Issue with static variables in Java

Question

I need an explanation for the output of the below code:

class Stats
{
    static int a = 10;
    int b = 20;
    void printMe()
    {
        System.out.println(a+b);
    }
}

public class Static
{
    public static void main(String args[])
    {
        Stats s1 = new Stats();
        Stats s2 = new Stats();
        s1.b = 30;
        s1.printMe();
        s1.a = 20;
        s2.printMe();
    }
}

Output: 40 40

I expected it to be 40 and 50 as there should be only one copy of static variable 'a' which is modified by through reference 's1' to 20.

Static means affecting the entire class. There is only one `Stats.a` and only one `Stats.b`. It is therefore good practice to refer to static variables by class name instead of an instance name. — Justin, Apr 27 '13 at 00:33
@gangqinlaohu If you use Stats, I get 40 and 50 as output but static variable I believe should have only one copy. So, even if I alter it using an object reference, that single copy must get modified? — , Apr 27 '13 at 00:37
The first (s1) is 30 + 10 and the second (s2) is 20 + 20. BTW, did you hear about a wonderful tool called debugger ? — Nir Alfasi, Apr 27 '13 at 00:37
@alfasin yes the debugger can help but understanding the concept with an explanation is better. — Luiggi Mendoza, Apr 27 '13 at 00:40
@alfasin A debugger, in cases like these, would only be an easier way of walking through code. I often don't use it, and just walk through my code in my head (because I'm to lazy to run it) — Justin, Apr 27 '13 at 00:43
@gangqinlaohu and Luiggi Mendoza: I also rarely use a debugger, but if I can't figure it out by walking through the code, I would definitely use one before posting this question. Voting to close as "too localized" — Nir Alfasi, Apr 27 '13 at 00:46
@alfasin I would say that it's a reasonable beginner Java question, but the OP probably should have searched the web for 'Java static variables' before asking the question. — Justin, Apr 27 '13 at 00:47
@gangqinlaohu oh he KNOWS what static means: see the last two lines of the question :) — Nir Alfasi, Apr 27 '13 at 00:49
@gangqinlaohu - There are as many Stats.b copies as there are instances of Stats. Only Stats.a is static. — Hot Licks, Apr 27 '13 at 00:52
The reason you get the second 40 is that you're adding the 20 from a and the 20 from b in the second instance of Stats, that second b never having been modified from its initial value of 20. — Hot Licks, Apr 27 '13 at 00:55

score 4 · Answer 1 · answered Apr 27 '13 at 00:37

a is static also called a "Class Variable", the value of a will be equal in all the instances of Stats,

so in the first call: a is equals to 10 and b is equals to 30 for s1, so a+b is equals to 40,

and in the second call: a is equals to 20 and b is equals to 20 for s2, so a+b is equals to 40

Issue with static variables in Java

1 Answers1