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constructors from exam cram

Krishna Priya
Greenhorn

Joined: Jun 08, 2000
Posts: 5
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
Ranch Hand

Joined: Aug 30, 2000
Posts: 3141
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.
------------------
Jane


Jane Griscti
SCJP, Co-author Mike Meyers' Java 2 Certification Passport
Varsha Dighe
Ranch Hand

Joined: Oct 14, 2000
Posts: 32
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
Enthuware Software Support
Ranch Hand

Joined: Sep 23, 2000
Posts: 3336
    
    8
This may help you :
http://enthuware.com/jqplus/FAJQuestions.html#DEFAULTCONSTRUCTOR
and
http://enthuware.com/jqplus/FAJQuestions.html#SUPERCONSTRUCTOR
-Paul.

------------------
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(Now Revised for the new Pattern) www.enthuware.com/jqplus
[This message has been edited by Paul Anil (edited October 21, 2000).]


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vijay krishna
Greenhorn

Joined: Oct 17, 2000
Posts: 8
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
Ranch Hand

Joined: Oct 14, 2000
Posts: 32
Thank you very much Paul and Vijay. I understood the concept now.
Varsha
 
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