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
