Why java static variable is not updating?

Question

I have a Class called Color in which there are three static Objects (instantiating with the same class itself) and a int type (called i) variable. When I Run the Class, the 'i' variable is increments in the Constructor, but it is not persisting in memory , please explain this below code

package test;
public class Color
{
     public static  Color RED = new Color();
     public static final Color BLUE = new Color();
     public static final Color GREEN = new Color();
     static int i=0;

     Color(){
         System.out.println("Incrementing 'i'  here  "+(++i));
     }

     public static void main(String[] args) {
         System.out.println("The 'i' variable is not incremented here, it still shows '0' ,  'i' is: "+ Color.i );  // line 14 
        Color x = new Color();
        System.out.println(x.i);
    }
}

The out is as follow:

Incrementing 'i'  here  1
Incrementing 'i'  here  2
Incrementing 'i'  here  3
The 'i' variable is not incremented here, it still shows '0' ,  'i' is: 0
Incrementing 'i'  here  1
1

Answer 1

When a class is loaded and linked, its static fields are all initialized to their default values. Once that is done, the static field initializers are executed in the order they appear in the file, thus completing the initialization of the class. All this happens before any code defined in that class executes. So what's happening here is:

1. RED, GREEN, BLUE, and i are initialized to their default values (null for the Color fields and 0 for i). Note that this is independent of any initializers.
2. The initializer for field RED (RED = new Color()) executes. As a side effect, i is incremented to 1.
3. The initializer for field BLUE executes and i is incremented to 2.
4. The initializer for field GREEN executes and i is incremented to 3.
5. The initializer for the static field i executes and i is assigned the value 0.
6. The main() method executes and produces the results consistent with i being 0 when main() begins execution.

You can read the gory details of all this in Chapter 12 of the Java Language Specification (JLS) and Chapter 5 of the Java Virtual Machine Specification.

You can get the output you expected simply by move the declaration of i to be ahead of the Color fields:

 static int i=0;
 public static  Color RED = new Color();
 public static final Color BLUE = new Color();
 public static final Color GREEN = new Color();

Then the output will be:

Incrementing 'i' here 1
Incrementing 'i' here 2
Incrementing 'i' here 3
The 'i' variable is not incremented here, it still shows '0' , 'i' is: 3
Incrementing 'i' here 4
4

Note that the final modifiers have no effect on the order of initialization here because GREEN and BLUE are not "constant variables" according to the definition of that term in the JLS. A constant variable (an unfortunate pairing of words) is a primitive or String variable that is both declared final and initialized to a constant expression. In this case, new Color() is not a constant expression and Color is not an appropriate type for a constant variable. See, for instance, §4.12.4 of the JLS.