Question

I am trying to learn about instanceof operator in java as per the link :- instanceof

But when i try to run their following code:-

class InstanceofDemo {
public static void main(String[] args) {

    Parent obj1 = new Parent();
    Parent obj2 = new Child();

    System.out.println("obj1 instanceof Parent: "
        + (obj1 instanceof Parent));
    System.out.println("obj1 instanceof Child: "
        + (obj1 instanceof Child));
    System.out.println("obj1 instanceof MyInterface: "
        + (obj1 instanceof MyInterface));
    System.out.println("obj2 instanceof Parent: "
        + (obj2 instanceof Parent));
    System.out.println("obj2 instanceof Child: "
        + (obj2 instanceof Child));
    System.out.println("obj2 instanceof MyInterface: "
        + (obj2 instanceof MyInterface));
}

}

class Parent {}
class Child extends Parent implements MyInterface {}
interface MyInterface {}

I get the following errors at compile time:

./Child.java:1: error: cannot find symbol
class Child extends Parent implements MyInterface{
                                      ^
  symbol: class MyInterface
InstanceOfDemo.java:9: error: cannot find symbol
System.out.println("obj1 instanceOf MyInterface" + (obj1 instanceof MyInterface));
                                                                    ^
  symbol:   class MyInterface
  location: class InstanceOfDemo
InstanceOfDemo.java:12: error: cannot find symbol
System.out.println("obj1 instanceOf MyInterface" + (obj2 instanceof MyInterface ));
                                                                    ^
  symbol:   class MyInterface
  location: class InstanceOfDemo
3 errors

Below is the directory structure where the src code is placed:-

-rw-rw-r-- 1 ankit ankit 174 Oct 25 15:36 Child.class
-rw-rw-r-- 1 ankit ankit  31 Oct 25 15:36 Child.java
-rw-rw-r-- 1 ankit ankit 920 Oct 25 15:41 InstanceOfDemo.class
-rw-rw-r-- 1 ankit ankit 637 Oct 25 15:41 InstanceOfDemo.java
-rw-rw-r-- 1 ankit ankit  25 Oct 25 15:20 MyInterface
-rw-rw-r-- 1 ankit ankit 186 Oct 25 15:36 Parent.class
-rw-rw-r-- 1 ankit ankit  16 Oct 25 15:18 Parent.java

Note: I removed the interface implementation from Child.java and InstanceOfDemo.java to compile and run the code.

Edit 1:- can i consider that instanceof operator can’t be used with Interfaces because the definiton of instanceof operator as per Oracle is as follows:-

The instanceof operator compares an object to a specified type. You can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface.

Answer 1

Your interface should also be in .java file so you need to put MyInterface in MyInterface.java

-rw-rw-r-- 1 ankit ankit  25 Oct 25 15:20 MyInterface<---Rename it as 
                                                         MyInterface.java

After this compile again and everything should work fine