Static variable initialization in Constructor java

Wrong - do it like this:

public class A {
       private static B b = null;

       static {
             b = new B();
       }

       public A() {
       }

       public static void main(String[] args) {
           b.func();
       }
    }

I think your question has two parts:

1) Why the value of static variable b was not initialized and though the value was initialized in the constructor?

Ans: First thing is that no constructor is called before main(). Constructors are called in main(). Whenever in main() you use new as :

public static void main(String args[]){
MyClass myclass= new MyClass()
}

then only constructor is called.

In your code static variable b was not initialized becoz u are initializing it in constructor A() but this constructor has never been called. You can call A() constructor in your code as:

public static void main(String[] args) {
          A a=new A(); // constructor gets called here.
           b.func();
       }

2) what is proper way of initializing a static variable?

The right way to initialize a static variable is to use static Initialization Blocks rather than to initialize them in constructors as shown in answer given by duffymo above.

static {
  b = new B();
 }

You can also use:

public class A {
       private static B b = new B();

       public A() {
       }

       public static void main(String[] args) {
           b.func();
       }
    }

Your question asked to help you "understand why" the behaviour was as it was. The reason is that the constructor for Class A is not called when you invoke a static method main().

If you were to instantiate an object of type A then the constructor would be called and you reference to B initialised.

I would recomend always instantiating a Class before executing it from static void main() as a matter of good practise. If you start using a frameworks (e.g. Spring), you are better off creating instances of your classes than just writing static methods which is akin to writing procedural code.

A solution without resorting to static initialisers and following the principle outline here, is...

public class A {
    private static B b = null;

    public A() {
        if (b == null)
          b = new B();
    }

    public static void main(String[] args) {
        A a = new A();
        a.callFunc();
    }

    public void callFunc() {
        b.func();
    }
}

As you can see you need a way to reference the b.func() method so I have added a a.callFunc() for this reason