About Question enthuware.ocpjp.v7.2.1078 :

[shareef.hiasat]

Why isn`t 1 1 a possible solution ; i read the explanation but i want super slow motion to understand this

thanks

This is a straight forward implementation on an thread unsafe class. Observe that count is a shared resource that is accessed by multiple threads and there are multiple issues in this code:

1. Since access to count is not synchronized, there is no guarantee that changes made by thread 1 will even be visible to thread 2. Thus, both the threads may print 0 and increment it to 1 even if they run one after the other. To understand this point, you need to read about topic of visibility guarantee provided by the Java memory model.

[admin]

Here is the super slow version http://java.dzone.com/articles/multithr ... ava-memory
and here is another good article explaining the whole thing: https://www.securecoding.cert.org/confl ... and+Memory

[shareef.hiasat]

Here is the super slow version http://java.dzone.com/articles/multithr ... ava-memory
and here is another good article explaining the whole thing: https://www.securecoding.cert.org/confl ... and+Memory

many thanks for the links

but my second question is 0 0 even possible solution ?! and why

Consider the following class:

Code: Select all

public class Counter {
    private int count;
    public void increment(){
        System.out.println(count++);
    }
 }

If two threads call the increment() method on the same Counter instance simultaneously, which of the following are possible outputs? (Assume that there are no other calls to the Counter instance.)

[admin]

Explained here: http://docs.oracle.com/javase/tutorial/ ... rfere.html

[goodness]

Scenario 1
Thread 1: Retrieve count. Print count; output is 0.
Thread 2: Retrieve count. Print count; output is 0.
Thread 1: Increment retrieved value; result is 1.
Thread 2: Increment retrieved value; result is 1.
Thread 1: Store result in count; count is now 1.
Thread 2: Store result in count; count is now 1.

Scenario 2
Thread 1: Retrieve count. Print count; output is 0.
Thread 1: Increment retrieved value; result is 1.
Thread 2: Retrieve count. Print count; output is 0.
Thread 1: Store result in count; count is now 1.
Thread 2: Increment retrieved value; result is 1.
Thread 2: Store result in count; count is now 1.

Scenario 3
Thread 1: Retrieve count. Print count; output is 0.
Thread 1: Increment retrieved value; result is 1.
Thread 1: Store result in count; count is now 1.
Thread 2: Retrieve count. Print count; output is 1.
Thread 2: Increment retrieved value; result is 2.
Thread 2: Store result in count; count is now 2.

[sir_Anduin@yahoo.de]

Hi,
I still cant understand how one result might be 0 as in any case the final asignment to count will be at least 1:
count = 0
temp = count +1 (temp =1)
count = temp.

so count must be at leat 1.

but as i tried it, the result is as you say...

[rohitbe]

I would like to add to this.
The output would be option B and D i.e 1 1 and 1 2 if and only if count is pre-incremented
public void increment(){
System.out.println(++count);
}

Correct me, if I am wrong

[admin]

Yes, in case of pre increment, the output would be either 1 1 or 1 2.