SCJP 1.6 ...Threads. Needs clarification of a Self Test ans from K&S Book

[Sumeet Chakraborty]

I am not able to get a clear explanation of the ans of the following question (Kathy Siera , Threads , Q.No 16).
Can anyone help me understanding the answer ... ?

Given

class Chicks {
synchronized void yack(long id){

for(int x=1; x<3; x++){
System.out.print(id +" ");
Thread.yield();
}
}
}

public class ChicksYack implements Runnable {

Chicks c;

public static void main(String[] args){
new ChickYack().go();
}

void go(){

c=new Chicks();
new Thread(new ChickYack()).start();
new Thread(new ChickYack()).start();
}

public void run(){
c.yack(Thread.currentThread().getId());
}
}



Which are true ?
A. Compilation Fails
B. The output could be 4 4 2 3
C. The output could be 4 4 2 2
D. The output could be 4 4 4 2
E. The output could be 2 2 4 4
F. An exception is thrown at runtime.

F is correct. When run() is invoked, it is with a new instance of ChicksYack and c has not been assigned to an object......


Can anyone explain me the above ?

[Cristian Senchiu]

Let's use only the relevant (for your question) part of code ( formatted so it's easier to read):

public class ChicksYack implements Runnable {

  Chicks c;

  public static void main(String[] args){
    new ChickYack().go();
  }

  void go(){
    c=new Chicks();
    new Thread(new ChickYack()).start();
    new Thread(new ChickYack()).start();
  }

  public void run(){
    c.yack(Thread.currentThread().getId());
  }
}

The two threads you start at line 11 and 12 do not use your current ChickYack instance you create at line 10.
Each creates its own, new instance: new Thread(new ChickYack()) ...
Since c is only declared (line 3) => it has the default value which is null.
That means, at line 16 c is null => a NullPointerException will be thrown.

[Sumeet Chakraborty]

Thanks for your reply .... I think i got the explanation.

Since it creates entirely new objects(in line 11,12) c will be null for these objects.

Thanks again ..