About Question enthuware.ocajp.i.v7.2.983 :

[mmontelatici]

The fifth answer is:

Code: Select all

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

And the explanation is:

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 is will still work fine with the explicit cast though.

But if I try to avoid the explicit cast on NetBeans i get:

incompatible types: Object cannot be converted to Runnable

So what?

Thanks

[admin]

Option 5 and the explanation are correct. You need to post the exact code that you have typed. There must be some difference because of which you are getting this message.

BTW, try to use command line to compile and run code while preparing for the exam. We do not recommend using IDEs for this purpose.

[German]

Hi Pual,

could you please explain why the following option is not valid:

Object o = a; Observer ob = (Observer) o ;

while this one is valid:

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

?

[admin]

Is the reference o pointing to an object of class Observer?

[German]

so, does it mean if it would be:

Code: Select all

Object o = b; Observer ob = (Observer) o;

then it would be valid as o pointing to Observer class?

[admin]

1. "o pointing to Observer class" is an incorrect way to say it. You should say, "o pointing to an instance of Observer".
2. Yes.

[alexandrehsantos]

In this case:

Code: Select all

Object o = a; Runnable r = o;

The compiler doesn't know because Object is superclass for all class? So Runnable is a Object not unlike, (Object o) though now being a reference for (a) the compiler does not check the actual reference variable, only the type declared on (Object o) ?

It is correct think that way?

[admin]

Yes, that is correct. Since the compiler doesn't execute any code it can only see if it is possible for a variable to point to an object to which it is being cast. If there is a possibility, it allows the cast, otherwise, it generates an error. For example,
String s = "some string";
Runnable r = (String) s; <-- this will not compile because the compiler knows that s can never point to an object that is-a Runnable.

HTH,
Paul.

[Roibeard]

does the explanatory text

'o' is declared as an Object. so same case as in choice 1.

mean the same thing as

'o' is declared as an Object which is Observable. so same case as in choice 1.


//for my notes. thanks.

[admin]

Option 1 and option 4 both are trying to assign o to a reference of a different type. The declared class of o is Object while the declared type of the target is Runnable or Observable. Both are wrong for the same reason as explained in the explanation, "Although o refers to an object which is Runnable but the compiler doesn't know about it. You have to do: Runnable r = (Runnable) o;"

HTH,
Paul.

[Mushfiq Mammadov]

Maybe Observable should be Observer.

[admin]

Fixed.
thank you for your feedback!