Problem with ioinning multiple Threads

Question

Hi,
I have multiple sub Threads T1, T2, T3.... started by main Thread T. After starting each sub Thread, I stored them in the HashMap. When finish starting all Threads, I want to join each sub Thread to my main Thread.
The problem is when T3 join with T, it hanged forever! Here is my code:
class MyThread extends Thread
private name;
MyThread(String tName)
name = tName;
public void run()
// doing something....
public HashMap startThreads()
HashMap tList = new HashMap();
for (int i=0; i<5; ++i)
String tName = "Thread" + i;
MyThread myT = new MyThread(tName);
myT.start();
tList.put(tName, myT);
public void joinThreads(HashMap tList)
Iterator iter = tList.values().iterator();
while (iter.hasNext())
Thread activeThread = (Thread)iter.next();
if (activeThread.isAlive()) {
System.out.println("Join alive thread " + activeThread.tName);
activeThread.join();
In my method:
... doing s/t
HashMap tMap = startThreads();
joinThreads(tMap);
... doing s/t else (never excecuted!!!)
My purpose is staring multi Threads at the same time and executing some code after they all die.
If I join my sub Thread right after creating it, then the other sub Thread couldn't started until subThread 1 dies!
If I don't join the sub Thread but remove it from the thread Map when it's not alive then my code executed perfectly. I want to apply join method with my mutiple Threads, how ????
Could some one give me a hint? Thanks.

Answer 1

Hi,
What I don't understand here is why the main Thread
does not join all the sub Threads ?If you're getting a ConcurrentModificationException while iterating the list of threads, then that's why.
Why main Thread
has to wait for sub Thread 1 to finish before joinning
next sub Threads?It has to join all of the threads. That means it has to wait for all of them to finish. In order to wait for all of the threads it has to wait for the first thread.
I'm 100% sure that my sub Threads in
the threadList are valid and alive since I already
veriffied with other methods.I don't see what that has to do with your problem.
I already set my joinThreads method as synchronized,
how come I still have ConcurrentModificationException?Because the list was modified concurrently.
How can I fix this error?Stop modifying the list concurrently. You haven't posted all of the relevant code, so I can't offer any clues about how to do that.