Inheritance Question

[Swati Singhal]

Hi,
I am a lil confused with this question. Appreciate if somebody could clarify.
Which of the following constructors must exist in Parent class?

  
public class Child extends Parent{
public Child(int i){ }
public Child(int i, int j){ 
super(i,j);
}
public void Child(int i, int j, int k) { }
}
a. public Parent(){}  
b. public Parent(int i){}  
c. public Parent(int i, int j){}  
d. public Parent(int i, int j, int k){}  

I thought the answer would be c, but it is both a & c.

[Gian Franco]

Hi Swati,

  
public class Child extends Parent{
public Child(int i){ }
public Child(int i, int j){ 
super(i,j);
}
public void Child(int i, int j, int k) { }
}
a. public Parent(){}  
b. public Parent(int i){}  
c. public Parent(int i, int j){}  
d. public Parent(int i, int j, int k){}  

Given the two constructor in the Child class both
a and c must be added, because for the latter an
explicit call is done with super(i,j). The former
is called by the default no-argument constructor
that JVM inserts in the Child(int i) constructor
as the first line of code.
You have to add the no-argument constructor
actually because you just had to add
public Parent(int i, int j). Normally the JVM
creates a default no-argument constructor
for you, but with one condition: there must NOT
be any other constructor.
HTH,
Gian Franco

[Richard Quist]

Originally posted by Swati Singhal:
Hi,
I am a lil confused with this question. Appreciate if somebody could clarify.
Which of the following constructors must exist in Parent class?

  
public class Child extends Parent{
public Child(int i){ }
public Child(int i, int j){ 
super(i,j);
}
public void Child(int i, int j, int k) { }
}
a. public Parent(){}  
b. public Parent(int i){}  
c. public Parent(int i, int j){}  
d. public Parent(int i, int j, int k){}  

I thought the answer would be c, but it is both a & c.

You need both because the Child constructor public Child(int i){ } has an implicit call to the compiler-provided, default, no-arg constructor of the parent (answer A).
Answer C is needed because of the explicit call to super(i,j); in the Child constructor public Child(int i, int j)
Also note that the method public void Child(int i, int j, int k) is NOT a constructor - since constructors don't have return types (and this method is declared to return void)
Hope this helps

[Vicken Karaoghlanian]

The compiler will do an implicit call to the no-args constructor in the super class parent.

class Parent {
Parent(int i) {
}
}
class Child extends Parent {
Child() {
// Here there is an implicit call to Parent() -- [I]super()[/I]
}
}

That is why this code doesn't compile, to allow the compilation, you must provide a no argument constructor in the super class, see code:

class Parent {
Parent() {
}
Parent(int i) {
}
}
class Child extends Parent {
Child() {
}
}