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instanceof operator

 
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hi all,
i am unable to understand what the following question is asking?
Given classes A,B and C where B is 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?
Select one right answer.
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 instanceof C))
Correct ans given is option b
pls tell me why b is correct and what is the question asking for?
rajashree.
 
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Posts: 42
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Dear Rajashree,
See the follwoing example...
Take A-Animal
B-Mammal
C-Cat..
You can create instances of Animal, Mammal , and cat .. like
Animal animal=new Animal();
Mammal mammal=new Mammal();
Cat cat=new Cat();
object o has actually been instantiated from class B as opposed to from A or C? ----means, Object O must be instantiate from only Mammal not from Cat and Animal..

a.) (o instanceof B) && (!(o instanceof A))// This option says that o is Mammal and O is not Animal.. This is not possible, if it is mammal it must be Animal.. Incorrect..
b.) (o instanceof B) && (!(o instanceof C))// o is Mammal and o is not Cat.. This Correct because Question is asking for only Object instantiated from Mammal and not Cat..
c.) !((o instanceof A) | | (o instanceof B))// o is not instantiated from Animal and Mammal.. Incorrect..
d.) (o instanceof B) //o is Mammal.. This seems to be right, but not.. o is Cat, this returns true.. but Question is asking for only o is instantiated from Mammal and not Cat..
e.) (o instanceof B) && !((o instanceof A) | | (o instanceof C))//o is Mammal and o is not Animal or o is Cat.. This wrong..

See the following code for more explanation..

 
Greenhorn
Posts: 25
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Hi,
The JLS abt. the instanceof operator says that.


15.20.2

Type Comparison Operator instanceof

The type of a RelationalExpression operand of the instanceof operator must be a reference type or the null type; otherwise, a compile-time error occurs. The ReferenceType mentioned after the instanceof operator must denote a reference type; otherwise, a compile-time error occurs.
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.
If a cast of the RelationalExpression to the ReferenceType would be rejected as a compile-time error, then the instanceof relational expression likewise produces a compile-time error. In such a situation, the result of the instanceof expression could never be true.


Your answer lies in the 2nd Para. The result of the RelationalExpression is true if the reference could be cast to the ReferenceType given that the lvalue(the relationalexpression) is not null.

Now lets analyze ur question


Given classes A,B and C where B is 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?


The hierarchy goes like this.
A---->B----->C
As per the language a subclass instance can be implicitly cast to a superclass instance.


Select one right answer.
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 instanceof C))
Correct ans given is option b
pls tell me why b is correct and what is the question asking for?
rajashree.



Hence an object o of any class be it A or B or C, will return true for the expression:
o instanceof A coz an object of a subclass gets casted to a superclass implicitly. Hence for the above question to return true the object o should be an instance of Class B and not an instance of any of its subclasses here it means C. Hence option b is the correct answer.
HIH
Arjun
 
rajashree ghatak
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Thanx Suresh and Arjun for ur explanations which have cleared my doubt.
rajashree.
 
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