Why superclass object cannot be implicitly converted to subclass object in Java?

Question

I have the following code:

class A {
}

class B extends A {
    public void fB(){};
}

According to Java rule:

Case 1:

Answer 1

Why case 1 is OK and case 2 is not OK: Because a Dog is an Animal, but not every Animal is a Dog.

class Animal { }
class Dog extends Animal { }
class Cat extends Animal { }

Dog fifi = new Dog();
Cat tommy = new Cat();

// OK because Dogs and Cats are Animals
Animal pet1 = fifi;
Animal pet2 = tommy;

// Not OK because an Animal is not always a Dog
Dog doggy = pet2;

Note that casting does nothing to the object; in particular, it doesn't do any kind conversion of objects. Casting is only telling the compiler "I have this object here, and I know better than you what it is; I want you to treat it as type X and not give me any error messages".

So in a line like this:

Dog doggy = pet2;

the compiler will complain because it cannot be sure that pet2 is actually a Dog; it only knows that it is an Animal - and not all Animals are Dogs. You can do a cast to tell the compiler to not complain about this:

// Tell the compiler that you want it to treat pet2 as a Dog
Dog doggy = (Dog)pet2;

But when you run the program, Java will still check if pet2 is really a Dog, and if it isn't, you get a ClassCastException.

(And the question in your title is exactly the opposite of what you mean, as biziclop noticed).