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Suspected Error in Answer for a Thread Question in "Practice Exams Java"

Kevin Simonson
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Joined: Oct 22, 2011
Posts: 114
Friday I posted an article to the Mock Exam Errata forum asking for an explanation for a published answer to a question in Practice Exams Java by Bert Bates and Kathy Sierra. It really looks to me like the answer is in error. Was that the right forum to post to, or should I have posted my article here or some other place?

Kevin Simonson
Suhas Mandrawadkar
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Joined: Jul 21, 2007
Posts: 72

Hi Kevin,

Could you, please, post more details ? If possible, the questions and options for answer as well


Regards, Suhas S. Mandrawadkar.
Certifications: SCJP 6, SCWCD 5, Oracle WebLogic Server Administrator, OCE Java EE 6 EJB Developer
Kevin Simonson
Ranch Hand

Joined: Oct 22, 2011
Posts: 114
Suhas Mandrawadkar wrote:Hi Kevin,

Could you, please, post more details ? If possible, the questions and options for answer as well


The article I posted to the Mock Exam Errata forum in its entirety is:

I'm looking at the PDF version of a book titled Practice Exams Java, with subtitles Questions . In-Depth Answers and OCP Java SE 6 Programmer Pracitce Exams. In parentheses below that second subtitle it says, "Exam 310-065," and the authors are listed as Bert Bates and Kathy Sierra. The purpose of the book appears to be to prepare software engineers to take the SCJP exam, which is what several software engineers at my company are preparing for. Each week we read one chapter from the Sun Certified Programmer for Java 6 Study Guide, by the same authors, and we meet for ninety minutes once that week to go over problems from the practice exams. This week (this Tuesday, in particular), it's my turn to lead the study group for the chapter on threads, so I went through the assessment tests and practice exams looking for problems I could use. One of those was problem #25 of Practice Exam 3. The problem with the answer provided for it, on pages 290 and 291 are (formatted by me for Notepad):

25. Given:

4. public class Stone implements Runnable {
5. static int id = 1;
6. public void run() {
7. try {
8. id = 1 - id;
9. if(id == 0) { pick(); } else { release(); }
10. } catch(Exception e) { }
11. }
12. private static synchronized void pick() throws Exception {
13. System.out.print("P "); System.out.print("Q ");
14. }
15. private synchronized void release() throws Exception {
16. System.out.print("R "); System.out.print("S ");
17. }
18. public static void main(String[] args) {
19. Stone st = new Stone();
20. new Thread(st).start();
21. new Thread(st).start();
22. } }

Which are true? (Choose all that apply.)

A. The output could be P Q R S
B. The output could be P R S Q
C. The output could be P R Q S
D. The output could be P Q P Q
E. The program could cause a deadlock.
F. Compilation fails.


Answer (for Objective 4.3):
[/] A, B, and C are correct. Since pick() is static and release() is
non-static, there are two locks. If pick() was non-static, only A would be
correct.
[X] D is incorrect because line 6 swaps the value of id between 0 and 1. There
is no chance for the same method to be executed twice. E and F are
incorrect based on the above.

A, B, and C I understand just fine. But I don't understand at all the assertion that there "is no chance for the same method to be executed twice." Access to and modification of static variable <id> isn't synchronized on line 8. The two times <run()> is executed, once for each thread, could happen in parallel, couldn't they? What's to keep each thread from reading a 1 value from <id> before either of them write to <id>?

In that case each would write a zero to <id>, which would cause <pick()> to be executed twice, which would result in output "P Q P Q". So it looks like D is a correct answer. Am I missing something here?

Kevin Simonson
Suhas Mandrawadkar
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Joined: Jul 21, 2007
Posts: 72

Either Thread A will modify "id" first or Thread B will. Both cannot modify it at the same time. Answers B and C are results of threads running in parallel. Else the answer would have been PQRS.
Kevin Simonson
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Joined: Oct 22, 2011
Posts: 114
Suhas Mandrawadkar wrote:Either Thread A will modify "id" first or Thread B will. Both cannot modify it at the same time. Answers B and C are results of threads running in parallel. Else the answer would have been PQRS.


Is "id = 1 - id;" an atomic statement then? If so, why is it atomic?

It seems to me more likely that this statement will be broken up into at least three steps, (1) reading <id> into a register, (2) subtracting the register from one, and finally (3) storing the result of the subtraction back in <id>.

What's to keep the first thread from reading <id> into one of its registers, and then the second thread interrupting to also read the contents of <id> into one of its registers, and then for each thread to do the subtracts and stores back into <id> in any order at all? In such a case, the value stored each time will be zero, and <pick()> will be called twice, resulting in output "P Q P Q".

The same result would occur if one thread did both steps (1) and (2), and then at that point the second thread interrupted and did step (1), and then the remaining steps followed in any order.

Kevin Simonson
dennis deems
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Joined: Mar 12, 2011
Posts: 808
This very question has been the subject of discussion a number of times. Did you try searching the forum?
Kevin Simonson
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Joined: Oct 22, 2011
Posts: 114
Dennis Deems wrote:This very question has been the subject of discussion a number of times. Did you try searching the forum?

Dennis, thanks for pointing me to this forum's search mechanism. I did a search on "Practice Exams Java Exam 3 25", and it led me to
"http://www.coderanch.com/t/531921/java-programmer-SCJP/certification/Threads-found-OCP-Java-Practice", which is a description of this very same controversy. Anybody interested in this question can click on this link and go down to the statements made at the bottom consecutively by Mike Simmons, Dennis himself, and Bert Bates (one of the authors of Practice Exams Java). Bert is "leaning towards thinking that there's a problem with the question," but as of early morning 27 March he hadn't "looked at it closely yet." I just posted a response to him asking him what the last word was on this question.

Kevin Simonson
Suhas Mandrawadkar
Ranch Hand

Joined: Jul 21, 2007
Posts: 72

Hmm, that is indeed an interesting thread to go through.
 
Don't get me started about those stupid light bulbs.
 
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