Waiting for multiple threads to complete.......

Question

Search the forum archives but have not found a definitive solution...
I have java app which spawns many threads (runnable objects) in the main() method. As one would expect, main exits before the threads finish but I don't want this to happen. I want to wait until all the threads have completed so I can use the results (stored in a vector) in the main thread....kindof like waitformultipleobjects in windows NT...
does any guru out there know how best to achieve this????
thanks

Answer 1

Have you tried using a ThreadGroup? I imagine something like the following. I think it is not so nice because of the empty while loop but you get the idea.
public class ThreadGroupTest {
     private static class Dummy extends Thread{
          private long sleepms;
          private String msg = null;
          public Dummy(ThreadGroup tg,long sleepms){
               super(tg,"test"+String.valueOf(sleepms));
               this.sleepms = sleepms;     
          public void run(){
               try {
                    Thread.sleep(sleepms);
               } catch (InterruptedException e) {
               this.msg="Slept "+String.valueOf(sleepms)+" millis.";
          public String getMsg() {
               return msg;
     public static void main(String[] args) {
          Dummy[] threads = new Dummy[10];
          ThreadGroup tg = new ThreadGroup("test");
          for(int i = 0;i<10;i++){
               threads[i] = new Dummy(tg,(i+1)*1000);
               threads.start();
          while(tg.activeCount()>0){
               System.out.println(tg.activeCount());
          for(int i=0;i<threads.length;i++){
               System.out.println(threads[i].getMsg());
Hope that helps.