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Instance Of operator

 
Ranch Hand
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When do instance of operator throw up compilattion error and when it evaluates to false?
 
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The instanceof operator is used to check if an object reference belongs to a certain type or not.
The type can either be an interface or a superclass.

The use of instanceof in java is as follows

Example
------- if(op1 instanceof op2) ....where op1 is a reference of any object & op1 is any class name.



During compile time the JVM will check whether the op1 is of type op2 or not.If it is not of same type then it will through compile time exception;
During runtime it will return true only if the Object which is referenced by op1 is not null & of same type
 
Ann Sebastian
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Could any body give an example in which
1.instance of operator throws compilation error
2.instance of operator returns false
 
Greenhorn
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Hi,

The instanceof operator returns true when the first operand is not null and it can be casted to a type of the second operand without throwing a ClassCastException.

If the first operand is null and it can be casted to a type of the second operand without throwing a ClassCastException, instanceof returns false.

If the first operand cannot be casted to a type of the second operand without throwing a ClassCastException, there is a compile-time error.

Here it is an example,

class Point { int x, y; }
class Element { int atomicNumber; }
class Element2 extends Element{}
public class ClsPruebas1 {


public static void main(String[] args) {
Point p = new Point();
Element e = new Element();

Element2 e2=new Element2();
Element2 e3=null;

System.out.println(e2 instanceof Element); //returns true
System.out.println(e3 instanceof Element); //returns false
System.out.println(e3 instanceof Point); //compiler-time error
}

}

Hope it helps,

Cristina
 
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Ann, a formal definition can be found here:JLS instanceof specification
HTH,
Sashi
 
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