Threads question (Resolved test question doubt)

[David Samer]

Greetings there community

I am hell confuse with the following test question and its explanation, from Kathy's Sierra and Bert's Book (Chapter 9 - Threads, page 784 )

Given :
	
3. class Chicks {
4. synchronized void yack(long id) {
5. for(int x = 1; x < 3; x++) {
6. System.out.print(id + " ");
7. Thread.yield();
8. }
9. }
10. }
11. public class ChicksYack implements Runnable {
12. Chicks c;
13. public static void main(String[] args) {
14. new ChicksYack().go();
15. }

16. void go() {
17. c = new Chicks();
18. new Thread(new ChicksYack()).start();
19. new Thread(new ChicksYack()).start();
20. }
21. public void run() {
22. c.yack(Thread.currentThread().getId());
23. }
24. }

Which are true? (Choose all that apply.)
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

Answer:
✓ F is correct. When run() is invoked, it is with a new instance of ChicksYack and c has
not been assigned to an object. If c were static, then because yack is synchronized, answers
C and E would have been correct.
A, B, C, D, and E are incorrect based on the above. (Objective 4.3)

:/

I see when run() is invoked, somehow takes instance of ChicksYack and , here the part I'm confuse with, it says c ... , why has not been assigned to an object? (What it does mean? ). (Isn't c instantiated already once thread invokes run()?)( Looks like it c weren't into the scope for run() but , no clue why )

Thanks in advance .

[Praveen Kumar M K]

In your main method, what would be the output if there were just this code,

public static void main(String[] args) {
ChicksYack chicksYack = new ChicksYack();
chicksYack.c.yack(Thread.currentThread().getId());
}

[David Samer]

Hey hello there Praveen,

Praveen Kumar M K wrote:In your main method, what would be the output if there were just this code,

public static void main(String[] args) {
ChicksYack chicksYack = new ChicksYack();
chicksYack.c.yack(Thread.currentThread().getId());
}

Hmm, that means chicksYack is trying to manipulate c object , but... wait.. if its inside main... its static method, so c should be static and initialized... is this the issue? then... hmm... no access.


EDIT: !oh! I do apologize, it seems this question were opened , found it through related topics -> http://www.coderanch.com/t/504261/java-programmer-SCJP/certification/non-static-object-as-lock (Yet a bit confuse all the explanation but I believe I get an idea thanks to your example Praveen (at least for me it's clearer than the whole thread there)).

[Praveen Kumar M K]

David Samer wrote:Hey hello there Praveen,

Praveen Kumar M K wrote:In your main method, what would be the output if there were just this code,

public static void main(String[] args) {
ChicksYack chicksYack = new ChicksYack();
chicksYack.c.yack(Thread.currentThread().getId());
}

Hmm, that means chicksYack is trying to manipulate c object , but... wait.. if its inside main... its static method, so c should be static and initialized... is this the issue? then... hmm... no access.

c need not be static and there is no access issue, however you are correct to point out that c should be initialized. We see that creating a new ChicksYack object does not inturn create a new Chicks object. Only a reference called c is created. Now, if we were to call the yack function on this reference, without initialization we would get a null pointer exception. Is that ok?

Now track back to the original program and see where a new Chicks object gets created and is assigned to "c". Let me know what you understand.

[Zhenyi Luo]

David Samer wrote:Greetings there community

I am hell confuse with the following test question and its explanation, from Kathy's Sierra and Bert's Book (Chapter 9 - Threads, page 784 )

Given :
	
3. class Chicks {
4. synchronized void yack(long id) {
5. for(int x = 1; x < 3; x++) {
6. System.out.print(id + " ");
7. Thread.yield();
8. }
9. }
10. }
11. public class ChicksYack implements Runnable {
12. Chicks c;
13. public static void main(String[] args) {
14. new ChicksYack().go();
15. }

16. void go() {
17. c = new Chicks();
18. new Thread(new ChicksYack()).start();
19. new Thread(new ChicksYack()).start();
20. }
21. public void run() {
22. c.yack(Thread.currentThread().getId());
23. }
24. }

Which are true? (Choose all that apply.)
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

Answer:
✓ F is correct. When run() is invoked, it is with a new instance of ChicksYack and c has
not been assigned to an object. If c were static, then because yack is synchronized, answers
C and E would have been correct.
A, B, C, D, and E are incorrect based on the above. (Objective 4.3)

:/

I see when run() is invoked, somehow takes instance of ChicksYack and , here the part I'm confuse with, it says c ... , why has not been assigned to an object? (What it does mean? ). (Isn't c instantiated already once thread invokes run()?)( Looks like it c weren't into the scope for run() but , no clue why )

Thanks in advance .

