About Question enthuware.ocajp.i.v7.2.1244 :

[admin]

The cast at line 2 is needed means you need the cast to be able to assign a to b. i.e. b = a; will not work. You need to add the cast i.e. b = (B[]) a;

HTH,
Paul.

[swapna@06]

The cast at line 2 is needed means you need the cast to be able to assign a to b. i.e. b = a; will not work. You need to add the cast i.e. b = (B[]) a;

HTH,
Paul.

Oh yeah, i got it. I was seeing the whole statement every time i read cast at line 2 is needed.
Thanks Paul.

[German]

Hi,

looks like 4th answer is correct as well - "The program will compile and run if the (B[ ] ) cast in the line 2 and the whole line 3 is removed." If yes, how to determine in such questions what option should be selected?

[admin]

Did you read the explanation? It explains exactly why the cast at line //2 is needed.

[German]

Yes, I read the explanation, it's clear, no questions here.
There was another question, - "if the cast at line //2" is correct statement from the first part of 4th answer, then the second part of 4th answer is "if the whole line 3 is removed" which is also looks like correct. Thus, 4th answer fit for this question as well as 3 and 5 or not?

[admin]

May be I am not able to understand your question. Cast at //2 is required. You can't remove that cast. So "if the cast at line //2 is removed" is not correct. No matter what you do with //3.

[German]

Sorry for delay,

looks like I didn't understand correctly option 4, in particular, that "is removed" applied to both "the whole line 3" and "Cast at //2". I thought "is removed" applied to "the whole line 3" only and didn't to "Cast at //2".

[lenalena]

Just to confirm, is it true that any super class (class that's higher in the hierarchy) can be cast to any subclass and pass compilation (with ClassCastException potentially at runtime)? Or are there cases where a super to subclass cast will throw a compile time error?

[admin]

Just to confirm, is it true that any super class (class that's higher in the hierarchy) can be cast to any subclass and pass compilation (with ClassCastException potentially at runtime)?

Yes, that is correct.

Or are there cases where a super to subclass cast will throw a compile time error?

Nope. Never.

Just keep in mind that it is not the object that you cast. Objects don't change their types. They remain what they are. It is the reference variables that you cast. You change the type of a reference variable when you cast it to another type. For example, SubKlass sk = (Subclass) k; Here, you are temporarily changing the type of k from Klass to SubKlass and assigning it to sk. The actual object pointed to by k will remain whatever type it was before.

HTH,
Paul.

[lenalena]

Just to confirm, is it true that any super class (class that's higher in the hierarchy) can be cast to any subclass and pass compilation (with ClassCastException potentially at runtime)?

Yes, that is correct.

Or are there cases where a super to subclass cast will throw a compile time error?

Nope. Never.

Just keep in mind that it is not the object that you cast. Objects don't change their types. They remain what they are. It is the reference variables that you cast. You change the type of a reference variable when you cast it to another type. For example, SubKlass sk = (Subclass) k; Here, you are temporarily changing the type of k from Klass to SubKlass and assigning it to sk. The actual object pointed to by k will remain whatever type it was before.

HTH,
Paul.

It's clear now, thank you.

[Javier]

Hi Admin!

I want to ask just one question:

a1=a; // the reference a1 now is pointing to the a object, right?
a=b;// the reference a now is pointing to the b object, right?

I would like to know after these to statements, why is not pointing a1 to the b object??
Why is there not conexion between a1 and b??

It is just because the references are pointing to objects and not to other references??

Thank you so much!!

[admin]

You have a fundamental problem in your understanding. a, a1, and b are not objects. They are references pointing to objects. (References don't point to other references. A reference can only point to an object or nothing i.e. null).

It's like this - let's say you have a TV (call it, TV1) and a remote (call it, a) that is pointing to that TV. The TV is the object and the remote is a reference pointing to that object.

You now buy another remote (a1) and set this remote to point to the same TV as your previous remote by doing a1 = a. Now you have two remotes pointing to the same TV i.e. TV1.

Now, you change your first remote(a) to point to another TV (TV2) that is pointed to by some other remote b by doing a = b. So now, you have remote b as well as remote a pointing to TV2.

But what happens to the remote a1? Nothing. The second remote will still be pointing to the same TV1 that it was set to point to earlier. Setting a to point to another remote doesn't affect what b is pointing to.

[Sergey]

it seems it might be helpful to you if you read about the difference between the type of a variable and the type of the object pointed to by a variable.

A a = new B();
A - type of a variable
B - type of the object

Am i correct?

[admin]

it seems it might be helpful to you if you read about the difference between the type of a variable and the type of the object pointed to by a variable.

A a = new B();
A - type of a variable
B - type of the object

Am i correct?

Correct.

[crazymind]

a1 fails at run time since a1 hold an instance of A not B ? but a hold an instance of B therefore it is valid.

[Dreamweaver]

can it be corrected like this? (I try it and run)

The line //1 will be allowed during compilation, since assignment is done from a subclass reference to a superclass reference.

The line //1 will be allowed during compilation and run time, since assignment is done from a subclass reference to a superclass reference.

[admin]

Yes, it will work even at run time.