instanceof Operator

[Siddharth Bhargava]

Hi All,

I didn't quite understand this statement :

"Even if the object being tested is not an actual instantiation of the class type on the right side of the operator, instanceof will still return true if the object being compared is assignment compatible with the type on the right."

given in the SCJP book by Kathy Sierra and Bert Bates.

    
    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{}

Output:

    obj1 instanceof Parent: true
    obj1 instanceof Child: false
    obj1 instanceof MyInterface: false
    obj2 instanceof Parent: true
    obj2 instanceof Child: true
    obj2 instanceof MyInterface: true

In the above code I didn't quite understand the output of obj2. Please do explain and clear my concept.

Thanks

[dhani kshirsagar]

As i understand, the statement means that 'if the object [i.e. obj2] is not at instance of the parent class, but that object [obj2] is type of parent class then instanceof will return true".

-dk

[Vijitha Kumara]

Siddharth Bhargava wrote:In the above code I didn't quite understand the output of obj2. Please do explain and clear my concept.

obj2 is referring to an object of "Child" at runtime. But "Child" extends "Parent" so it inherits everything from the "Parent", hence instanceof returns true (this is what assignment compatible means in the below statement).

"Even if the object being tested is not an actual instantiation of the class type on the right side of the operator, instanceof will still return true if the object being compared is assignment compatible with the type on the right."

[Campbell Ritchie]

Never mind about "child" and "parent" (which are not the official Java terms anyway): look in the Java Language Specification and follow the link to §5.2 and see exactly what instanceof does. You will have to follow lots of links to find the whole story, and it is by no means easy to read.

You know that passing null to instanceof always results in false, don't you?