it's polymorphism or a upcasting

[Alexandra Zhang]

Here is the code:

Class Animal {}
Class Dog extends Animal {}

Class DogTest {
   public static void mian () {
        Dog d = new Dog {};
        Animal a1 = d; //why here is upcasting but not polymorphism?
        Animal a2 = (Animal) d;
    }
}

In line "Animal a1 = d;" my understanding is it's a polymorphism, because d is a newly declared dog, and a1 is assigned to be it. But in the guide book, it says this line is an implicit upcasting. What's wrong with my uderstanding?

Thanks.

[Wouter Oet]

Casting and polymorphism are 2 completely different things. That line uses implicit upcasting and polymorphism.
If your unsure what polymorphism is you could read this.

[Sebastian Janisch]

The polymorphism comes in force upon invoking a method on Animal when it's dynamic type is Dog. If the method is overridden in Dog, the Dog's version gets executed, even though the static type is Animal. And this is called Polymorphism.

[Wouter Oet]

Learn your dog polymorphish