all subgroups of the thread are also put into group.

    Syntax: public int enumerate(Thread[] list, boolean recurse).
    Returns: the number of threads placed into the array.
    Exception: SecurityException - if the current thread does 
    not have permission to enumerate this thread group.
    
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    // Java code illustrating enumerate(Thread[] group, boolean recurse)
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                    System.out.println("Exception encounterted");
                }
            }
            System.out.println(Thread.currentThread().getName() +
                  " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
      
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting one");
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting two");
      
            // returns the number of threads put into the array
            Thread[] group = new Thread[gfg.activeCount()];
            int count = gfg.enumerate(group, true);
            for (int i = 0; i < count; i++) 
            {
                System.out.println("Thread " + group[i].getName() + " found");
            }
        }
    }

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    Output:

    Starting one
    Starting two
    Thread one found
    Thread two found
    one finished executing
    two finished executing
    
  7. int enumerate(ThreadGroup[] group): The subgroups of the evoking thread group are put into the group array.

    Syntax: public int enumerate(ThreadGroup[] group).
    Returns: the number of thread groups put into the array.
    Exception: SecurityException - if the current thread does 
    not have permission to enumerate this thread group.
    
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    // Java code illustrating enumerate(ThreadGroup[] group) method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                    System.out.println("Exception encounterted");
                }
            }
            System.out.println(Thread.currentThread().getName() + 
                   " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
      
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting one");
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting two");
      
            // returns the number of threads put into the array
            ThreadGroup[] group = new ThreadGroup[gfg.activeCount()];
            int count = gfg.enumerate(group);
            for (int i = 0; i < count; i++) 
            {
                System.out.println("ThreadGroup " + group[i].getName() +
                    " found");
            }
        }
    }

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    Output:

    Starting one
    Starting two
    ThreadGroup child thread found
    two finished executing
    one finished executing
    
  8. int enumerate(ThreadGroup[] group, boolean all): The subgroups of the invoking thread group are put into the group array. If all is true, then all subgroups of the subgroups(and so on) are also put into group.

    Syntax: public int enumerate(ThreadGroup[] group, boolean all)
    Returns: the number of thread groups put into the array.
    Exception: SecurityException - if the current thread does 
    not have permission to enumerate this thread group.
    
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    // Java code illustrating enumerate(ThreadGroup[] group, boolean all)
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                    System.out.println("Exception encounterted");
                }
            }
            System.out.println(Thread.currentThread().getName() +
                   " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
      
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting one");
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting two");
      
            // returns the number of threads put into the array
            ThreadGroup[] group = new ThreadGroup[gfg.activeCount()];
            int count = gfg.enumerate(group, true);
            for (int i = 0; i < count; i++) 
            {
                System.out.println("ThreadGroup " + group[i].getName() + 
                    " found");
            }
        }
    }

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    Output:

    Starting one
    Starting two
    ThreadGroup child thread found
    two finished executing
    one finished executing
    
  9. int getMaxPriority(): Returns the maximum priority setting for the group.

    Syntax: final int getMaxPriority().
    Returns: the maximum priority that a thread in this thread group can have.
    Exception: NA.
    
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    // Java code illustrating getMaxPriority() method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                    System.out.println("Exception encounterted");
                }
            }
            System.out.println(Thread.currentThread().getName() +
                   " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            // checking the maximum priority of parent thread
            System.out.println("Maximum priority of ParentThreadGroup = "
                               + gfg.getMaxPriority());
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting one");
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting two");
        }
    }

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    Output:

    Maximum priority of ParentThreadGroup = 10
    Starting one
    Starting two
    two finished executing
    one finished executing
    
  10. String getName(): This method returns the name of the group.

    Syntax: final String getName().
    Returns: the name of this thread group.
    Exception: NA.
    
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    // Java code illustrating getName() method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                    System.out.println("Exception encounterted");
                }
            }
            System.out.println(Thread.currentThread().getName() + 
                 " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting " + t1.getName());
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting " + t2.getName());
        }
    }

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    Output:

    Starting one
    Starting two
    two finished executing
    one finished executing
    
  11. ThreadGroup getParent(): Returns null if the invoking ThreadGroup object has no parent. Otherwise, it returns the parent of the invoking object.

    Syntax: final ThreadGroup getParent().
    Returns: the parent of this thread group. 
    The top-level thread group is the only thread group 
    whose parent is null.
    Exception: SecurityException - if the current thread 
    cannot modify this thread group.
    
