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Thread: Threading question....

  1. #1
    Junior Member
    Join Date
    Jul 2009
    SomeWhere in the world
    Thanked 11 Times in 6 Posts

    Question Threading question....

    I got this sample from JavaPassion site.It's great one for beginners
    it's very good sample about getting current Threads information

    public class DisplayAllThreads {
        public static void main(String[] args) {
            // Start three threads first.  They should belong
            // to a same ThreadsGroup.
            new SimpleThread("Boston").start();
            new SimpleThread("New York").start();
            new SimpleThread("Seoul").start();
            Thread[] tarray = findAllThreads();
            for (int i=0; i<tarray.length;i++){
                System.out.println("Thread " + tarray[i].getName()
                + " in thread group " + tarray[i].getThreadGroup().getName());
        // Create an array of all threads in the system.
        public static Thread[] findAllThreads() {
            ThreadGroup group = Thread.currentThread().getThreadGroup();
            ThreadGroup topGroup = group;
            while (group != null) {
                topGroup = group;
                group = group.getParent();
            int estimatedSize = topGroup.activeCount() * 2;
            Thread[] slackList = new Thread[estimatedSize];
            int actualSize = topGroup.enumerate(slackList);
            Thread[] list = new Thread[actualSize];
            System.arraycopy(slackList, 0, list, 0, actualSize);
            return list;
    class SimpleThread extends Thread {
        public SimpleThread(String str) {
        public void run() {
            for (int i = 0; i < 5; i++) {
                // System.out.format("%d %s%n", i, getName());
                try {
                    sleep((long)(Math.random() * 1000));
                } catch (InterruptedException e) {}
            System.out.format("DONE! %s%n", getName());

    All i need to know Why when getting total number of active thread He multiply 2
    this line...

     int estimatedSize = topGroup.activeCount() * 2;

  2. #2
    Super Moderator Json's Avatar
    Join Date
    Jul 2009
    Warrington, United Kingdom
    My Mood
    Thanked 156 Times in 152 Posts

    Default Re: Threading question....

    There is an array called slackList created which needs to be able to hold the amount of threads needed, therefore since the activeCount() method only returns an estimate he then multiply that estimate by 2 to make sure he can store all the threads in his slackList array. Then he enumerate() through them which will give him the correct number of threads. Then he creates a new list with that correct number of threads and copies the slackList into it and returns it.

    // Json

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