fundamental Code Samples

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How do I use the instanceof keyword?

To check whether an object is of a particular type (class or interface type) you can use instanceof operator. The instanceof operator is used only for object reference variable. x instanceof y can be read as x is-a y.

The instanceof returns true if the reference variable being tested is of the type being compared to. It will still return true if the object being compared is assignment compatible with the type on the right.

For interface type, an object is said to be of a particular interface type (meaning it will pass the instanceof test) if any of the object's superclasses implement the interface.

package org.kodejava.example.fundametal;

public class InstanceofDemo {
    public static void main(String[] args) {
        Body body = new Body();
        Hand hand = new Hand();
        Nail nail = new Nail();
        Shoes shoe = new Shoes();

        if (body instanceof Man) {
            System.out.println("body is a Man");
        }

        if (hand instanceof Man) {
            System.out.println("hand is a Man too");
        }

        if (hand instanceof Body) {
            System.out.println("hand is a Body");
        }

        //
        // it should be return false
        //
        if (hand instanceof Nail) {   
            System.out.println("hand is a Nail");
        } else {
            System.out.println("hand is not a Nail");
        }

        if (nail instanceof Man) {
            System.out.println("nail is a Man too");
        }

        if (nail instanceof Hand) {
            System.out.println("nail is a Hand");
        }
        if (nail instanceof Body) {
            System.out.println("nail is a Body too");
        }

        // 
        // it should return false, cause Shoes is not implements Man
        //
        if(shoe instanceof Man){ 
            System.out.println("shoe is a Man");
        } else {
            System.out.println("shoe is not a Man");
        }

        //
        // compile error. cannot test against class in different 
        // class hierarchies.
        //
        if(shoe instanceof Body){ }  

    }

}

interface Man {
}

class Body implements Man {
}

//
// indirect implements Man
//
class Hand extends Body {   
}

// 
// indirect implements Man
//
class Nail extends Hand {  
}

class Shoes {
}

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