JQPlus question on "instanceof"

[Angela Narain]

Given the following class definitions the following expression :
(obj instanceof A) && !(obj instanceof C) && !(obj instanceof D)
correctly indentifies whether the object referred to by obj was created by instantiating class B rather than classes
A,C, D ?
class A
class B extends A {}
class C extends B {}
class C extends D {}
The answer is False
Now is the obj is an instance of class B, the instanceof check
done with C and D should return false.
That is ( obj instanceof C ) = false
( obj instanceof D ) = false
( obj instanceof A ) = true
Then how does the expression evaluate to false ???
Can someone explain ..

[Axel Janssen]

Hi Angela,
because it could have been instantiated by new A(), too.
Object obj = new A();
(obj instanceof A) -> true
!(obj instanceof C) --> true
!(obj instanceof D) --> true
hope this helps,
Axel

[This message has been edited by Axel Janssen (edited July 02, 2001).]

[Angela Narain]

But the question states that it "obj was created by instantiating class B "
So still how does the expression evaluate to false ??

[Dave Vick]

Angela
The expression, as Axel pointed out, evaluates to true. The Answer to the question is false. Meaning that the code will not tell if the object was instantiated as an instance of B, but not A, C, or D. It will tell you if it was not C or D, but the way it is written it won't tell you if it was A or B, just that it was one of those two.

hope that helps
Dave