Thread Null pointer problem

I got my program to compile, but I'm getting this null pointer error. Can anyone help me figure out why? I'd appreciate it.
public class printOutThreadGroups { public static void main( String args[] ) { printOutThreadGroups t = new printOutThreadGroups(); t.printOutThreadGroups(); } public void printOutThreadGroups() { //create a thread Thread t = new Thread(); //get the current thread Thread tt = t.currentThread(); //get the thread group of current thread ThreadGroup tg = tt.getThreadGroup(); ThreadGroup topMost = null; //find the topmost thread group while(tg != null) { topMost = tg; tg = tg.getParent(); } //get an estimate of active thread groups under topMost int groupCount = topMost.activeGroupCount(); //get an enumeration of thread groups under topMost ThreadGroup[] tgArray = new ThreadGroup[groupCount]; topMost.enumerate(tgArray, true); //for every thread group under topMost, print out the active threads System.out.println("Top Most ThreadGroup: \""+topMost.getName()+ "\" has the following Thread Groups:"); System.out.println("**********************"); for(int i=0; i<tgArray.length; i++) { ThreadGroup aThreadGroup = tgArray[i]; int count = aThreadGroup.activeCount(); //get an enumeration of threads in aThreadGroup Thread[] tArray = new Thread[count]; aThreadGroup.enumerate(tArray, true); System.out.println("Thread Group: \""+aThreadGroup.getName()+ "\" has the following threads: "); String indent = " "; for(int j=0; j<tArray.length; j++) { Thread aThread = tArray[j]; System.out.println(indent +" \""+aThread.getName()+"\""); indent += " "; } System.out.println("**********************"); tArray = null; } tgArray = null; t = null; } } <br /> [errors]<br /> Microsoft Windows 2000 [Version 5.00.2195]<br /> (C) Copyright 1985-2000 Microsoft Corp.<br /> C:\myjava>cd assignment3<br /> C:\myjava\assignment3>cd problem1
C:\myjava\assignment3\problem1>java printOutThreadGroups
Top Most ThreadGroup: "system" has the following Thread Groups:
**********************
Thread Group: "main" has the following threads:
"main"
Exception in thread "main" java.lang.NullPointerException
at printOutThreadGroups.printOutThreadGroups(printOutThreadGroups.java:5
5)
at printOutThreadGroups.main(printOutThreadGroups.java:9)
C:\myjava\assignment3\problem1>
[/errors]
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Thanks, Dianne

The NullPointerException message tells you that it originates from line 55 of your file. This is really useful information, and much easier for you to find than for us. So whe you post an error like this, it's a good idea to point out for us exactly which line is throwing the error. By inspection though, I'm pretty sure it's the line
<pre> System.out.println(indent +" \""+aThread.getName()+"\"");</pre>
The reason is that aThread is null - at least, it may be null. Note that the ThreadGroup activeGroupCount() method only returns an estimate of the number of active groups - which means the number of groups in the array after enumerate() is called, may be different. And if enumerate returns fewer groups than the original estimate, then your array will have nulls in the remaining slots. Instead, save the return value from enumerate() to find out how many groups were actually returned, and use this to control your for loop through the array. Or, check each element to see if it's null before you access it.