Java Constructors

[Sidharth Pallai]

Why the code doesn't get compiled? Can anyone explain me.

class Y{
Y(String s){}
}
class Z extends Y{
Z(){}
}

[Madhu Sagar]

You are creating a sub class by using superclass.
Inheritance rule is that

The superclass should have a default constructor then only we can create a sub class for it.

So now compile this.
class Y{
Y(){}
Y(String s){}

}
class Z extends Y{
Z(){}
}

[Sidharth Pallai]

Hi Madhu,
Why should i need to declare a default constructor in the super, if there exist a implicit default constructor declaration in the super.

[Sagar Rohankar]

Why should i need to declare a default constructor in the super, if there exist a implicit default constructor declaration in the super.

No when you declare your own constructor in any class , the compiler no longer provide its own default constructor !!

If thats clear,

Now see this code :

  class Y
{
Y(String s){}
}
class Z extends Y
{
Z(){[B]super("now-this-run");[/B]}
}  

when you call the constructor of sub-class , the compiler execute the firs line as the call to super class constructor ..

in your original code , you are not providing the default constructor like this :

  class Y
{
  Y(){}
} 

and when compiler sees this code :

  class Z extends Y{
Z(){}
}  

it replace first line of the subclass constructor like this :

 class Z extends Y{
Z(){[B]super(); // compiler insert it , for you !![/B]}
}   

so there is no default constructor for super class , hence compiler raise an error !!

Hope this help !

[Sidharth Pallai]

Thanks a lot Sagar. I understood the stuff hence corrected myself.

[Rob Spoor]

Originally posted by Sagar Rohankar:
No when you declare your own constructor in any class , the compiler no longer provide its own default constructor !!

Ding ding ding! We have a winner!

The compiler only inserts a default constructor if you don't provide any constructors at all. And all that constructor does is call its parent's class' constructor without parameters, like so:

public class X {}
 
// similar to
 
public class X
{
    public X() { super(); }
}