Doubt in instanceof operator

[Kayalvizhi Umashankar]

Given the following class definitions, which expression identifies whether the object referred to by obj was created by instantiating class B rather than classes A, C and D?


class A {}
class B extends A {}
class C extends B {}
class D extends A {}

Options:
a) obj instanceof B
b) obj instanceof A && !(obj instanceof C)
c) obj instanceof B && !(obj instanceof C)
d) !(obj instanceof C || obj instanceof D)
e) !(obj instanceof A) && !(obj instanceof C) && !(obj instanceof D)

I thought the answer was option e) but turned out to be option c).

Can anyone explain.

Thanks in advance

Kayal

[Pavel Kubal]

It can't be e), because object of class B is instance of class A.

[srikanth reddy]

hi umashankar,

i think the question means ..

for B b=new B;
then since class B extends class A..then b is instanceof b is true as well
as b is instanceof a is also true ...so i think there could be multiple
answers..
friends do correct me if iam wrong ...

thanks & regards

srikanth

[Santana Iyer]

obj instanceof B && !(obj instanceof C)

if obj was created instantiating B

obj instanceof B is true
obj instanceof C is false

true && !(false)
true && true
true (Yes obj refers to an Object of B)

now if obj was created instantiating C
obj instanceof B is true // C isa B
obj instanceof C is true

true && !(true)
false

[Emilia Ipate]

Hi Kayalvizhi,

I must say I definitely agree with Pavel.
The question assumes you have a line �obj = new B()�; where obj is declared as B or A.
So, if obj is of type B, then

the test obj instanceof A will always return true (as B extends A),

obj instanceof B will return true (obj is of type B),

obj instanceof D will return false(B has nothing to do with D)

obj instanceof C will return false(as C extends B and obj is of type B)

=> obj instanceof B && !(obj instance of C)

[s mahen perera]

Hi Umashankar,

I can remember this Question ,, i did this when i was preparing for SCJP . It is a very good Q which tests whether the candidate knows the mechanics of "instanceof" operator
Ok. I will explain how the answer was c)

Answer a) --> wrong ! since obj can be an object created from the class C, and still obj instanceof B returns true !! (Since class C extends Class B)

Answer b) --> if obj instanceof A evaluates to true, that means the obj can be an object created from any of the classes A,B,C or D . For obj instanceof C to be false , then obj can be an object created from any of the classes A, B or D. so for "obj instanceof A && !(obj instanceof C)" to evaluate to true, then obj can be an object created from either A,B or D. So this answer is also wrong !!

Answer c) --> if obj instanceof B evaluates to true, then obj can be an object created by either class B or class C. and for "obj instanceof C" to be false, then obj should be an object created by A,B or D . hence, for obj instanceof B && !(obj instanceof C)
to be true , the object ob must be created from class B


i think u can follow my argument style similarly for the other options Good luck.

Cheers
Mahen

[Steve Morrow]

Santana's got the best explanation so far.

The question asks for a way to determine if the object was created as a "new B()". Naturally, we'll want to know if "obj instanceof B". But from the class definitions, we can see that C extends B and therefore a new C() object is also an instanceof B. So, we'll need to exclude C objects from our test.

This leads to the statement "obj instanceof B && !(obj instanceof C)" (answer c).

[srikanth reddy]

thanks uma for the nice question ...
uma for the option e its u cant instantiate it by A,C,D...even with B also ..so the only option is C as explained by santana ....

thanks & regards

srikanth

[Sergei Iakhnin]

Option e) also wouldn't compile unless obj was declared as A.