Thread: synchronization

hi could someone please help me understand how interference might occur in this program ?

class ThreadTest implements Runnable {
static String printString = null;
private String threadName;
ThreadTest(String s) { threadName = s; }
public void run() {
int i = 0;
while (i < 100) {
if (printString == null) {
printString = threadName + (++i);
} else {
System.out.println(printString);
printString = null;
}
}
}
public static void main(String[] args) {
Thread t1 = new Thread(new ThreadTest("a"));
Thread t2 = new Thread(new ThreadTest("b"));
t1.start();
t2.start();
}
}

its meant to print a1 to a100 and b1 to b100...

I think you first need to understand concept of threads very clearly........
lets try this way
if the output is as you expected....what's the use of setting priority of a thread?