Nullpointer Exception and Threads

Question

Consider the following code:

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;                                //.....(1)
      public static void main(String[] args) {
                       new ChicksYack().go();
      }
      void go() {
                      c = new Chicks();        //........(2)
                      new Thread(new ChicksYack()).start();
                      new Thread(new ChicksYack()).start();
      }
      public void run() {
                      c.yack(Thread.currentThread().getId());
      }
}

When i run this code, I am getting a Null Pointer Exception that I have not initialized variable c. But didn't i initialized it at line ....(2)? I am having trouble getting this concept. Does threading has a part to play in this exception?

Which line gives the NPE?

Code-Apprentice
– Code-Apprentice

2013-06-28 23:59:43 +00:00
Commented Jun 28, 2013 at 23:59 — Code-Apprentice
– Code-Apprentice, Commented Jun 28, 2013 at 23:59

jlordo · Accepted Answer · 2013-06-29 00:02:15Z

4

Look at this line:

new Thread(new ChicksYack()).start();
           ^^^^^^^^^^^^^^^^

The attribute c of the newly created ChicksYack object is never initialized. In the go() method you only initialize c for the current (this) object.

That's why you get an NPE in the run() method. A good solution would be to initialize that variable in a default constructor for ChicksYack.

answered Jun 29, 2013 at 0:02

jlordo

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1 Comment

SimonC Over a year ago

Or even better, just initialise it at the point of declaration (since it doesn't require any external state).

tom · Accepted Answer · 2013-06-29 00:02:35Z

0

In the go() method, you're instantiating two new ChickYack objects, which have a null c. You should put the c = new Chicks() in your ChicksYack constructor.

answered Jun 29, 2013 at 0:02

tom

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