在java中引用变量downcasting的澄清

Question

Below is the sample code

class Animal {
    void makeNoise() {System.out.println("generic noise"); }
}

class Dog extends Animal {
    void makeNoise() {System.out.println("bark"); }
    void playDead() { System.out.println("roll over"); }
}

class DogTest {
    public static void main(String [] args) {
        Animal animal = new Animal();
        Dog d = (Dog) animal; 
        d.makeNoise();
    }
}

Above code compiles well but when i try to run it, i get

java.lang.ClassCastException

My assumption is it should print "generic noise" because at run time it should invoke actual Animal object makeNoise() method without giving any exception.

Answer 1

You can't cast an arbitrary Animal instance to a Dog instance, unless the instance you are casting is actually a Dog or a sub-class of Dog .

It would make more sense for the Animal class to be abstract, since an actual Animal that can be instantiated should be a specific Animal, such as Dog , Cat , etc...

Answer 2

If you had:

Animal animal = new Dog();
Dog dog = (Dog) animal;

there would be no problem.

You need to understand the difference between compile-time type and runtime type . In the code snippet above, the compile-time time of animal is Animal , while the runtime type is Dog . This is why there's no problem casting to Dog (note that the actual casting happens at runtime).

In your code, however, the runtime type of animal is Animal (same as the compile-time type). This means that the animal will point to an object on the heap, that supports the features of an Animal (which is makeNoise() ), but not the features of a Dog and that's why the ClassCastException happens.

Answer 3

All answers here are technically correct. But I want to explain this from an other point of view.

First off, you have defined one Animal and one Dog . So far everything is fine. The problem comes when you (within DogTest ) create an Animal and cast ti as Dog . Why? Not every Animal is a Dog . I mean, an Animal can be a Bird , a Cat , an Elephant ... These Animal s do not "bark". Said that, you do can cast a Dog as an Animal because all Dog s are Animal s.

I hope I have clarified a bit your mind :)

Answer 4

It won't print anything, because you won't reach the line with d.makeNoise() . The java.lang.ClassCastException is thrown in the earlier line, when you try to cast to Dog something, which is not Dog .

Still, without this line, you'll get the expected result: "generic noise".

Answer 5

Best possible downcasting i can explain by re-writing your code as follows :-

public interface  Animal {
            public void show();
        }


        public abstract class Dog implements Animal {

            public void display(){
                System.out.println("Dog Called");
            }
        }

        public class Impl extends Dog{

            @Override
            public void show() {
                System.out.println("Impl Called");  
            }
        }


            class DogTest {
            public static void main(String [] args) {
               Animal a = new Impl();
               /**
                * child class's method tried to be accessed with the help of
                * parent class's object using downcasting of object
                * */
               ((Dog)a).display();
            }
    }

However real time such requirements can fulfilled without using explicit downcasting of objects instead with the help of Factory Design Pattern in which we create object without exposing the creation logic to the client and refer to newly created object using a common interface.

Complete factory pattern can be pulled from here