Static variables initialization process

Question

I have the following code:

public class StaticKindOfThing {
    static int a =getValue();
    static int b = 10;
    public static int getValue()
    {
        return b;
    }

    public static void main (String []args)
    {
        System.out.println(a);
    }
}

I am aware that default variables are set to 0, however does not it happen at runtime? From the above code it appears that the default initialization to 0 happens before runtime.. otherwise getValue should give a compilation error or runtime exception not finding the value. So my question is. Does the variable static int b = 10; get the 0 default value at compilation time?

Rahul Tripathi · Accepted Answer · 2013-09-01T12:34:20.930

2

It gets value which you provide i.e, 10. Static variables are loaded at runtime

When you fire up a JVM and load the class StaticKindOfThing, then static blocks or fields(Here a, b) are 'loaded' into the JVM and become accessible.

From here:-

It is a variable which belongs to the class and not to object(instance)

Static variables are initialized only once , at the start of the execution .

These variables will be initialized first, before the initialization of any instance variables

A single copy to be shared by all instances of the class

A static variable can be accessed directly by the class name and doesn’t need any object

EDIT:-

Please go through the Detailed Initialization Procedure

edited Sep 01 '13 at 12:34

answered Sep 01 '13 at 12:06

Rahul Tripathi

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the output is 0 actually not 10 – Rollerball Sep 01 '13 at 12:16
It is because you have not put the value of a in static block. try to put that in a static block and see the result.!! – Rahul Tripathi Sep 01 '13 at 12:17
Try this:- static int a; static{ a = getValue(); } – Rahul Tripathi Sep 01 '13 at 12:18
I would like to know the exact moment in which the variable b get set to 0. It does get set to 0 as the result of my code is 0 – Rollerball Sep 01 '13 at 12:19
when it gets set to 0. – Rollerball Sep 01 '13 at 12:20
When your class is loaded then it is set that is at the run time. I would recommend you to read this:- http://docs.oracle.com/javase/tutorial/java/javaOO/initial.html – Rahul Tripathi Sep 01 '13 at 12:23
before inlined initialization all the variables get set first at 0 right? otherwise does not make sense – Rollerball Sep 01 '13 at 12:27
I agree with your view +1 – Maxim Shoustin Sep 01 '13 at 12:28
See the example given by Adriaan Koster to explain in this question:= http://stackoverflow.com/questions/4343760/when-is-static-variable-loaded-in-java-runtime-or-compile-time – Rahul Tripathi Sep 01 '13 at 12:28
@Rollerball:- I have added the detailed initialization procedure doc on how the initialization takes place. Hope that helps for you!!!! – Rahul Tripathi Sep 01 '13 at 12:35
I think that this 0 is set at compile time. as a final variable is. I will go through the detailed initial procedure however thanks – Rollerball Sep 01 '13 at 12:37

Maxim Shoustin · Answer 2 · 2013-09-01T12:12:20.020

1

No, it gets value you provided, aka 10.

In your case even you write:

static int a;

the result will be 0. Because you didn't give any value.

Sometimes you can write static block like:

static {
  //...
}

to be sure that this block runs first before class initiation.

The static initializer block is executed only once when the class is loaded into the JVM like static variable.

Try this one that will do what you think:

public class StaticKindOfThing {

static int a;
static int b = 10;

static{
    a = getValue();
}


public static int getValue()
{
    return b;
}

public static void main (String []args)
 {
    System.out.println(a);
 }
}

Output: 10

edited Sep 01 '13 at 12:12

answered Sep 01 '13 at 12:05

Maxim Shoustin

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I would like to know the exact moment in which the variable b get set to 0. It does get set to 0 as the result of my code is 0 – Rollerball Sep 01 '13 at 12:18
When you run .class file, all static int variable without any provided value loaded into JVM with 0 – Maxim Shoustin Sep 01 '13 at 12:21
the thing is that b has a provided value, is 10. – Rollerball Sep 01 '13 at 12:22
so why I get 0 instead of 10 – Rollerball Sep 01 '13 at 12:24
@Rollerball:- Because your class is loaded and 'a' is the first static variable. – Rahul Tripathi Sep 01 '13 at 12:26
so it's safe to say that before the inilined initialization process all the variables present are first set to 0 – Rollerball Sep 01 '13 at 12:29
yep, all primitives have initial values – Maxim Shoustin Sep 01 '13 at 12:30
here we are, and I think that those values are passed are embedded in the bytecode (compilation time) as it happens for a final variable. – Rollerball Sep 01 '13 at 12:32
because in "non initializing" code (in the main method for instance) primitive variables do need to be initialized – Rollerball Sep 01 '13 at 12:33

Static variables initialization process

2 Answers2