Enthuware

Option 2 says: Here you are explicitly telling the compiler that o refers to an object that is Runnable.

Option 5 says: 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

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

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: Option 5: Here variable o refer to an object a which implement Runnable.

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

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.

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.

maxmurks wrote: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?

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.

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?

Sergey wrote:

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.

maxmurks wrote: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"

Currently answer 4 is phrased like this Could answer 4 be phrased like this: Because unlike previous answers these 2 statements are not related.

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.

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?

Yes, your class diagram is correct.