Java multithreading query

sathish j :

Hi I am a newbie in concurrent programming domain, I was testing the below code and seems the while loop is not terminating for threads. Could someone help explain whats happening here.

public class PrimePrinter{
    public long counter = 0;
    public synchronized long getCounter() {
        return counter++;
    }
    public Thread makeThread() {
        Runnable rn = new Runnable() {          
            /* (non-Javadoc)
             * @see java.lang.Runnable#run()
             */
            @Override
            public void run() {
                while (counter < 100) {
                    try {
                        Thread.sleep(1000);
                        System.out.println(Thread.currentThread().getName() + " : " +getCounter());
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }                   
                }
            }
        };
        return new Thread(rn);
    }
    public static void main(String[] args) {
        int n = 10;
        Thread[] threads = new Thread[10];
        PrimePrinter pp = new PrimePrinter();
        for(int i = 1; i < n; i++) {            
            threads[i] = pp.makeThread();
            threads[i].start();
        }
    }
}

Last few lines of output

Thread-4 : 81
Thread-5 : 87
Thread-7 : 91
Thread-5 : 97
Thread-2 : 95
Thread-4 : 98
Thread-6 : 96
Thread-8 : 90
Thread-1 : 93
Thread-3 : 92
Thread-0 : 94
Thread-2 : 99
Thread-6 : 107
Thread-3 : 103
Thread-0 : 104
Thread-1 : 105
Thread-8 : 106
Thread-5 : 102
Thread-4 : 101
Thread-7 : 100

Maurice Perry :

Each thread waits a second after having tested that counter < 100, during this second, other threads can increment the counter.

UPDATE: You could do something like this:

            while (true) {
                long current = getCounter();
                if (current >= 100) {
                    break;
                }
                try {
                    Thread.sleep(1000);
                    System.out.println(Thread.currentThread().getName() + " : " + current);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }                   
            }