Why casting is not possible from child to parent

user_18571422 0
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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.

late-binding downcast polymorphism java
1 Answers

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.

user_32292980
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