instanceof with an interface

Question

If I try to use instanceof operator with wrong class I'm getting a compile error (" Animal can not be converted to String") but with an interface I'm not getting compile time error.

For eg: In line 10 I'm getting a compile error because Animal is not a subclass of String. But in line 14 I'm not getting a compile error even though Animal does not implement List interface.

class Animal {

}

public class InstanceOf {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
         Animal a = new Animal();
        if (a instanceof String ){  //line 10
            System.out.println("True");
        }

         if (a instanceof List ){ //line 14
            System.out.println("True");
        }
    }

}

Consider this: `class B extends Animal implements List`. Now this: `Animal a = new B();` — njzk2, Apr 10 '16 at 19:25
@Tunaki You also have the "Dupehammer®" - why didn't you close it? (Likely: You are as hesitant as me ... :-/ ) — Marco13, Apr 11 '16 at 08:38

Eran · Accepted Answer · 2016-04-10T11:44:17.030

19

a can never be an instanceof String, hence the compilation error.

a can be an instance of List if some sub-class of Animal would implement the List interface and you would assign an instance of such sub-class to a. Therefore the compiler allows it.

From the JLS :

If a cast (§15.16) 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.

edited Apr 10 '16 at 11:44

answered Apr 10 '16 at 11:38

Eran

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Is java not allowed multiple inheritance is the root cause? At the same time any subclass can implement an Interface whenever it wants? – nantitv Apr 10 '16 at 11:46
3

@nantitv Yes. Since Animal is not a sub-class of String, any sub-class of Animal cannot be a sub-class of String. Though in your specific snippet, it wouldn't make a difference even if multiple inheritance was allowed, since String is final and cannot be sub-classed. – Eran Apr 10 '16 at 11:50
Thanks . String I just took it for an example. – nantitv Apr 10 '16 at 11:51
@nantitv if you tell the compiler that there will be no subclasses of `Animal`, it will in fact be able to infer that `instanceof` is pointless: simply replace `class Animal{}` by `final class Animal{}`. – CompuChip Apr 10 '16 at 19:42

Suganthan Madhavan Pillai · Answer 2 · 2016-04-10T12:09:39.813

Just an experiment I did from this question.

class Animal {}
interface AnimalA {}
class AnimalB{} 

class AnimalC extends Animal,AnimalB {} //not possible
class AnimalD extends Animal implements AnimalA{} //possible   

public class InstanceOfTest {

    public static void main(String[] args) {
        Animal a = new Animal();
        if(a instanceof AnimalA) { //no compile time error
            System.out.println("interface test");
        }
        if(a instanceof AnimalB) { //compile time error
            System.out.println("interface test");
        }
        if(a instanceof List) { //compile time error
            System.out.println("interface test");
        }
        if(a instanceof ArrayList) { //compile time error
            System.out.println("interface test");
        }
    }
}

So as @Eran said, as Animal is not a sub-class of AnimalB non of its sub-class can become and instance of AnimalB. But on the other any of its sub-class can implement interface List.

instanceof with an interface

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