not able to access static data

[chandana nannapaneni]

The question is interesting.

I have 2 classes class1 and class2. class1 has method "method1" and class2 has a method "method2".
method1 has a static variable i. method1 is constantly running and always updates i. Now I run method2 parallely with method1.
In method2, when i try to print value of i as "class1.i" , I always get the value "0".
I am expecting method2 to output the current value of i but it is always printing 0(even though method1 is updating the value of i).

Can anyone help me with this?

Thanks,
Chandana.

[Steve Luke]

Perhaps if you show the code which causes the error, we could show you the problem.

My guess is that you have a class like this:

public class Class1 {
    public static int i;

    static void method1() {
        int i = 14;
        while (...) {
            ...
            i = 2 * i -3; //or some other code which would make i change
            ...
        }
    }
}

Then, you have a second class which tries to access i directly, and find that i is still 0.

When I make a simple example as above can you see why the second class might see only 0?

[Rob Spoor]

Also, try making the static field volatile. If you're updating and reading it from different threads without synchronization you'll get stale data without the volatile keyword.

[chandana nannapaneni]

Thanks Steve and Rob for the immediate reply.

Let me explain the problem more clearly.

I have class "class1" as follows,

public class class1{
static int i;
public static void main(String args[]){
for(;;)
{
i++;
System.out.println("in class1: value of i is"+i);
}
}
}

I have class "class2" as follows,

public class class2{
public static void main(String args[]){
for(int j=0;j<10;j++)
System.out.println("in class2: value of i is"+class1.i);
}
}


Now I open Command prompt, and run class1.
Next I open another command prompt and run class2.
the value of i in class1 changes but the output of class2 is always zero.

There is no concept of threads used in this question.
Can you help me with this?

Chandana.

[Henry Wong]

Now I open Command prompt, and run class1.
Next I open another command prompt and run class2.

When you do this, you are running two different JVMs with two different programs.

Static means that there is one copy per JVM -- not one copy across all JVMs...

Henry

[chandana nannapaneni]

Thanks Henry.
But what should I do if I want to access a variable across different JVMs? How should I declare a variable that I want to use across all JVMs?

[Rob Spoor]

You would need inter-process communication:
- sockets (or RMI for a higher level protocol)
- files
- database
- ...

That's definitely not a beginner's topic anymore. This problem has been encountered multiple times; a search in the intermediate forum should yield several results.

[David Newton]

You'd have to use some sort of shared memory/messages/etc. system--you're basically saying you want to share memory across multiple applications.

[chandana nannapaneni]

Thanks Rob and David. You replies helped me solve my problem.