Hi,

The following code is an example from a book* where it shows some
errors in concurrent programming. I've actually typed and tried this
one, and it actually does what the author says but I do not understand
why. In my (obviously wrong) opinion the program should have run
without error.

It's a EvenGenerator which generates even integers in multiple
threads, and a EvenChecker, who checks if  the number generated is
even?

*Bruce Eckel, Thinking in Java 4th ed, p.1150-1153

<code>

import java.util.concurrent.*;

abstract class IntGenerator {
    private volatile boolean canceled = false;
    public abstract int next();
    public void cancel() {
        canceled = true;
    }
    public boolean isCanceled() {
        return canceled;
    }
}

public class EvenGenerator extends IntGenerator {
    private int currentEvenValue = 0;
   
    public int next() {
        ++currentEvenValue;  //danger point here
        ++currentEvenValue;
        return currentEvenValue;
    }
    public static void main(String[] args) {
        EvenChecker.test(new EvenGenerator());
    }
}

class EvenChecker implements Runnable {
    private IntGenerator generator;
    private final int id;
    public EvenChecker(IntGenerator g, int ident) {
        generator = g;
        id = ident;   
    }
    public void run() {
        while(!generator.isCanceled()) {
            int val = generator.next();
            if(val % 2 != 0) {
                System.out.println(val + " not
even!");
                generator.cancel();
            }
        }
    }
    public static void test(IntGenerator gp, int count) {
        ExecutorService exec =
Executors.newCachedThreadPool();
        for(int i = 0; i < count; i++)
            exec.execute(new EvenChecker(gp, i));
        exec.shutdown();
    }
    public static void test(IntGenerator gp) {
        test(gp, 10);
    }
}

</code>

Output: (an example)
89476993 not even!
89476993 not even!

My problem is, even the thread scheduler stops an EvenGenerator.next()
after the first ++currentEvenValue, next() doesn't return after the
second one, so I don't understand how the val variable (which is the
return value  of next() ) in EvenChecker gets an odd number?

Thanks