Clarification request on question ID enthuware.ocajp.i.v7.2.983

[aarulselvam]

Option 2 says:

Code: Select all

Object o = a; Runnable r = (Runnable) o;

Here you are explicitly telling the compiler that o refers to an object that is Runnable.

Option 5 says:

Code: Select all

Object o = b; Runnable r = (Runnable) b;

Since b is declared of a type that indirectly implements Runnable, the compiler can figure out that b will always point to an object that is assignable to a Runnable. Therefore, explicit cast is not required here. It will still work fine with the explicit cast though.

My Confusion:

While Option 5 says that the compiler can figure out that b will always point to an object of Runnable. I was wondering why not the same applicable for Option 2? Why does Option 2 require explicit casting?

I am not an experienced JAVA developer and trying to learn. May be I am missing something here. Appreciate if any one can assist me to understand.

With regards,
Arul

[admin]

Let me ask you a question. Can variable o refer to an object that doesn't implement Runnable?

[aarulselvam]

In the question, variable o refer to the object that implement Runnable. Let me reproduce the question here:

Consider the following classes:

class A implements Runnable{ ...}
class B extends A implements Observer { ...}

(Assume that Observer has no relation to Runnable.) and the declarations :   

A a = new A() ;   
B b = new B();

Which of the following Java code fragments will compile and execute without throwing exceptions?

Option 2:

Code: Select all

Object o = a; Runnable r = (Runnable) o;

Option 5:

Code: Select all

Object o = b; Runnable r = (Runnable) b;

Here variable o refer to an object a which implement Runnable.

My query is that casting (Runnable) required for option 2?

[admin]

Yes, it is required. I asked you the question to make it easy for you to learn. But anyway, cast is required because it is possible for o to point to any object so the compiler needs to be assured that it will point to a Runnable at run time.

[maxmurks]

The thing is: The explanation for Option 5 says, that the explicit cast is not required in option 5. Which I'm quite sure is wrong.

[admin]

The thing is: The explanation for Option 5 says, that the explicit cast is not required in option 5. Which I'm quite sure is wrong.

Did you try compiling the code?

[maxmurks]

Yes, I tried compiling it. Turns out I should try reading the options correctly first.

I read (and therefore typed) Option 5 like

Object o = b; Runnable r = (Runnable) o;

instead of

Object o = b; Runnable r = (Runnable) b;

So kind of exactly like Option2.

Sorry about that.

[Sergey]

Although, at run time, o does point to a Runnable, the compiler doesn't know about it

how could we know that o does point to a Runnable?

[admin]

Although, at run time, o does point to a Runnable, the compiler doesn't know about it

how could we know that o does point to a Runnable?

Because of the statement Object o = a;
and the fact that a points to an object of class A, which implements Runnable.

[Sergey]

Yes, I tried compiling it. Turns out I should try reading the options correctly first.

I read (and therefore typed) Option 5 like

Object o = b; Runnable r = (Runnable) o;

instead of

Object o = b; Runnable r = (Runnable) b;

So kind of exactly like Option2.

Sorry about that.

I did the same mistake. But i think, the question should be "very hard" rather than "easy"

[flex567]

Currently answer 4 is phrased like this

Code: Select all

Object o = b; Runnable r = (Runnable) b;

Could answer 4 be phrased like this:

Code: Select all

Runnable r = (Runnable) b;

Because unlike previous answers these 2 statements are not related.

[admin]

No, the option is fine as it is. You may see options that don't make much sense in practice but that test your Java knowledge nevertheless.

[Mikhail Volkov]

Hello! Please, tell me - did I paint correct schema for to solve this question? http://joxi.ru/52aEWXTEa8G720
I think, that when I have correct schema it is easier to find correct answer.
But I'm not sure...

In the fact I find very strange - make Interface as Type of reference... For Why?

[admin]

Yes, your class diagram is correct.