It looks like c has been assigneed to an object, but that one belongs to the main thread. When it tries to run c.yack(Thread.currentThread().getId()), it needs a "c" which has been assigned to the new thread. In other words, "c" has not been initialized in the context. On the other hand, if c is static, it belongs to the class, not any individual instance, so we only need to instantiate it once.

Hope it answers your question.

Eric

[David Samer]

Praveen Kumar M K wrote:

c need not be static and there is no access issue, however you are correct to point out that c should be initialized. We see that creating a new ChicksYack object does not inturn create a new Chicks object. Only a reference called c is created. Now, if we were to call the yack function on this reference, without initialization we would get a null pointer exception. Is that ok?

Got it!.

Praveen Kumar M K wrote:
Now track back to the original program and see where a new Chicks object gets created and is assigned to "c". Let me know what you understand.

As for what I see, tracking back, line 17 is the one in where Chicks object is created at the same time assigned to "c" . Fine, so far, but now it comes the confuse part for me. I suppose program keeps goinng so after line 17 , it creates another 2 object, but this time, both are Threads ,which both gets started but. In which point , can I know it will invoke run() and call yack(long id) method , as well as on which object?.

=================

Hey there Eric ;)

Praveen Kumar M K wrote:
It looks like c has been assigneed to an object, but that one belongs to the main thread. When it tries to run c.yack(Thread.currentThread().getId()), it needs a "c" which has been assigned to the new thread. In other words, "c" has not been initialized in the context. On the other hand, if c is static, it belongs to the class, not any individual instance, so we only need to instantiate it once.

Hope it answers your question

Wow!, I would say I'm a bit more confuse. I don't understand the part highlighted in black :/


=======

Thanks both for taking your time answering.

[Praveen Kumar M K]

David Samer wrote:

As for what I see, tracking back, line 17 is the one in where Chicks object is created at the same time assigned to "c" . Fine, so far, but now it comes the confuse part for me. I suppose program keeps goinng so after line 17 , it creates another 2 object, but this time, both are Threads ,which both gets started but. In which point , can I know it will invoke run() and call yack(long id) method , as well as on which object?

Right, there are 2 new ChicksYack objects. But have we called the go method on these objects to initialize their internal Chicks objects? Nope, their "c" is still null.

Its a kind of catch-22 situation. If you actually did something like

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

you would have a recursive overflow..and without the call you would have a null pointer exception! A possible change would be as Zhenyi Luo suggested, to make c static.

[David Samer]

Praveen Kumar M K wrote:

Right, there are 2 new ChicksYack objects. But have we called the go method on these objects to initialize their internal Chicks objects? Nope, their "c" is still null.

Its a kind of catch-22 situation. If you actually did something like

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

you would have a recursive overflow..and without the call you would have a null pointer exception! A possible change would be as Zhenyi Luo suggested, to make c static.

Oh! , that's the trick then. That was hell clear for understand it. It also has helped me out for understand Zhenyi's answer, reading it again a couple of times.

Thank you so much Praveen!

Can I take advantage for ask something else regarding question?

I copied code and paste it in my IDE. Then I did the needed change for get it working ( made c static and a new -> static Chicks c = new Chicks(); ) and what's my surprise the output is not what answer says but 9 9 10 10 instead. Is there any way we can know how to get these numbers? If someone else try its in their computer would get different numbers?

[Praveen Kumar M K]

The thread "id" AFAIK is just some autogenerated number uniquely identifying a thread. I think if you run your program again maybe you'll get a different output.

[David Samer]

Praveen Kumar M K wrote:The thread "id" AFAIK is just some autogenerated number uniquely identifying a thread. I think if you run your program again maybe you'll get a different output.

As I thought then. Thanks Praveen for all your help, it has been good and appreciated.

[Praveen Kumar M K]

You are most welcome David! :-)