Consturctor Invocation,

[Ritu Kapoor]

Please look at the following code. The output is YXYZ. But as per my knowledge, instance variables are created before constructor is called. So when Z() is invoked, a variable of Y is created and in return its constructor is called. Now when constructor of Z() is called, it first creates the parent class i.e X now before invoking its constructor it should create object of Y ie line number 3. and finally constructor of class x and hence z should be invoked. so the result should be YYXZ. Can anyone please explain me.


code :

    
 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();
            }
}

[ December 26, 2006: Message edited by: Ritu Kapoor ]

[Barry Gaunt]

Please edit your post and put

tags around your program. That will preserve your code formatting and make it more readable.

[Chandra shekar M]

hi

The java compiler will interpret the code you have writen as below

class X {
Y b;
X(){
b = new Y();
System.out.print("X");
}
}

class Y {
Y(){
System.out.print("Y");
}
}

public class Z extends X {


Y y;
Z(){
y = new Y();
System.out.print("Z");
}
public static void main(String[] args){
new Z();
}}


so the output will yxyz not onley with respect to object creation but even with respect to assignments say if we have code like

public class edf{

int i=10;
int j;
String str=new String();
edf(){}
.....
}


the compiler will consider the above code AS


public class edf{

int i;
int j;
String str;
edf(){
i=10;
str=new String();
}
.....
}

Observe carefully that the order we wrote the code that is i,j, and str are maintained same in the constructor but the code at class level will be moved in to constructor only when we have definition if only declared then compiler will not move it inside in this case see int j it is not moved inside the constructor

Thanks

[Sandeep Rana]

Hi Ritu,
You are corrct that instance variable are created before constructor is called. But the way you described is wrong.
In your code when Z is created... Now before this x should be created. Now your rule (instance variable must be created first), so Y is the output then finally X is printed. Now control come back to class Z again same rule (instance variable must be created firstf) so Y is printed and again finally Z is printed.
This you can check it by commenting intance variable one at a time. Hope this might be useful to you

Sandeep Rana

[av bharathiraja]

Sandip,

Still im not clear can you just point out which Y is printed from which class(X or Z). As per my understanding it should be like the foll.

1. z() invokes y() [Prints Y]
2. then x() ---> x invokes y() [Prints Y]
x() [prints x]
3. then come back to z to print z

[Henry Wong]

Originally posted by av bharathiraja:
Sandip,

Still im not clear can you just point out which Y is printed from which class(X or Z). As per my understanding it should be like the foll.

1. z() invokes y() [Prints Y]
2. then x() ---> x invokes y() [Prints Y]
x() [prints x]
3. then come back to z to print z

1. z() constructor calls it super constructor x()
2. x() constructor instantiates it y()
3. y() prints Y
4. x() constructor then prints X
5. z() constructor then instantiates its y()
6. y() prints Y
7. z() constructor than prints Z

Henry

[donald rieck]

Wow, the Z() will instantiate(new) all of its class classes before it completes its own constructor.

I did not know this.

[Ritu Kapoor]

Thanks to all of you especially Henry Wong.