Constructor call

[Gunjan Kumar]

Hi

I executed this code


class Just {
int a = 10 ;

Just ( ) {
call ( ) ;
}

void call ( ) {
System . out . print( "a = " + a + " ") ;
}
}

class Q05 extends Just {
int b = 16 ;

Q05 ( ) {
call ( ) ;
}

void call ( ) {
System.out.print( " b = " + b + " " ) ;
}
public static void main ( String args [ ] ) {
new Q05 ( ) ;
}
}

I got b=0 b=16 but i was expecting a=10 b=16 because main method is creating object of subclass so first super class constructor will get called and in turn super class constructor is calling call() method. so it should call call() of just class.why it is calling call() of subclass method,

Please clarify my doubt.

Source:Voodoo mock exam Test3

[Tina Tibrewal]

This is because the object that is created is of the subclass type.
This is what is polymorphism.

[Chandra Bhatt]

Hi Gunjan,

class Just {
int a = 10 ;
 
Just ( ) {
call ( ) ;
}
 
void call ( ) {
System . out . print( "a = " + a + " ") ;
}
}
 
class Q05 extends Just {
int b = 16 ;
 
Q05 ( ) {
call ( ) ;
}
 
void call ( ) {
System.out.print( " b = " + b + " " ) ;
}
public static void main ( String args [ ] ) {
new Q05 ( ) ;
}
}

Q05 overrides the call() method of its parent class Just.
When you create new object of subclass Q05, in its constructor super() is
automatically placed by the compiler.

OK now in the parent class method call() is called, and because the subclass
overrides the call() method, the subclass version of the call() method is called;

Real concern:
See the order of initialization in any book:
Until the parent class completes, member variables of the subclass get their
default value, (default means 0 to int, 0.0 to float, null to a reference variable and so on)

Second output is b=16, and that is explicit value because at this time,
subclass constructor call the call() method, and explicit value of the b is
printed (is has been initialized before the subclass constructor completes)

Please place the code in between

tags.



Regards,
cmbhatt

[Tommaso Nuccio]

Exactly,

if you debug it using breakpoints, the compiler will tell/show you that during the call of the method call() in the superclass constructor the this object is of type Q05.

Good luck.

[Gunjan Kumar]

Thanks to all of you.

I got the point

[Gaurav Pavan Kumar Jain]

HI Folks

In above code

b=16 I am ok with that
but b=0 i cannot understand.