instanceoff

[Ivan Ivanoff]

Hi all,
JQ+ :
956597418360
=============================================
(obj instanceof C && !(obj instanceof D) )
correctly identifies whether the object referred to by obj was created by instantiating class
C rather than classes A, B and D?
class A {}
class B extends A {}
class C extends B {}
class D extends C {}
Answer :True .
==============================================
(obj instanceof C) - i can see its true ,
!(obj instanceof D) - i can see its true ,
but how come obj in not an instance form A and D classes ?

[Corey McGlone]

I'm not sure that I quite understand what you're asking. The key to the instanceof operator is to realize that it will return true if the object in question is of the same type or a descendent type of the class given. Therefore, in your example, if you had an object of type C, this would be your following results:

obj instanceof A; // false
obj instanceof B; // false
obj instanceof C; // true
obj instanceof D; // true

Therefore, if you wanted to check to see if a given object was of type C only, you'd have to do the following:

if (obj instanceof C && !(obj instanceof D))
{
   // This object is of type C
}

Check out the JLS, §15.20.2 Type Comparison Operator instanceof for more information.
I hope that helps,
Corey

[Ivan Ivanoff]

but if we create
A obj = new C();
then
(obj instanceof A) - true ! ???

please help

[Corey McGlone]

Yes, that's right, because, according to the JLS, this is what the instanceof operator does:

At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast (�15.16) to the ReferenceType without raising a ClassCastException. Otherwise the result is false.

Therefore, if the object that is used is the exact type it is compared to, or a descendent of that type, it returns true.
Corey

[Ivan Ivanoff]

Thank you all ,
so it means that JQ+ question 956597418360 is not in the correct form or it's my mistake ?
please help.

[Corey McGlone]

No. The question and answer looks fine to me. Given the inheritance structure, the given test will correctly determine if an object is of type C.
Corey

[Ivan Ivanoff]

ohh - thank you Corey McGlone.
I was reading this question wrong .
Thanks