Please Explain working of this program.

[Prateek Singla]

Hi, I am new to this form. I came up across this code in Head First Java, if someone could explain my small doubt, I would really appreciate it.
Code goes like this-

class StaticSuper {

static{

System.out.println("super static block");

}

StaticSuper(){

System.out.println("super constructor");

}

 

}

 

public class StaticTests extends StaticSuper{

static int rand;

static {

rand = (int)(Math.random()*6);

System.out.println("static block "+rand);

}

StaticTests(){

System.out.println("constructor");

}

public static void main(String[] varargs){

System.out.println("in main");

StaticTests st = new StaticTests();

}

}

I got output as-
super static block
static block 10
in main
super constructor
constructor

What I could not understand is.., why there is "super constructor" in second last line. As the code is neither called explicitly or implicitly...

[Phil English]

The no argument superclass constructor is called implicitly from the subclass constructor where you do not instruct otherwise using this() or super(args)

Take a look at the note towards the bottom of this page

[Prateek Singla]

So.. Is it like.. first statics, gets initialized irrespective of the fact from where they come from in inheritance tree and then, main method code starts and then before any object is instationized the no argument constructor runs...

[Winston Gutkowski]

Prateek Singla wrote:So.. Is it like.. first statics, gets initialized irrespective of the fact from where they come from in inheritance tree and then, main method code starts and then before any object is instationized the no argument constructor runs...

If you don't call another constructor yourself; and that's the important part. If you explicitly call a superclass contructor yourself then that will be run instead - and before the constructor for the subclass (which is why it MUST be the first statement in your constructor).

As to instantiation, it gets a bit tricky (at least it used to be; it's possible that the various changes to the memory model have simplified things), but in general you can take it as read that if you write new Whatever(), the object is not fully instantiated until the constructor for Whatever has completed (and don't forget that it may have to call superclass constructors first).

To be honest, the nuts and bolts are probably best left to memory architects, who are a rare breed of geek. If you concentrate on writing clear, uncomplicated, dumb code, the chances are you will never need to worry about it.

Winston