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instanceof operator confusion

vernon franks
Greenhorn

Joined: Jul 28, 2013
Posts: 1
Howdy yallll javacoffeecuppajoe here.....

I am learning intermediate Java and I am having confusion over one aspect of the instanceof operator. I know what it is used for and how to use it. What I am confused about though is why the operator cannot test for siblings in a class tree. Suppose I have a Class A, and Class B extends Class A. Then I make another class, Class C extends class A. Class B and Class C are siblings of Class A. It seems you cannot test instances of one sibling against the type of the other sibling. For instance:



The compiler complains of "Incompatible conditional operand types B and C" or something like that.


Why?
Jeanne Boyarsky
author & internet detective
Marshal

Joined: May 26, 2003
Posts: 30598
    
154

Welcome to CodeRanch!

Because there is no chance of it being true at runtime. Instanceof will only compile if it might be true. If it can never be true, it is unreachable code. The compiler recognizes this and tells you. The solution being to fix the if condition to something possible or get rid of it.


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Jesper de Jong
Java Cowboy
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Joined: Aug 16, 2005
Posts: 14196
    
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Note that it will work if you change the type of the variable instance_B to for example Object (or any other superclass of C to which you can also assign a B):


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