Using non-static object as lock in Threads PRoblem

[Mohit G Gupta]

KB book self test
Question 16

 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 ChicksYack().go();
   }
     void go() {
    c = new Chicks();
    new Thread(new ChicksYack()).start();
    new Thread(new ChicksYack()).start();
  }
  public void run() {
    c.yack(Thread.currentThread().getId());
  }
}
    

i ran the above code and got the following error:
Exception in thread "Thread-0" java.lang.NullPointerException
at ChicksYack.run(ChicksYack.java:20)
at java.lang.Thread.run(Thread.java:619)
Exception in thread "Thread-1" java.lang.NullPointerException
at ChicksYack.run(ChicksYack.java:20)
at java.lang.Thread.run(Thread.java:619)


Answer as given in the book:
When run() is invoked, it is with a new instance of ChicksYack and c has
not been assigned to an object. so,Runtime Exception


Queries
1.But c has been to an object on line 15,then why is there a Runtime Exception
2.If I change Chicks c to static Chicks c,the code run fines.


can anyone explain ?
I cannot get the answer as given in book

[Henry Wong]

mohitkumar gupta wrote:
1.But c has been to an object on line 15,then why is there a Runtime Exception

Yes, but there are three objects -- one at line 12, one at line 16, and one at line 17. Is the assignment of "c" at line 15, the same variable that is used at line 20?

mohitkumar gupta wrote:
2.If I change Chicks c to static Chicks c,the code run fines.

Can you explain the difference between a static and instance variable? and how the previous question may be affected by this?

Henry

[Abimaran Kugathasan]

This question has already discussed recently.

[Mohit G Gupta]

i was not able to find this question in the posts.please refer to that link if anyone finds it.


Chicks c is an instance variable of ChicksYack.
each ChicksYack object will have it's own copy of c.
line 12,16,17 have a new ChicksYack object created.

when the line 16,17 are run,then these threads would start and their run methods would be called.The run() method contains call to yack function through c object.
Since c has been assigned to a Chicks object on line 15.

i think that code would run
but i am getting a runtime error.
is yack function is not been called as c doesnot refers to any object ?
but c is been assigned to object on line 15.

please explain

[Henry Wong]

mohitkumar gupta wrote:
Chicks c is an instance variable of ChicksYack.
each ChicksYack object will have it's own copy of c.
line 12,16,17 have a new ChicksYack object created.

when the line 16,17 are run,then these threads would start and their run methods would be called.The run() method contains call to yack function through c object.
Since c has been assigned to a Chicks object on line 15.

i think that code would run
but i am getting a runtime error.
is yack function is not been called as c doesnot refers to any object ?
but c is been assigned to object on line 15.

please explain

As already hinted at, in my last post, the "c" variable that has been assigned at line 15, is not the same "c" variable used by the run() method, by the newly created threads. Work it out on paper. It's straightforward.

Henry

[Abimaran Kugathasan]

mohitkumar gupta wrote:
please explain

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

Here, ChicksYack is a class, which HAS - A Chick object. So for every instance of the ChicksYack class, there should be a Chick object. In line 12 you create a ChicksYack object, and call go() method on that instance. But the Chick object associted with ChicksYack object is not created yet(simply it's null), and in the go() method, you create a Chick object and assign that object to your already created ChicksYack object's member field c(to the field variable c). And, in the go() method, you create another two ChicksYack objects(in lines 16,17), to pass them to Threads.

And if I ask you, do those object, which you created in the lines 16,17(the instance of ChicksYack class), have the Chick object? Where did you create those objects? Check it again. If don't get it, ask again!

[Shanky Sohar]

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;   //c is the refernce variable of Chicks....
   public static void main(String[] args) {   
     new ChicksYack().go();   
   }   
     void go() {   
    c = new Chicks();   //object of chicks has been created but it will remain within the limits of this method only
    new Thread(new ChicksYack()).start();   
    new Thread(new ChicksYack()).start();   
  }   
  public void run() {   
    c.yack(Thread.currentThread().getId());   //here again c is the reference variable of chicks and there is no object associated with it so it throws null pointer exception
  }   
}   

[Jim Hoglund]

I'll take a swing at it too. Each instance of ChicksYack will have its own 'c' member
variable of type Chicks. Three ChicksYack objects are created: #1 in the main method,
#2 and #3 in the go() method. When go() is called, it instantiates 'c' for ChicksYack
object #1. This 'c' is not used. It then starts ChicksYack #2 and #3 in their own threads.
The trick is that after each start(), the scheduler calls the run() method (not the go()
method) and run() tries to use 'c' without instantiating it.

Jim ... ...

[Mohit G Gupta]

Object created in the lines 16,17(the instance of ChicksYack class), have the Chick object c.it is initialized to as
c = new Chicks();
THen why is it not accessible when run method of THreads created on line 16,17 is called.

[Abimaran Kugathasan]

{EDIT: This posted before mohitkumar gupta had edited his post above this.}

[Jim Hoglund]

mohitkumar : As I said above - please read carefully - there are three (3) separate
ChicksYack objects created, each with their own copy of 'c'. But only one Chicks object
is created. "c = new Chicks()" happens only if go() is called. This happens only once.

Jim ... ...

[Unmesh Chowdhury]

According to your ChicksYack class definition, there is a Chicks type reference variable in every instance of ChicksYack classes which is c.

When you create an instance of ChicksYack at line 12 the c reference variable of the instance gets null typically, after that, you invoke go() method on this instance, so in the go method the instance which is created at line 12 is involved. Now when you assigned a Chicks object in c at line 15, the involved instance’s c reference variable gets the value which was previously null. Now at line 16 and 17 you have created two instances of ChicksYack and their Chicks type references (individual c) are gets null typically and they are not assigned elsewhere but in both threads method is invoked on that references (those are not still refereed any actual Chicks object on the heap by their c reference variables) individually at line 20.

[Abimaran Kugathasan]

mohitkumar gupta wrote:Object created in the lines 16,17(the instance of ChicksYack class), have the Chick object c.it is initialized to as
c = new Chicks();
THen why is it not accessible when run method of THreads created on line 16,17 is called.

OK, in which instance you've called the go() method? And in which instances, you called the start() method and consequently the run() method? Please go through the this thread(post) again.

[jay sugrue]

The answer to this question also says that if c were marked static and because yack is synchronized then the answers would be:

C: The output could be 4 4 2 2

E: The output could be 2 2 4 4

Is this because c does get initialized in the go method and if it was a static variable (just one copy) then the invocation of yack would
now be on an initialized variable, no matter the instance used ?

[Himai Minh]

 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 ChicksYack().go();
   }
     void go() {
    c = new Chicks();             //c object
    new Thread(new ChicksYack()).start();  // ChicksYack object 1
    new Thread(new ChicksYack()).start();  //ChicksYack object 2
  }
  public void run() {
    c.yack(Thread.currentThread().getId());
  }
}
 

When ChicksYack object 1 and 2 are created, they encapsulate Chick objects, c.
When new Thread(new ChicksYack()) starts run() method, the ChicksYack's c tries to call yack method. But this c is not initialized to any object. It is refering to null.
The c object in the code is not encapsulated by ChicksYack object 1 or 2.

[David Samer]

Hello everyone.

I hope this link : http://www.coderanch.com/t/596424/java-programmer-SCJP/certification/Threads-Resolved-test , can answer the doubts (in which I also asked the same question )