Devaks's PracticeExam1 Q:44 and Q72

[Balaji Bang]

HI

This Question is from Devaka's Practice Exam1 Q.No:72 . Output is "ABC" .Agreed. I read the long discussion on this question. . Here Synchronization wont work as it is on different objects. Okay. Please check the MThread class(below XMap class) from PracticeExam-1 Q NO 44 which was giving compiler error but now modified....

public class XMap implements Runnable{

     static String s="ABC";

     public void run(){
          synchronized(this){
               System.out.print(s);
          }
     }

     public static void main(String args[])throws Exception{
          Thread td=new Thread(new XMap());
          synchronized(td){
               td.start();
               Thread.sleep(1000);
               s="XYZ";
          }
     }
 
}    

OUTPUT : ABC

public class MThread extends Thread{

     static int x=0;

    public  void run(){
    	 while(x==0){}
         System.out.println("A");
     }

     public static void main(String argv[]){
          try{
        	  MThread mt=new MThread ();
               mt.start();
               Thread.sleep(1000);
               System.out.print("B");
               x=1;
          }catch(Exception e){ }
     }

}

OUTPUT: BA (is coming)

I felt XMap and MThread are similar. only difference is that XMap implements Runnable and synchronization blocks are there(but synchronization wont work as it is on different objects) but MThread extends Thread And no synchronization blocks in this class. in both Thread.sleep() is there but why here first main is getting finished and then only run completes. Really I am in Confusion

[Ruben Soto]

Banu, try taking out the while(x==0){} statement in the run() method, and see what happens. If you don't see what's going on after a couple of minutes post another message and I will try to explain.

[Balaji Bang]

Ya I got it . It is because of while loop. when x becomes 1 then only it comes out of while loop.


1. while runs
2. B prints and x=1
3. comes out of while loop and A prints....

Thank you Ruben...

[Ruben Soto]

Yeah, you got it Banu. You are actually implementing a "lock" in your code. But this is a very bad way to implement a lock, because the thread that is running the while loop is wasting valuable CPU cycles.

[Balaji Bang]

Yes the lock is implemented through while loop...

[Abhi vijay]

Can anyone explain me what is happening in this program? From where do locks come into play and the while loop does not have a body of execution?

[Ruben Soto]

Abhi, when I said "lock" I didn't mean a real lock. What's going on is that the while() loop will continue executing until x is changed by main and its iteration condition becomes false.

[Abhi vijay]

Ok, Ruben.