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Why such behaviour?

3 replies on 1 page. Most recent reply: Mar 2, 2006 7:51 PM by Sanjay Perera

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Sanjay Perera

Posts: 42
Nickname: angel6
Registered: Apr, 2004

Why such behaviour?

Posted: Mar 1, 2006 7:52 PM

Parent.java
-------------
class Parent {
int i = 111;
Parent() {
this.returnVal();
System.out.println("SUPER:NO ARG");
}

public void returnVal(){
System.out.println(i);
System.out.println("Super method");
}
}

Child.java
------------
class Child extends Parent {
int i=55;
Child(){
this.returnVal();
System.out.println("Child Empty construct");
}

public void returnVal(){
System.out.println(i);
System.out.println("Child method");
}
public static void main (String args[]){
Child df = new Child();
}
}

Question A
-------------
When running the Child program the out put will be...

0
Child method
SUPER:NO ARG
55
Child method
Childt Empty construct

Why the child method was called even when the Parent's contructor was calling the Parent's returnVal() method? Please explain the concept and the reason.
Question B
-------------
If it called child’s returnVal() then why it didn’t print 55 at first place (Child’s i). From where it got 0? Even the Parent class’s i is initialized to 111. Please explain the concept and the reason.

Question C
------------
How to make the method returnVal() of Parent class to be called from the Parent’s constructor using above two programs. Without making the Parent’s returnVal method private.

Question D
-------------
Does anyone know any online tutorial which teaches this behaviour?

Matthias Neumair

Posts: 660
Nickname: neumi
Registered: Sep, 2003

Re: Why such behaviour?

Posted: Mar 2, 2006 2:36 AM

repost your code using the java tags as shown on the right of the input field.

Matthias Neumair

Posts: 660
Nickname: neumi
Registered: Sep, 2003

Re: Why such behaviour?

Posted: Mar 2, 2006 6:29 AM

Ok, here is what happens.

Call of Child constructor
1. Child's class variables are declared
2. Child constructor calls the constructor of it's parent class
2.1 Parent's class variables are declared
2.2 Parent's constructor calls the constructor of it's parent class, which is Object
2.2.1 //I don't care what happens in Object
2.3 Parent's class variables are initialized
2.4 Parent's constructor is executed
2.4.1 Child's returnVal overrides parent's method, so it is called instead of Parent's returnVal. The class variables in Child are not yet inizialized, just declared, so the value of i is default = 0
3. Child's class variables are initialized
4. Child's returnVal is called, returns 55

Sanjay Perera

Posts: 42
Nickname: angel6
Registered: Apr, 2004

Re: Why such behaviour?

Posted: Mar 2, 2006 7:51 PM

Thanks, sorry for the not formmating it!!

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