instanceOf doubt...

[Siri Naray]

Hi,
I have a doubt in using instanceOf.
In K&B page 113 (Reference variable casting) the following code is given.

if(animal instanceOf dog)
{
dog d = (dog) animal;
}
assuming that dog ISA animal.....

instanceOf operator is used to see if a class is a subclass of a superclass. but can we do the opposite?
Here dog ISA animal but vice versa is false. So how can "animal instanceOf dog" return true???
Can anyone explain??
Thanks
Sirisha

[marc weber]

Originally posted by Sirishaaaaa Ghatty:
...Here dog ISA animal but vice versa is false...

A reference of type Animal could refer to a Dog if the object was originally created as a Dog and its reference was upcast to type Animal. The instanceof test can be used to check this before attempting to downcast (which would cause a runtime exception if wrong).

class Animal {}
 
class Dog extends Animal {}
 
class DogGone {
  public static void main(String[] args) {
    Animal a1 = new Dog(); //implicit upcast
    System.out.println("a1 a Dog: " + (a1 instanceof Dog));
    Dog d1 = (Dog)a1; //a1 is a Dog, so downcast okay
    
    Animal a2 = new Animal();
    System.out.println("a2 a Dog: " + (a2 instanceof Dog));
    //Dog d2 = (Dog)a2; //a2 is NOT a Dog, so RUNTIME EXCEPTION
  }
}

[Siri Naray]

thanks marc, i got the answer.