Stopping thread with sentinel does not work

Question

I'm new to multithreading in java and I can't figure out on how to stop a thread. I have an ArrayList of threads which I want to stop. Here is a snippet of my code:

class PrimeThread implements Runnable {
    public void run() {
        while(flag) {
        //do some stuff
        }
    }
    public void cancel() {
        flag = false;
    }
...
}
Class MainClass {
    public static void stopPrimeThreads() {
        for(int i=0; i<primeThreadList.size(); i++) primeThreadList.get(i).cancel();
    }
}

I try to stop the thread by setting the flag variable to false. The flag variable is a volatile boolean declared in the class itself. I also tried to use an outer class variable as sentinel but that did not work either. Also a non volatile variable does not do the job. When I use the deprecated stop() method I get the behavior I want, but I don't want to use a deprecated method. Does anybody has an idea on how to fix this problem? Am I missing something fundamental? Thanks in advance.

Answer 1

Make internal loop something like

while (flag) {
   // smaller step 1
   if (!flag) break;
   // so on
}

As a side-note, if your internal loop takes very long time, it may indicate you using some sub-optimal algorithm. Though, if you working with very large numbers, long calculation times are unavoidable.