What is the output(synchronized block and thread)? : Questions « Thread « SCJP

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class MyClass {
    synchronized void yack(long id) {
      for(int x = 1; x < 3; x++) {
        System.out.print(id + " ");
        Thread.yield();
      }
    }
 }
 public class MainClass implements Runnable {
   MyClass c;
   public static void main(String[] args) {
     new MainClass().go();
   }
   void go() {
     c = new MyClass();
     new Thread(new MainClass()).start();
     new Thread(new MainClass()).start();
   }
   public void run() {
     c.yack(Thread.currentThread().getId());
   }
 }