Mock question on polymorphism

[Jean Vilart]

Hi,

I found this question in a mock exam and I can't find the answer:

10. interface A { public int getValue() }
11. class B implements A {
12. public int getValue() { return 1; }
13. }
14. class C extends B {
15. // insert code here
16. }
Which three code fragments, inserted individually at line 15, make use
of polymorphism? (Choose three.)

A. public void add(C c) { c.getValue(); }
B. public void add(B b) { b.getValue(); }
C. public void add(A a) { a.getValue(); }
D. public void add(A a, B b) { a.getValue(); }
E. public void add(C c1, C c2) { c1.getValue(); }

According to me, all answers make use of polymorphism. But, we have to choose three. So which three and WHY???

Thanks in advance.
Jean

[Ameen khan]

I think the answer is B,C,D
because Interface A and Class B references can hold object of Class C.

[Akhilesh Trivedi]

How about A,B and E? :roll:

[Jean Vilart]

Dear Ameen Khan, thank you for your answer but I don't understand it.

Dear Akhilesh Trivedi, I am as most interested in the right answer as in its explanation. Could you please elaborate why you think those are the right answers.

Thanks

[Ameen khan]

Options A and E have Reference of type C. it means the only object of type C can be passed in method "add" not object of type B.
So if Reference and object belongs to same type there is no polymorphism.

[Jean Vilart]

aaaaaaaaah ok, now it makes sense !
Thanks a lot for the clear explanation.

[Akhilesh Trivedi]

It is really catchy to have getValue() inside another method add() than to have it simply redefined/overridden as it is in C. Why? let us see...

    
interface A
{
public int getValue();
}
class B implements A
{
public int getValue()
{
return 1;
}
}
class C extends B
{
 // insert code here, let me insert the first option A
 public void add(C c)
 {
  c.getValue();
  }
 public static void main(String args[])
 {
            // my ultimate aim here is to avoid polymorphism by all means..
 }
 }

I go for

  
C c = new C();
c.add(c);

and I am done! !!
first option is out, there is no polymorphism here... only inheritance

Option B,

    
interface A
{
public int getValue();
}
class B implements A
{
public int getValue()
{
return 1;
}
}
class C extends B
{
 // insert code here
 public void add(B b)
 {
  b.getValue();
  }
 public static void main(String argsp[])
 {
 B b = new B();
 C c = new C();
 c.add(b);
 }
 }

Any polymorphism in above case?? All is right at its place I believe.

Option C

  
interface A
{
public int getValue();
}
class B implements A
{
public int getValue()
{
return 1;
}
}
class C extends B
{
 // insert code here
 public void add(A a)
 {
  a.getValue();
  }
 public static void main(String argsp[])
 {
 //whatever I do I am to store either 
 //B or C's object in the parameter a
 //and that is polymorphism.
 }
 }
 

Option D will also be counted on same basis.

Option E same as one have three C objects c1, c2 an c.add(c1,c2) in main.


I feel only C and D are ok... any suggestions? :roll:
[ August 07, 2007: Message edited by: Akhilesh Trivedi ]

[V Gala]

hi Akhilesh
In option 2
if we do c.add(c) then also there is polymorphism
I think correct option are B,C and D

[Akhilesh Trivedi]

Originally posted by Viral Gala:
hi Akhilesh
In option 2
if we do c.add(c) then also there is polymorphism
I think correct option are B,C and D

Yes viral! But the main() method part was nowhere supplied in the question, I put it to explain that there is a possibility that polymorphism can be bypassed. It was nowhere mentioned that I should use same object for calling the method and for passing as parameter.