Objects and casting

[Jason Attin]

Hi guys, I have absolutely no idea what goes on in this code: Glenn, Mitchell. OCAJP Oracle Certified Associate Java SE 8 Programmer Practice Exams (Kindle Locations 11714-11716). Enthuware. Kindle Edition.

Consider the following classes :

interface I{ } 
class A implements I{ } 
class B extends A { } 
class C extends B{ } 
And the following declarations: 
A a = new A(); B b = new B();

Identify options that will compile and run without error. Select 1 option
A. a = (B)( I) b;
B. b = (B)( I) a;
C. a = (I) b;
D. I i = (C) a;

I just have no idea what I've read here. Is there any sort of easy way to get my head around this mess of double casting objects to other references etc? Generally speaking I've noticed that I have quite a few problems with these kind of questions, and I'm not sure how to tackle them really, I usually draw the hierarchy on a piece of paper but still, it's questions like these that really confuse me. How do I know that one or the other compiles and run? Is ther some kind of general rule I can apply? he correct answer is A by the way

[Jason Attin]

To add to the above here is another example on the same note:

Which statements, when inserted in the code below, will cause an exception at run time?

class B {} 
class B1 extends B {} 
class B2 extends B {} 
public class ExtendsTest{ public static void main( String args[]){ 
B b = new B(); 
B1 b1 = new B1(); 
B2 b2 = new B2(); // insert statement here 
} } 

Select 1 option
A.

b = b1; 

B.

b2 = b; 

C.

b1 = (B1) b;

D.

b2 = (B2) b1;

E.

b1 = (B) b1;

I mean, is there a way to be sure which one will cause issues? Like, I know I can do A and B ( B needs an explicit cast, that's obvious) but C,D and E are not so obvious to me.

[Paul Anilprem]

This discussion should be helpful : http://enthuware.com/forum/viewtopic.php?f=2&t=1050

HTH,
Paul.

P.S. Not sure if you are aware of this but just in case - While going through a question on your kindle, you can click on the question id to view a discussion on that question. You may find the existing discussion helpful. If there is no existing discussion on a particular question, you can post your query.

[Jason Attin]

OK thanks, I have bought the book from amazon but I don't have a kindle so I'm browsing it on my laptop.

[Jason Attin]

as a side note, I compiled and run the first example, and it seems that both A and B compile and run with no problem, here is the code:

interface I{ } 
class A implements I{ } 
class B extends A { } 
class C extends B{
public static void main(String[] args) {
		A a = new A();
		B b = new B();
		a = (B)(I) b;
		b = (B)(I) a;
		/* a = (I) b;
		a = (I) b; */
	}
}

[Jeanne Boyarsky]

Jason Attin wrote:OK thanks, I have bought the book from amazon but I don't have a kindle so I'm browsing it on my laptop.

That book is a book of only practice exams. I think Paul is suggesting you buy an actual study guide. Practice questions are intended to supplement a study guide, not replace it. A study guide explains the things you are asking about. For example, Mala Gupta and I are authors of too different study guides for this exam.

[Jason Attin]

I've got your book Jeanne :-)

[Paul Anilprem]

Jason Attin wrote:OK thanks, I have bought the book from amazon but I don't have a kindle so I'm browsing it on my laptop.

The link to forum feature works on browser as well. Just click on the question id to view the appropriate discussion thread for that question.

HTH,
Paul.