Synchronizing on an int shouldn't be necessairy, should it?

Question

The following example consists of 2 threads that both increment a static int by 1 and decrement it by 1.
Both threads are stopped abruptly after half a second.
What i don't understand is why there are other results possible than 0, 1 or 2.
Synchronizing one a shared object helps but this shouldn't be necessairy. What is going one here?
import java.io.*;
import java.util.*;
public class Test {
    static int a1   = 0;
    public static void main(String[] args) throws Exception {
       Thread t1 = new Thread(new Runnable() {
          public void run() {
             while (true) {a1++; a1--;}
       Thread t2 = new Thread(new Runnable() {
          public void run() {
             while (true) {a1++; a1--;}
       t1.start();
       t2.start();
       Thread.sleep(500);
       t1.stop();
       t2.stop();
       System.out.println("Finished: " +a1);
    } //main
} // Test

Answer 1

Found this:
"In order to provide good performance in the absence
of synchronization, the compiler, runtime, and cache
are generally allowed to reorder ordinary memory
operations as long as the currently executing thread
cannot tell the difference. (This is referred to as
within-thread as-if-serial semantics.) Volatile reads
and writes, however, are totally ordered across
threads; the compiler or cache cannot reorder
volatile reads and writes with each other.
Unfortunately, the JMM did allow volatile reads and
writes to be reordered with respect to ordinary
variable reads and writes, meaning that we cannot use
volatile flags as an indication of what operations
have been completed."
http://www-106.ibm.com/developerworks/java/library/j-j
tp02244.html#6.0
So it looks like it can be done at any level :(
/KajActually, that looks like it prohibits reordering a++/a-- if a is volatile, which the OP said he did.
I've also heard though, that the JMM is broken and that some VM's don't implement volatile correctly. I don't know which ones and what problems they have, but that may have something to do with what he's observing.