现有: ’ 1. class Waiting implements Runnable { 2. boolean flag=false; 3.public synchronized void run() { 4. if (flag) { 5. flag=false; 6.System.out.print ("1"); 7. try { this.wait(); ) catch (Exception e) { } 8.System.out.print ("2"); 9. } 10. else { 11. flag=true; 12. System.out.print ("3"); 13. try{Thread.sleep (2000); } catch(Exception e) {} 14. System.out.print ("4"); 15. notify(); 16. } 17. } 18. public static void main (String [] args) { 19. Waiting w=new Waiting(); 20. new Thread (w) .start(); 21. new Thread (w) .start(); 22. } 23. } 以下哪两确的?(选两项)