upcasting confusion while checking instanceof

Question

class Animal
{

}

    class Dog extends Animal
    {

    }

    class main
    {
      public static void main(String args[])
    Animal g= new Dog();
    System.out.println(g instanceof Dog);      // 1st case

    System.out.println(g instanceof Animal);   // 2nd case

}

QUESTION: why the output is true in both cases ?

Shouldn't Dog extend Animal, though? – martijno Aug 14 '12 at 12:27 — martijno, Aug 14 '12 at 12:27
I added `extends Animal`, the OP probably just forgot it... – Radu Murzea Aug 14 '12 at 12:28 — Radu Murzea, Aug 14 '12 at 12:28

score 5 · Accepted Answer · answered Aug 14 '12 at 12:25

5

Because the object that is referenced, at run-time, by local variable g is of type Dog (and thus also an Animal, because Dog extends Animal, though that's missing from your example).

answered Aug 14 '12 at 12:25

Arnout Engelen

6,709
1
25
36

score 4 · Answer 2 · edited May 23 '17 at 12:16

4

This is polymorphism in action. See here and here.

If you want to avoid this behaviour, use getClass() instead of instanceof. See my answer here for an example.

edited May 23 '17 at 12:16

Community

1
1

answered Aug 14 '12 at 12:25

Radu Murzea

10,724
10
47
69

though your example does not show your usage, be wary anytime you use this kind of construct on a class hierarchy. If you find yourself consistently checking the type in an if statement, you might want to consider putting that behavior inside a method which can be implemented differentl for each subclass. – Matt Aug 14 '12 at 14:31

upcasting confusion while checking instanceof

2 Answers2