Co variant returns

[harmeet saini]

Hi follwing is the problem from certPal

Given this code snippet, what can be inserted in the fragment ?

   
class Marmaduke
{}
public class Cert extends Marmaduke 
{
    public static void main(String [] args)
    {}
    public Marmaduke go()
    {
        // Insert code fragment here
    }
}
 

And the otipns are
1)return new Object();
2)return this;
3)return new Marmaduke();
4)return null;
5)return"";

the correct options are 2,3,4 and the expalnation given is "Co variant returns are allowed in java language as of JDK1.5

I am not able to understand the explanation and also why option 1 is not correct

Thanks in advance

[Deepak Bala]

You cannot return an Object when the expected return type is a subclass of Object.

As for the co-variant return type, of what class type is the instance 'this' ? What is the difference between 'this' and Marmaduke ?

[Ankit Garg]

I don't think this has anything to do with co-variant return types. Its just polymorphism. The return type of go method is Marmaduke, so you cannot return an Object from it. I would be like doing this

Marmaduke m = new Object();  //error

(PS: The spelling of otipns is great )

[Deepak Bala]

I don't think this has anything to do with co-variant return types. Its just polymorphism

Whoops. That is indeed correct. Will make a minor change to the answer's explanation

[Deepak Bala]

oh and thanks