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    // Java code illustrating getParent() method
    import java.lang.*;
    class NewThread extends Thread {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) {
                try {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) {
                    System.out.println("Exception encounterted");
                }
            }
            System.out.println(Thread.currentThread().getName()
                                                + " finished executing");
        }
    } public class ThreadGroupDemo {
    public static void main(String arg[]) throws InterruptedException,
            SecurityException
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting " + t1.getName());
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting " + t2.getName());
      
            // prints the parent ThreadGroup 
            // of both parent and child threads
            System.out.println("ParentThreadGroup for " + gfg.getName() +
                            " is " + gfg.getParent().getName());
            System.out.println("ParentThreadGroup for " + gfg_child.getName() 
                            + " is " + gfg_child.getParent().getName());
        }
    }

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    Output:

    Starting one
    Starting two
    ParentThreadGroup for Parent thread is main
    ParentThreadGroup for child thread is Parent thread
    one finished executing
    two finished executing
    
  12. void interrupt(): Invokes the interrupt() methods of all threads in the group.

    Syntax: public final void interrupt().
    Returns: NA.
    Exception: SecurityException - if the current thread is not 
    allowed to access this thread group or any of the threads in the thread group.
    
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    // Java code illustrating interrupt() method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                  System.out.println("Thread " + Thread.currentThread().getName()
                                       + " interrupted");
                }
            }
            System.out.println(Thread.currentThread().getName() + 
                 " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting " + t1.getName());
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting " + t2.getName());
      
            // interrupting thread group
            gfg.interrupt();
        }
    }

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    Output:

    Starting one
    Starting two
    Thread two interrupted
    Thread one interrupted
    one finished executing
    two finished executing
    
  13. boolean isDaemon(): Tests if this thread group is a daemon thread group. A daemon thread group is automatically destroyed when its last thread is stopped or its last thread group is destroyed.

    Syntax: public final boolean isDaemon().
    Returns: true if the group is daemon group. Otherwise it returns false.
    Exception: NA.
    
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    // Java code illustrating isDaemon() method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                  System.out.println("Thread " + Thread.currentThread().getName()
                                       + " interrupted");
                }
            }
            System.out.println(Thread.currentThread().getName() + 
                    " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting " + t1.getName());
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting " + t2.getName());
      
            if (gfg.isDaemon() == true)
                System.out.println("Group is Daemon group");
            else
                System.out.println("Group is not Daemon group");
        }
    }

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    Output:

    Starting one
    Starting two
    Group is not Daemon group
    two finished executing
    one finished executing
    
  14. boolean isDestroyed(): This method tests if this thread group has been destroyed.

    Syntax: public boolean isDestroyed().
    Returns: true if this object is destroyed.
    Exception: NA.
    
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    // Java code illustrating isDestroyed() method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                  System.out.println("Thread " + Thread.currentThread().getName()
                                       + " interrupted");
                }
            }
            System.out.println(Thread.currentThread().getName() + 
                    " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException, Exception
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting " + t1.getName());
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting " + t2.getName());
      
            if (gfg.isDestroyed() == true)
                System.out.println("Group is destroyed");
            else
                System.out.println("Group is not destroyed");
        }
    }

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    Output:

    Starting one
    Starting two
    Group is not destroyed
    one finished executing
    two finished executing
    
  15. void list(): Displays information about the group.

    Syntax: public void list().
    Returns: NA.
    Exception: NA.
    
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    // Java code illustrating list() method.
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                  System.out.println("Thread " + Thread.currentThread().getName()
                                       + " interrupted");
                }
            }
            System.out.println(Thread.currentThread().getName() + 
                  " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException, Exception
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting " + t1.getName());
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting " + t2.getName());
      
            // listing contents of parent ThreadGroup
            System.out.println("\nListing parentThreadGroup: " + gfg.getName() 
                  + ":");
            // prints information about this thread group
            // to the standard output
            gfg.list();
        }
    }

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    Output:

    Starting one
    Starting two
    
    Listing parentThreadGroup: Parent thread:
    java.lang.ThreadGroup[name=Parent thread, maxpri=10]
        Thread[one, 5, Parent thread]
        Thread[two, 5, Parent thread]
        java.lang.ThreadGroup[name=child thread, maxpri=10]
    one finished executing
    two finished executing
    
  16. boolean parentOf(ThreadGroup group): This method tests if this thread group is either the thread group argument or one of its ancestor thread groups.

    Syntax: final boolean parentOf(ThreadGroup group).
    Returns: true if the invoking thread is the parent 
    of group(or group itself). Otherwise, it returns false.
    Exception: NA.
    
