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I have a class Animal which has one child Dog.

class Animal {
    public void makeSound() {
        System.out.println("Animal sound");
    }
}

class Dog extends Animal {
    @Override
    public void makeSound() {
        super.makeSound();
    }
}

public static void main(String[] args) {
    Dog dog = (Dog) new Animal(); // It compiles, but throws runtime exception.
    dog.makeSound();
}

Here if we will cast child to parent, it will compile, but it will throw runtime exception - ClassCastException. Of course, there can be a case when the child will have a field or method which the parent doesn't. That's why we can't cast Dog to Animal. Also, there's no IS-A relationship. But why the cast is not possible, in case parent and child classes have only the same fields and methods?
I've got asked this question in the interview)

Thank you.

User_214534
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    What is your actual question? – Bohemian Mar 24 '22 at 18:33
  • Why downcasting is not possible in case parent and child are structurally the same? – User_214534 Mar 24 '22 at 18:35
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    The terms you are looking for here are [structural type systems](https://en.wikipedia.org/wiki/Structural_type_system) and [nominal type systems](https://en.wikipedia.org/wiki/Nominal_type_system). It is not possible to cast `Animal` to `Dog` simply because Java's type system is designed to be nominal. – Sweeper Mar 24 '22 at 18:51
  • Sorry, I've edited my question – User_214534 Mar 24 '22 at 18:54
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    You can cast explicitly (`Animal a = new Animal(); Dog dog = (Dog) a`) which will throw a runtime error if `a` is not in fact a dog, see https://www.geeksforgeeks.org/upcasting-vs-downcasting-in-java/ but the compiler won't automatically cast it for you – Esther Mar 24 '22 at 18:54
  • Yeah, this will throw classcastexception – User_214534 Mar 24 '22 at 18:56
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    https://stackoverflow.com/questions/380813/downcasting-in-java – Esther Mar 24 '22 at 18:58
  • The cast from Animal would work if the reference was concretely of type Dog, e.g. `Animal a = new Dog(); Dog d = (Dog) a;`. – Andy Turner Mar 24 '22 at 19:10
  • Java will only allow objects that are in fact dogs (are instances of `Dog` or a subclass of `Dog`) to be cast to `Dog`. Which fields the class has (or hasn’t) is not taken into consideration. It reflects real life: in reality you cannot make an animal into a dog no matter how similar it is to a dog. – Ole V.V. Mar 24 '22 at 19:29
  • Actually, there are 3 checks. The first 2 checks performs the compiler and the last check JVM. When we write like this `Dog d = (Dog) new Animal()` it passes the first 2 checks of the compiler, but in the runtime, JVM recognizes that this cast is impossible, so in the 3rd check we get classcastexception. – User_214534 Mar 24 '22 at 21:24

1 Answers1

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When the compiler sees that you are trying to cast the Animal object into a Dog, it "knows" that Dog is underneath Animal in the hierarchy, so it is possible that the animal is a dog, so it allows it. However, at runtime, it turns out that it is just an Animal, and is not a Dog, so the cast fails.

This is a weird case in which it is obvious in reading the program that the Animal is just an Animal and not a Dog because it was just created with "new Animal()", but the compiler doesn't know that--it just knows that it is an Animal. Compilers are smart, but they can't know everything. I hope that makes sense.

ferzle
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  • You're right. I answered this question like this, that it's not correct technically because Animal is not a dog, the dog is animal and child class can have other methods/fields, which animal doesn't, but they were not satisfied with this answer)) And after that, they modified the question like assume both child and parent has the same fields/methods... Thank you – User_214534 Mar 24 '22 at 21:27