Threads. Preemption not working.

[Dillinger4]

Once T3 starts it should run to completion with all current running threads yielding until then. This does not seem to be the case though. Any ideas why??? Thanks.

public class PreemptTest{
 public static void main(String[] args){
  Thread t1 = new Thread(new T(), "t1"); 
  Thread t2 = new Thread(new T(), "t2"); 
  Thread t3 = new Thread(new T(), "t3"); 
  t1.start(); 
  t2.start();
  t3.setPriority(Thread.currentThread().getPriority() + 1); 
  t3.start();
 }
}

class T implements Runnable{
 private int square; 
 
 public void run(){
  for(int i = 1; i < 5; i++){
   square = i * i; 
   System.out.println("Thread " + Thread.currentThread().getName() +
	" has a priority of " + Thread.currentThread().getPriority() +
	 ": " + square);   
  }
 } 
}