Question

This code should produce even and uneven output because there is no synchronized on any methods. Yet the output on my JVM is always even. I am really confused as this example comes straight out of Doug Lea.

public class TestMethod implements Runnable {

private int index = 0;

    public void testThisMethod() {
        index++;
        index++;
        System.out.println(Thread.currentThread().toString() + " "
                    + index );

    }

    public void run() {
        while(true) {
            this.testThisMethod();
        }
    }

    public static void main(String args[]) {
        int i = 0;
        TestMethod method = new TestMethod();
        while(i < 20) {
            new Thread(method).start();
            i++;
        }
    }
}

Output

Thread[Thread-8,5,main] 135134

Thread[Thread-8,5,main] 135136

Thread[Thread-8,5,main] 135138

Thread[Thread-8,5,main] 135140

Thread[Thread-8,5,main] 135142

Thread[Thread-8,5,main] 135144

Answer 1

First off all: as others have noted there’s no guarantee at all, that your threads do get interrupted between the two increment operations.

Note that printing to System.out pretty likely forces some kind of synchronization on your threads, so your threads are pretty likely to have just started a time slice when they return from that, so they will probably complete the two incrementation operations and then wait for the shared resource for System.out.

Try replacing the System.out.println() with something like this:

int snapshot = index;
if (snapshot % 2 != 0) {
  System.out.println("Oh noes! " + snapshot);
}