Java polymorphism casting - runtime error ClassCastException

Question

I have this code here:

public class code {

    public static void main(String args[]){

        Animal ani1 = new Animal();
        Animal ani2 = new Dog();

        Dog d1 = (Dog)ani1; //runtime error ClassCastException
        Dog d2 = (Dog)ani2;
    }
}

class Animal {
    public String name = "animal";
    public void talk(){
        System.out.println("Animal");
    }
}

class Dog extends Animal {
    public String name = "dog";
    public void talk(){
        System.out.println("Dog");
    }
}

I am not understanding why a runtime error is occurring and downcasting in general. There are 2 objects being created, an Animal object and a Dog object. Each is referenced to by a variable of type Animal .

I think Java will not assign d1 an object of type Animal because d1 was declared as type Dog . But what is confusing me is that both ani1 and ani2 are type Animal , and should be able to be downcasted to Dog right? The only difference is that one is referencing an Animal object and the other a Dog object.

Does Java know that ani1 is referencing an Animal object and will not allow d1 to be assigned a variable that references an Animal object?

Answer 1

What VGR said was right, you cannot change what something is. Casting is only here to help our code during compilation recognize what it is.

Consider the ff example:

Animal ani1 = new Cat();
Animal ani2 = new Dog();

Dog dog1 = (Dog) ani1; // will produce a runtime error
Dog dog2 = (Dog) ani2;

Just like the given example, you cannot simple change Cat to Dog by simply casting. Dog will always be an Animal but an Animal can't always be a Dog .

Answer 2

The comment by VGR is correct, but let me elaborate for your specific situation. A Dog is an Animal, but an Animal is not necessarily a Dog. You could create Dog objects and pass them around as Animal objects, but not the other way around. A common way to handle this would be the following:

public static void main(String args[]) {        
    Dog dog = new Dog();
    makeMeTalk(dog);
}

public static void makeMeTalk(Animal animal) {
    animal.talk();
}

The output of this would be: Dog .