Threadgroup - nullpointer expcetion (help!)

Question

Please can anybody explain why the following code throws a nullpointer exception in the //PROBLEM HERE line?
My JBuilder debugger shows that when i is 1, my threads array has 101 threads in it, but when I try to do a Thread t = threads[i] t ends up as null, so the System out dies with a nullpointer ...
Where am I going wrong? Is it my understanding of thread groups?
Many thanks for any help...
Ryan Harris
public class Threads {
    public Threads() {
    public static void main(String[] args) {
        Threads t = new Threads();
        t.showThreadGroup();
    void showThreadGroup(){
        ThreadGroup tg = Thread.currentThread().getThreadGroup();
        //Make some threads!
        for (int i = 0; i < 100; i++) {
            Thread t = new Thread(tg, "Thread-number-" + i);
            t.run();
        Thread[] threads = new Thread[ tg.activeCount() ];
        System.out.println("Thread group: " + tg.getName() );
        // Put all the active threads in an Array (and return the number of threads there)
        System.out.println(tg.enumerate(threads) + " threads enumerated");
        for (int i = 0; i < threads.length; i++) {
            Thread t = threads;
// PROBLEM HERE vvv
// ** When i = 1, the JBuilder debugger shows threads[] has 101 threads in it, but t = null! **
System.out.println(" " + (i+1) + " called " + t.getName() + " priority " + t.getPriority());

Answer 1

Interesting problem: If I call t.start() it all works
nicely if I run it in JBuiler. If I debug it, it
prints this:
Java HotSpot(TM) Client VM warning: Setting of
property "java.compiler" is ignored
Thread group: main
1 threads enumerated
1 called main priority 5
Hit uncaught exception java.lang.NullPointerException
Strange? anybody have any ideas?It's probably a timing issue - you've allocated as many threads as were active at the time you called activeCount(), but by the time you call enumerate(threads), some of the threads were probably finished running, so part of your array will have null values. For example, maybe 100 threads were active, but 2 finished, so it only filled in 98 elements of your array.