how can a child thread notify a parent thread its status?

[scriptlearner at gmail.com]

My parent thread keeps a counter for the number of free child workers
(say 100) and initializes some child threads and call child.start().
Once the number of free child workers reach 0, the parent thread will
wait until some at least one child thread finishes and then it will
initialize another child thread.
My question is, how can a child thread notify the parent that it's
done so that the parent can call join() on it?  I am not sure how a
child thread can send a signal to its parent while it may not even
know anything about it's parent.  Can you guys please provide some
suggestions?  Some code samples will be nice.  Thanks.