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Khalid's Q6.23 pg 271

 
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Given three classes A, B, and C, where B is subclass of A and C is subclass of B, which one of these boolean expressions is true when an object denoted by reference o has actually been instantiated from class B as opposed to from A or C?
choices:
1. (o instanceof B)
2. (o instanceof B) && (!(o instanceof C))
I think both these answers are right, but the ans given is 2.
Why?
--Cathy.
 
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Originally posted by Cathy Song:
Given three classes A, B, and C, where B is subclass of A and C is subclass of B, which one of these boolean expressions is true when an object denoted by reference o has actually been instantiated from class B as opposed to from A or C?
choices:
1. (o instanceof B)
2. (o instanceof B) && (!(o instanceof C))
I think both these answers are right, but the ans given is 2.
Why?
--Cathy.


I agree with you Cathy.
You can also look at it this way without thinking about the classes.
&& is a short-circuit operator. So for (2) to be true, then the 1st expression must be true, otherwise, it will immediately return false and ignore the 2nd expression. Now the 1st expression in (2) is (o instanceof B). And if this is true, then (1) must also be true.
[ October 14, 2003: Message edited by: Alton Hernandez ]
 
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Hi Cathy, I have the first edition (2000) of Khalid�s book.
Question 6.22 on page 213 says
Given three classes a, B and C, where B is a subclass of A and C is a subclass of B, which one of these boolean expressions correctly identifies when an object o has actually been instantiated from class B as opposed to from A or C?
(a) (o instanceof B) && (!(o instanceof A))
(b) (o instanceof B) && (!(o instanceof C))
(c) !((o instanceof A) || (!(o instanceof B))
(d) (o instanceof B)
(e) (o instanceof B) && !((o instanceof A)) || (o instance of C))
Since option (d) is true if o = new B() and if o = new C(), option (d) does not identify an object as being created from class B.
 
Cathy Song
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I see your point Marlene. So we have to find the correct expression, given o is an object of B and only B.
Do you think the questions in the certification exam are as tricky as this one? :roll:
 
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