Enthuware

You are telling me, that the answer #4 is correct:
! ( !(o instanceof B) || (o instanceof C))

But there is a || used, which means it is not an instance of C OR is a instance of B. But it won´t verify if both things are true...Is this answer now really correct?

Yes, it is correct. Please try it out. As the explanation says, option 3 and option 4 are actually same.

HTH,
Paul.

goblingift wrote:You are telling me, that the answer #4 is correct:
! ( !(o instanceof B) || (o instanceof C))

But there is a || used, which means it is not an instance of C OR is a instance of B. But it won´t verify if both things are true...Is this answer now really correct?

If you remember maths had something called de morgan's law for sets

( A ∪ B)’ = A’ ∩ B’
( A∩B)’ = A’ ∪ B’

and the the second answer is simple the negated answer of the first answer.

I am not satisfied with answer number 4, still.
Lets assume o is an object of class C.
Then the expression

! ( !(o instanceof B) || (o instanceof C))

stops after the first evaluation
! ( !(o instanceof B)
because of the || operator

The expression ! ( !(o instanceof B)
returns true (C is a B)

so the object can be an instance of class C , too.

Then: When i create an instance of class C and use the following expression

! ( !(o instanceof B) || (o instanceof C))

it returns false.
Why ?
Very strange ...for me

If o points to an object of class C, then the expression evaluation will proceed as follows -
! ( !(o instanceof B) || (o instanceof C))
! ( !(true) || (o instanceof C))
! ( false || (o instanceof C)) //since the first operand of || is false, the second operand i.e. o instanceof C will be evaluated.
! ( false || true)
! ( true)
false.

You should now work out the evaluation of the expression for other cases in a similar fashion.

Your explanation makes a lot of sense, now. To simplify the original question, with respect to each statement, if o is b then it should return true, but if o is c it should return false. Right? Or am I missing something.

o is b and o is c doesn't make sense. Do you mean o is B and o is C?
You might want to check out Section 11.3.3 The instanceof operator of OCAJP Java 8 Programmer Certification Fundamentals. It explains this in detail.