Object Construction Flow

[M Sharma]

PROGRAM 1
----------

class C
{
C()
{
System.out.print("C");
}
}
class A 
{
        C c = new C();
A() 
{
this("A"); 
System.out.print("A"); 
}
        A(String s) 
{
 System.out.print(s); 
}
}
class B extends A 
{
        B() 
{ 
super("B"); 
System.out.print("B"); 
}
        public static void main(String[] args) 
{
                new B();
        }
}
[B]This program gives the output : CBB[/B]

-----------------------------------------------
PROGRAM 2
-----------------------------------------------

class X 
{
Y b = new Y();
        X() 
{ 
System.out.print("X"); 
}
}
class Y 
{
        Y() 
{ 
System.out.print("Y"); 
}
}
public class Z extends X 
{
        Y y = new Y();
        Z() 
{ 
System.out.print("Z"); 
}
        public static void main(String[] args) 
{
                new Z();
        }
}
[B]This program gives the output YXYZ[/B]

Flow of program 1
----------------------
When the object of class b is constructed new B();
it calls the default constructor of class B which in-turn
calls superclass constructor but before the superclass constructor
executes it constructs an object of class C which calls default
constructor of class C printing "C"
The constructor call returns to constructor of class A and "B" gets printed and then call returns back to constructor of class B where again b"B" gets printed
So the output is CBB
Flow of program 2
----------------------
BUT IN CASE OF PROGRAM 2 the behaviour or i should say the flow seems to be bit different.
When the object of class Z is constructed...
First member varaible of class Z is initialized i.e... Y y = new Y();
So this leads to calling the constructor of class Y which in turn prints Y on the screen
Then the control return back to the constructor of class Z where an implicit call to the default constructor of superclass X gets called.
Now here the diffence is that in this case FIRST X IS PRINTED FROM THE CONSTRUCTOR OF CLASS X AND THEN THE CLASS MEMBER Y b = new Y(); gets executed
So in program 1 first the class member is initialized and then the constructor is executed but in case of program 2 first the constuctor is executed and then the class level member is initialized ...... why
Why is that difference in execution....... pls help !!!
[ September 23, 2003: Message edited by: Manish Sachdev ]

[Uma Balu]

Hi,

When the object of class Z is constructed...
First member varaible of class Z is initialized i.e... [b]Y y = new Y();

I think, it will first initialize the super class members, call super class constructor only then, initialize Z members and call its constructor.
Have a look at this,

 
class X 
{
   Y b = new Y("member in X");
   X() 
   { 
     System.out.println("X"); 
   }
}
class Y 
{
   Y(String s) 
   { 
     System.out.println("Y " + s); 
   }
}
public class Z extends X 
{
   Y y = new Y("member in Z");
   Z() 
   { 
     System.out.println("Z"); 
   }
   public static void main(String[] args) 
   {
     new Z();
   }
}
 

Rgds,
Uma...

[M Sharma]

Thanx a ton Uma. Your explaination made the flow very clear to me.
Thank you very much !!!