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    // Java code illustrating parentOf() method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                  System.out.println("Thread " + Thread.currentThread().getName()
                                       + " interrupted");
                }
            }
            System.out.println(Thread.currentThread().getName() + 
                    " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException, Exception
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting " + t1.getName());
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting " + t2.getName());
      
            // checking who is parent thread
            if (gfg.parentOf(gfg_child))
                System.out.println(gfg.getName() + " is parent of " +
                   gfg_child.getName());
        }
    }

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    Output:

    Starting one
    Starting two
    Parent thread is parent of child thread
    two finished executing
    one finished executing
    
  17. void setDaemon(boolean isDaemon): This method changes the daemon status of this thread group. A daemon thread group is automatically destroyed when its last thread is stopped or its last thread group is destroyed.

    Syntax: final void setDaemon(boolean isDaemon).
    Returns: If isDaemon is true, then the invoking group is 
    flagged as a daemon group.
    Exception: SecurityException - if the current 
    thread cannot modify this thread group.
    
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    // Java code illustrating setDaemon() method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                  System.out.println("Thread " + Thread.currentThread().getName()
                                       + " interrupted");
                }
            }
            System.out.println(Thread.currentThread().getName() + 
                   " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException, Exception
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            // daemon status is set to true
            gfg.setDaemon(true);
      
            // daemon status is set to true
            gfg_child.setDaemon(true);
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting " + t1.getName());
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting " + t2.getName());
      
            if (gfg.isDaemon() && gfg_child.isDaemon())
                System.out.println("Parent Thread group and "
                                   + "child thread group"
                                   + " is daemon");
        }
    }

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    Output:

    Starting one
    Starting two
    Parent Thread group and child thread group is daemon
    one finished executing
    two finished executing
    
  18. void setMaxPriority(int priority): Sets the maximum priority of the invoking group to priority.

    Syntax: final void setMaxPriority(int priority).
    Returns: NA.
    Exception: SecurityException - if the current thread 
    cannot 
    modify this thread group.
    
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    // Java code illustrating setMaxPriority() method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                  System.out.println("Thread " + Thread.currentThread().getName()
                                       + " interrupted");
                }
            }
            System.out.println(Thread.currentThread().getName() + 
                     " [priority = "
               Thread.currentThread().getPriority() + "]
                    finished executing.");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException, Exception
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
            gfg.setMaxPriority(Thread.MAX_PRIORITY - 2);
            gfg_child.setMaxPriority(Thread.NORM_PRIORITY);
      
            NewThread t1 = new NewThread("one", gfg);
            t1.setPriority(Thread.MAX_PRIORITY);
            System.out.println("Starting " + t1.getName());
            t1.start();
            NewThread t2 = new NewThread("two", gfg_child);
            t2.setPriority(Thread.MAX_PRIORITY);
            System.out.println("Starting " + t2.getName());
            t2.start();
        }
    }

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    Output:

    Starting one
    Starting two
    two [priority = 5] finished executing.
    one [priority = 8] finished executing.
    
  19. String toString(): This method returns a string representation of this Thread group.

    Syntax: public String toString().
    Returns: String equivalent of the group.
    Exception: SecurityException - if the current thread 
    cannot modify this thread group.
    
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    // Java code illustrating toString() method
    import java.lang.*;
    class NewThread extends Thread 
    {
        NewThread(String threadname, ThreadGroup tgob)
        {
            super(tgob, threadname);
            start();
        }
    public void run()
        {
      
            for (int i = 0; i < 10; i++) 
            {
                try 
                {
                    Thread.sleep(10);
                }
                catch (InterruptedException ex) 
                {
                  System.out.println("Thread " + Thread.currentThread().getName()
                                       + " interrupted");
                }
            }
            System.out.println(Thread.currentThread().getName() + 
                  " finished executing");
        }
    public class ThreadGroupDemo 
    {
        public static void main(String arg[]) throws InterruptedException,
            SecurityException, Exception
        {
            // creating the thread group
            ThreadGroup gfg = new ThreadGroup("Parent thread");
            ThreadGroup gfg_child = new ThreadGroup(gfg, "child thread");
      
            // daemon status is set to true
            gfg.setDaemon(true);
      
            // daemon status is set to true
            gfg_child.setDaemon(true);
            NewThread t1 = new NewThread("one", gfg);
            System.out.println("Starting " + t1.getName());
            NewThread t2 = new NewThread("two", gfg);
            System.out.println("Starting " + t2.getName());
      
            // string equivalent of the parent group
            System.out.println("String equivalent: " + gfg.toString());
        }
    }

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    Output:

    Starting one
    Starting two
    String equivalent: java.lang.ThreadGroup[name=Parent thread, maxpri=10]
    one finished executing
    two finished executing
    

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