java - The 'instanceof' operator behaves differently for interfaces and classes

Question

I would like to know regarding following behavior of instanceof operator in Java.

interface C {}

class B {}

public class A {
    public static void main(String args[]) {
        B obj = new B();
        System.out.println(obj instanceof A);      //Gives compiler error
        System.out.println(obj instanceof C);      //Gives false as output
    }
}

Why is it so? There is no relation between interface C and class B, but it gives false whereas in case of obj instanceof A it gives compiler error?

Answer 1

Because Java has no multiple class inheritance it's absolutely known during the compilation that obj object of type B cannot be subtype of A. On the other hand it possibly can be subtype of interface C, for example in this case:

interface C {}

class B {}

class D extends B implements C {}

public class A {
    public static void main(String args[]) {
        B obj = new D();
        System.out.println(obj instanceof C);      //compiles and gives true as output  
    }
}

So looking only at obj instanceof C expression compiler cannot tell in advance whether it will be true or false, but looking at obj instanceof A it knows that this is always false, thus meaningless and helps you to prevent an error. If you still want to have this meaningless check in your program, you can add an explicit casting to the Object:

System.out.println(((Object)obj) instanceof A);      //compiles fine