Why does notifyAll() raise IllegalMonitorStateException when synchronized on Integer?

Question

Why does this test program result in a java.lang.IllegalMonitorStateException?

public class test {
    static Integer foo = new Integer(1);
             


        

Answer 1

Incrementing the Integer makes the old foo disappear and be replaced with a brand new object foo which is not synchronized with the previous foo variable.

Here is an implementation of AtomicInteger that erickson suggested above. In this example foo.notifyAll(); does not produce a java.lang.IllegalMonitorStateException beause the AtomicInteger Object is not refreshed when foo.incrementAndGet(); is run.

import java.util.concurrent.atomic.AtomicInteger;

public class SynchronizeOnAPrimitive {
    static AtomicInteger foo = new AtomicInteger(1);
    public static void main(String[] args) {
        synchronized (foo) {
            foo.incrementAndGet();
            foo.notifyAll();
        }
        System.out.println("foo is: " + foo);
    }
}

Output:

foo is: 2