Reference Type and Object Type Changing Java

Question

Given:

public class X implements Z {

    public String toString() { return "I am X"; }

    public static void main(String[] args) {
        Y myY = new Y();
        X myX = myY;
        Z myZ = myX;
        System.out.println(myZ);
    }
}

class Y extends X {

    public String toString() { return "I am Y"; }
}

interface Z {}

What is the reference type of myZ and what is the type of the object it references?

A. Reference type is Z; object type is Z.

B. Reference type is Y; object type is Y.

C. Reference type is Z; object type is Y.

D. Reference type is X; object type is Z.

In this situation, I know that the object type is for sure Y, because I can test it with the .getClass() method. But I'm unsure of how to check what the reference type is. I'm assuming it is Z but that assumption is based on gut feeling and not logic.

Can anyone elaborate on what the reference type might be and why?

Thank you.

I would go with C, the object was create with new Y(); and myZ is declared as Z — Aviram Segal
– Aviram Segal, Commented Dec 20, 2012 at 14:07

Sergey Kalinichenko · Accepted Answer · 2012-12-20 14:08:13Z

5

The type of the object reference is defined statically at the point of its declaration:

Z myZ = ...

Therefore, the type of the reference is Z, so "C" should be the right answer.

answered Dec 20, 2012 at 14:08

Sergey Kalinichenko

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2 Comments

Nikolay Kuznetsov Over a year ago

what is the meaning of reference type overall? Why do we need it?

NickL Over a year ago

The reference declares the interface of the object. if type X had methods getA(), getB() and getC(). And type Y only has getA() and getB(), then by changing the interface (reference type) from X to Y, you could only get access to getA() and getB() even though the object is of type X.

Aviram Segal · Accepted Answer · 2012-12-20 14:09:59Z

2

The Object was created with new Y(); so the object type is Y

myZ was declared as Z (Z myZ = ...;) so the reference type is Z

Hence, the right answer is C

answered Dec 20, 2012 at 14:09

Aviram Segal

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Comments

0x6900 · Accepted Answer · 2013-10-06 09:04:32Z

0

The object is referenced to Z type

Z myZ = myX;

but originally it was created as Y type,

Y myY = new Y();

Hence, ans is obviously C.

edited Oct 6, 2013 at 9:04

answered Oct 6, 2013 at 8:01

0x6900

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Comments

thedarkside ofthemoon · Accepted Answer · 2013-12-10 13:33:13Z

0

I know it is confusing because of the wrong answer that is marked as the true one; I have also met it. The truth is as they say, the reference is of type Z and the object type is Y, so C is the correct answer.

answered Dec 10, 2013 at 13:33

thedarkside ofthemoon

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