Java Variable Scope - Instance and class variables

Question

I am new to Java and trying to learn by myself. I wrote the below code and I was wondering why the output is not as I expect. Below is the code I have written:

public class Roughwork {

    public static int classVar = 25;

    public void getValue(int a){
        classVar = a;
        System.out.println(classVar);

    }

    public static void main(String[] args) {
        Roughwork test = new Roughwork();
        System.out.println(classVar);
        test.getValue(30);
        System.out.println(classVar);
    }

}

and the output of this program is:

25
30
30

I expected the output to be

25
30
25

What exactly is happening and What I have to do to get my expected output?

Remove the `static` keyword. Then go and read about what it does. — christopher, Jun 21 '15 at 13:45
@user2810706 you're changing the value of the variable. Why are you wondering that it was changed? — Maroun, Jun 21 '15 at 13:48
@MarounMaroun: The value I have changed is in (getValue) method, so I was thinking it will be confined to that method and the final classVar value will be taken as 25 instead of 30 — user2810706, Jun 21 '15 at 13:51
@user2810706 `static` variables are shared among all instances. — Maroun, Jun 21 '15 at 13:52

wvdz · Accepted Answer · 2015-06-21T14:31:16.573

3

ClassVar is declared static.

public static int classVar = 25;

This means that it is not tied to an instance of Roughwork. It is a global variable if you will. You can call this variable even from other classes in your application like this:

Roughwork.classVar

To get your expected behaviour, change your code to this:

public class Roughwork {

    public int classVar = 25;

    public void getValue(int a){
        classVar = a;
        System.out.println(classVar);

    }

    public static void main(String[] args) {
        Roughwork test = new Roughwork();
        System.out.println(test.classVar);
        test.getValue(30);
        Roughwork test2 = new Roughwork();
        System.out.println(test2.classVar);
    }

}

edited Jun 21 '15 at 14:31

answered Jun 21 '15 at 13:59

wvdz

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That doesn't seem to be working.. I still get 25 30 30 – user2810706 Jun 21 '15 at 14:18
It should actually give a compile time error if you remove the static keyword. I'll add a fixed code example. – wvdz Jun 21 '15 at 14:28

score 0 · Answer 2 · answered Jun 21 '15 at 13:57

It's because you change it to 30 and don't change it back to 25. (static means it is the same for all instances of class)

    System.out.println(classVar); //25
    test.getValue(30);            //30
    System.out.println(classVar); //30 - you don't change variable back to 25

To fix it add classVar = 25 after test.getValue or remove static modifier on classVar definition.

score 0 · Answer 3 · edited May 23 '17 at 12:22

A static variable is shared by all instances of the class. and in case of instance variable each instance of class have different copy.

Static variable memory allocate at compile time, They are loaded at load time and initialized at class initialization time and in case of instance variable everything is done at run time.

To make this example interesting: Create new instances of Roughwork, try to change the value and give it a try.

Roughwork test = new Roughwork();
Roughwork test2 = new Roughwork();
System.out.println(classVar);

test.getValue(30);
test2.getValue(25);
System.out.println(classVar);

Reference this thread for example to understand clearly:Class Variable & Instance Variable

Java Variable Scope - Instance and class variables

3 Answers3