confusion about instane of operator

[Basit56]

Hi,
Hope you all will be fine. In the following code where instance of operator use. Can i call that "a instance of B" means that "a is a super class of B" .

    class A { }
    class B extends A {
        public static void main (String [] args) {
            A myA = new B();
            m2(myA);
        }
        public static void m2(A a) {
            if (a instanceof B)
                ((B)a).doBstuff(); // downcasting an A reference to a B reference
        }
        public static void doBstuff() {
            System.out.println("'a' refers to a B");
       }
    }

I read that if variable on left side of "instance of" passes IS A relationship to the variable on the right then instance of result is true.So if here i say that "a instance of B" means "a IS A B" then it's not true because here "B IS A A" because B extends A.

Similarly in the code

    interface Face { }
    class Bar implements Face{ }
    class Foo extends Bar { }

"Bar instance of Foo" return false why ? above "B extends A" and "a instance of B" return true. Here If we replace on the third line Foo with B and Bar with A then it's the same code as above but above it's return true here false. Can any one clear my confusion about instance of operator.

Thank you

[JosAH]

Here is what the JLS has to say about it.

kind regards,

Jos

[Tolls]

"instanceof" simply asks if something is an instance of the class on the right.

In your case, B extends A so an instance of B is an instance of A. In the case of the reference "a", that is a reference to an object of class B. So instanceof returns true.

In the second example, a reference to an object of class Foo is an instance of Foo, Bar and Face.

[JavaJuJitZu]

I think the confusion you have is a class vs object problem.

the operator doesn't test whether a class is another class, it test whether an object is a instance of a class.

So saying Bar instance of Foo is only valid when you have a variable name of type Bar named Bar.

 Bar variableName;

Here we have declared a variable name of type bar. Currently it holds nothing as is not-instantiated
We now can instanciate

 variableName = new Bar()

we create a new instance of Bar and now try our test

 variableName instanceof FOO

this will ofcourse return false as we have assigned a new instance of Bar;

 variableName = new FOO()

because foo extends bar we can assign an instance of foo to variable that is of type Bar

 variableName instanceof FOO

Here instanceof returns true as the instance assigned to the variable is of type FOO.

Hope that helps.

[Basit56]

Hi,
Thanks to all.JavaJuJitZu the way you answered me is very good. Although Tolls said the same thing but you cleared me with a very simple example :).

Thank you to all :).