constructors from exam cram

[Krishna Priya]

Hi All
Could anyone of you please give me the correct answer for the folowing code:
Given the following class definition:
1. public class deriveddemo extends demo {
2. int M,N,L;
3. public deriveddemo(int x,int y) {
4. M=x;N=y;
5. }
6.public deriveddemo(int x) {
7. super(x);
8. }
9. }

which of the following constructor signatures must exist
in the demo class for deriveddemo to compile correctly?
a.public Demo(int a,int b)
b.public Demo(int c)
c.public Demo()
I think the correct answer is only b.But the correct answer
is b and c.Howcome c is correct ans?
Thanks and Regards
Priya

[Jane Griscti]

Hi Krishna,
When you create an object the Runtime goes through a series of initialization steps. One of which is to invoke super(). If the base class does not have a default constructor the initialization fails. (see JLS§12.5).
Hope that helps.
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Jane

[Varsha Dighe]

Jane,
Does that mean one has to have a explicitely declared constructor (with no arguments) defined in a class, if we wish to extend the class ?
-Varsha

[Paul Anilprem]

This may help you :
http://enthuware.com/jqplus/FAJQuestions.html#DEFAULTCONSTRUCTOR
and
http://enthuware.com/jqplus/FAJQuestions.html#SUPERCONSTRUCTOR
-Paul.

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[This message has been edited by Paul Anil (edited October 21, 2000).]

[vijay krishna]

Varsha,
Any constructor in sub class 1st calls default constructor super() (if there is no "super" or "this" specified).
If there is such a constructor in SUB CLASS then "Yes", you have to have one default constructor in super class.
OTHERWISE You dont have to have default constructor in super calss.
Is that clear?

[Varsha Dighe]

Thank you very much Paul and Vijay. I understood the concept now.
Varsha