Thread

[Ependi Jusdi]

i though it will always finish with a.i=0, but it's not, come somebody help me???

class A {
static int i=0;
public synchronized void aMethod(){ ++i; }
public void bMethod(){
synchronized(this){ --i; }
}
}
public class Question52 implements Runnable {
A a = new A();

public static void main(String[] args) throws Exception{
Thread t1 = new Thread(new Question52());
Thread t2 = new Thread(new Question52());
t1.start(); t2.start();
t1.join();
t2.join();
System.out.println("a.i="+ A.i +" ");
}
public void run(){
for(int i=0;i<100000000;i++){
a.aMethod();
a.bMethod();
}
}
}

[Robi Iskandar]

Maha, Ajith can you explain to me, I also stack, I think this question is similar to Valentine Mock Exam Q52.
Pls help us.
Thx.

[Corey McGlone]

Alright - this one is a bit tricky, but I believe I've found the answer. They key is to notice that each thread has it's own instance of an A object. Therefore, both threads could be executing a.aMethod() or a.bMethod() at the same time. Since i is a staic variable, both instances share that variable. Let's take this case:
0. A.i = 0
1. We create two threads, t1 and t2
2. We start both threads, t1 and t2
3. t1 invokes a.aMethod()
4. t1 increments i (i = 1)
5. t1 loses its timeslice before it can assign 1 to i (i it still 0)
6. t2 invokes a.aMethod()
7. t2 increments i (i = 1)
8. t2 assigns 1 to i
9. t2 loses its timeslice to t1
10. t1 assigns 1 to i
11. t1 invokes a.bMethod()
12. t1 decrements i (i = 0)
13. t1 assigns 0 to i
14. t1 loses its timeslice to t2
15. t2 invokes a.bMethod()
16. t2 decrements i (i = -1)
17. t2 assigns -1 to i
From this, you can see that, after one cycle of each thread invoking aMethod and bMethod, the value of i is no longer 0. One of the increments was lost due to the fact that the increment was performed but the thread was timesliced out prior to tha assignment being performed.
Similarly, you could propose a case in which a decrement would be lost, resulting in a positive value for i.
I doubt this conditions would occur very often. That's why the for loop is so large. You need to execute this code many times in order to get the timeslices to occur just right even a few times.
I hope that makes sense. :roll: If you have any more questions, please let me know.
Corey
[ June 04, 2002: Message edited by: Corey McGlone ]

[Ependi Jusdi]

got it Corey, many thanks...
Regards,
au

[Francisco A Guimaraes]

Is it possible that in the exam would appear a question like "Does the code below always print A.i=0?
[ June 04, 2002: Message edited by: Francisco A Guimaraes ]

[Corey McGlone]

Originally posted by Francisco A Guimaraes:
Is it possible that in the exam would appear a question like "Does the code below always print A.i=0?
[ June 04, 2002: Message edited by: Francisco A Guimaraes ]

It is possible, but I have a feeling this question is a little more difficult than anything you'd see on the real exam.
Of course, on the exam, questions are multiple choice, so they'd probably give you a number of other options, such as "Compiler Error", etc.
Corey