How a non-static init blocks can assign values to a static instance variables?

[Joey Sanchez]

 public class Main{
     static int a;
     static int b;
 
     static{
         a=1;
         b=2;
     }
     {
         a=3;
         b=4;
     }
     public static void main(String[] args) {
         Main main = new Main();
         System.out.println(a+""+b);
     }
 }

The output is 34

[Tommy Delson]

See my comments below...

 public class Main{
     static int a; //Static members - all instance or object shares the same static members which are a & b
     static int b; 

     static{
         a=1; //Initialization of static member a & b in a static initialized block
         b=2;
     }
     {
         a=3; //No problem - Initialization of static member a & b in an instance initialized block
         b=4; //You can assign a value to a static members a & b in an instance initialized block or method.
                 //VS - you can not access an instance members from a static methods directly in main() unless with a reference variable prefix with the dot. 
                 //Moreover, you can not initialize or access instance members from static methods or initialized block.
                 //Try it in coding and see how they behave...I'll follow up with update code to confirm the concept stated above.
     }
     public static void main(String[] args) {
         Main main = new Main();
         System.out.println(a+""+b); //a & b are static members so they're not required a reference variable to access it. 

     }
 }

The output is 34

Simply the new values over wrote the old values since all instance share the same static members a & b.

If you still doubting go back to Chapter 1 on Declarations and Access Control and reread to reinforce your knowledge and clear your doubt.

Hope it help...

[Joey Sanchez]

I had already looked up in the book but I wasn't capable of finding anything about in it.

Chapter 1, explain slightly about static
Chapter 3, Neither Initialization Blocks example nor Variable Scope cover this case

class SmallInit{
   static int x;
   int y;
   static{ x = 7;}   // static init block
   { y = 8;}            // instance init block
}

Thanks you very much, you help me a lot.

[Tommy Delson]

You're welcome, I'm glad that you have grabbed the concept.

The book doesn't provide all the information in details, we need to step out of the box and dig deeper...

[Yui Huang]

I have seen some mock exam question about this. The following code would throw java.lang.NullPointerException.

public class Init {
   static int[] a;
   {a[0]=2;}
   public static void main (String[] arg)
  {
	Init i = new Init();
  }
}

And the following throws java.lang.ExceptionInInitializerError when the class is loaded.

public class Init {
   static int[] a;
   static {a[0]=2;}
   public static void main (String[] arg)
  {
	Init i = new Init();
  }
}

[Martin Petransky]

FYI.. in both cases you are trying to change state of a non-existing object. As the array was never created (initialized if you like) you are actually saying:

null[0] = 2;