Usage of static variables in Java

Question

I knew that Static variables are created and initialized only one time I.e when the class is loaded and not per object.

In the code given below what I am trying to do is: I declared an int variable "a" as static in class A and assigned it a value of 5. When the main is executed I changed its value to 6 and made the program to loop infinitely. So currently "a" value is 6.

Now what I tried to do is I tried to access this variable from other class class B when class A is still looping and I expected that 6 should be printed when I ran class B (because by the time I ran class B "a" value is changed to 6) but surprisingly it still gave me 5.

Why is this happening even though I declared "a" as static? Unable to figure out what's wrong with this approach.

class A{
    static int a=5;

    public static void main(String args[]){
        System.out.println(A.a+"");
        a=6;
        while(true){
        }
    }
}

class B{    
    public static void main(String args[]){
        System.out.println(A.a+"");
    }
}

You are running two separate programs via A.main and B.main. You'd have to use threads within the same process to demonstrate what you expect to see. — Steve Townsend
– Steve Townsend, Commented May 21, 2012 at 17:42
I dont think threads are necessary. In B; `pubic static void main ( String [ ] args ) { A . main ( args ) ; System . out . println ( A . a + "" ) ; } — emory
– emory, Commented May 21, 2012 at 17:43

Jeremy · Accepted Answer · 2012-05-21 17:56:04Z

4

The problem you are having is that you are running two different JVM processes. What you should be doing is running each of your "main" methods in its own thread. This way they share (and therefore can modify) the same memory. Here is your example modified to use threads:

public class Main{
    public static int v = 5;

    public static void main(String[] args) throws Exception{
        new Thread(new A()).start();
        Thread.sleep(5000);
        new Thread(new B()).start();
    }
}

class A implements Runnable{
    public void run(){
        System.out.println(Main.v);
        Main.v = 6;
    }
}

class B implements Runnable{
    public void run(){
        System.out.println(Main.v);
    }
}

Side note: While this example does demonstrate what is expected from the question, there is still a lot of things wrong in terms of writing a correct Java program that uses multiple threads. Do your reasearch.

edited May 21, 2012 at 17:56

answered May 21, 2012 at 17:50

Jeremy

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1 Comment

user1406427 Over a year ago

ya u r right i tried to load the two programs in different processes that's the mistake. Thank You!

nsfyn55 · Accepted Answer · 2012-05-21 17:44:25Z

1

Your are running two java processes and two programs. The second classloader loads class A and sets its static variable to 5 but there is no call in class B to invoke that static main() method defined in class A that increments it to 6.

answered May 21, 2012 at 17:44

nsfyn55

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1 Comment

user1406427 Over a year ago

ya u r right i tried to load the two programs in different processes that's the mistake. Thank You!

OmniOwl · Accepted Answer · 2012-05-21 17:41:04Z

0

You never really run the main loop in A.

Just make a method to change the value.

answered May 21, 2012 at 17:41

OmniOwl

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3 Comments

user1406427 Over a year ago

i ran Class A first so "a" value should be changed to 6. After this, while A is struck in a while loop i tried to access "a" in class B i got 5 as answer.

OmniOwl Over a year ago

Where do you run "A" first? You would have to run those two in the same class, to get what you want. They are still two separate objects right now, that has nothing to do with each other. So you are just accessing the variable, not really changing it before you access it with B.

user1406427 Over a year ago

ya u r right i tried to load the two programs in different processes that's the mistake. Thank You!

emory · Accepted Answer · 2012-05-21 17:42:21Z

0

That is because the main method of A is never invoked.

answered May 21, 2012 at 17:42

emory

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