Understanding how variable point to value in Java

Question

I have a below code. The variable c and d are class variables and initially they were pointing to value 0, but when I did c=a* a; and d =b* b*b they printed value as 25 and 64 which is correct. so what I think now is that the c and d are now pointing to value 25 and 64 and they are class variables, so if I do j=c+d; it should give me 89 as j value, but it is giving me 0... why? I know if I use static with c and d variable it will give me 89 value... but why I should use static as c and d are global variables and there values are now updated to 25 and 64. Please let me know. Thanks.

public class BaiscSum {

    int a=5;
    int b=4;
    int c;
    int d;
    int j;

    public void square() {
        c=a*a;
        System.out.println(c);   
    }

    public void cube() {
        d=b*b*b;
        System.out.println(d);
    }

    public void sum() {
        j=c+d;
        System.out.println(j);
    }

    public static void main(String[] args) {
        BaiscSum squ= new BaiscSum();
        squ.square();

        BaiscSum cub = new BaiscSum();
        cub.cube();

        BaiscSum su = new BaiscSum();
        su.sum();    
    }
}

variable c and d are NOT class variables. Class variables have the key word static — OPK
– OPK, Commented Apr 12, 2015 at 18:53
These aren't class variables. They're instance variables. Why do you think they're class variables? — user2357112
– user2357112, Commented Apr 12, 2015 at 18:53
user2357112 - Thanks for clearing. As these variables are used inside the class, they are global and I thought they are class variable, so what is the difference between class variable and instance variable ?? — Deepa Singh
– Deepa Singh, Commented Apr 13, 2015 at 14:34

Moishe Lipsker · Accepted Answer · 2015-04-12 18:55:35Z

6

You are using three separate instances of your class. This means that squ, cub, and su each have their own version of the class. Instead, use the same one instance, so that all changes will happen to the same instance.

public static void main(String[] args) {
    BaiscSum sum= new BaiscSum();
    sum.square();
    sum.cube();
    sum.sum();
}

answered Apr 12, 2015 at 18:55

Moishe Lipsker

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1 Comment

Deepa Singh Over a year ago

Thank you Moishe Lipsker

singhakash · Accepted Answer · 2015-04-13 14:54:22Z

3

If I do j=c+d; it should give me 89 as j value, but it is giving me 0. why?

Because you are referencing sum() with su object but you have not called cube() and square() on su instead you called it with cub and squ respectively.

Change to

BaiscSum su = new BaiscSum();
su.square();
su.cube();
su.sum();

It will give you the expected output.

Since all the variables you are using are instace so every object will have its own set of variable on their respective memory spaces and if you changed any one objects variable it will not effects other objects value.

This could clear more

enter image description here

As an explanation every object is assigned a seperate memory and so changes to the variable of one objects doesn't effect changes to other objects variable unless they are static. So in the third object c and d is not initialized for su object so jvm uses default value if int ie 0 giving you a sum of zero.

edited Apr 13, 2015 at 14:54

answered Apr 12, 2015 at 18:53

singhakash

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2 Comments

singhakash Over a year ago

@DeepaSingh my pleasure and if you find my or Moishe Lipsker answer helpful consider accepting any one

singhakash Over a year ago

@DeepaSingh To mark an answer as accepted, click on the check mark beside the answer to toggle it from hollow to green.Also check the link

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