Why can't a superclass object be implicitly converted to a subclass object in Java?

Question

I have the following code:

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

According to the Java rule:

Case 1:

 B b = new B(); A a = b; ((B)a).fB(); 

Case 2:

 A a = new A(); B b = a; 

According to the Java rule, case 1 is ok and case 2 is not normal . Why is case 2 not normal ? And what this line actually does ((B)a).fB(); (I mean what's going on inside)?

Answer 1

Why is case 1 okay, and case 2 is not OK: since Dog is Animal , but not every Animal is 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 for the object; in particular, it does not carry out any transformation of objects. Casting says only to the compiler: "I have this object here, and I know better than you what it is, I want you to treat it like type X and not tell 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 Dog ; he only knows that it is Animal - and not all Animal - Dog s. You can throw to tell the compiler not to 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 Dog valid, and if not, you get a ClassCastException .

(And the question in your name is exactly the opposite of what you mean, as the bizicop noted).

Answer 2

Well, think of two people: a doctor and an engineer. A doctor can heal, an engineer can build. Both are human.

Now here is your example.

 Person p = new Person(); // somebody that does not know to do anything special. Doctor d = (Doctor)p; // do you think that casting makes person that does not know anything to be a doctor? 

Do you want a person who has been “abandoned” to contact you to be the doctor you prefer to be a real doctor?

I believe the answer is clear. Every doctor is a person (case 1), but not every person is a doctor, because not everyone can treat. The same applies to the class hierarchy. The subclass inherits the properties of the superclass and probably adds it. Therefore, not every instance of a superclass can be attributed to a subclass.

Answer 3

Case 1 is fine, because (B) the part that you explicitly tell the compiler:

even if you only know that a has type A, I tell you that it has type B

therefore, object a is treated as if it were type B

Case 2 is not appropriate because the compiler cannot safely assign b to a. That would be accepted if you wrote

 A a = new A(); B b = (B)a; 

but it will throw a runtime exception

Answer 4

Suppose 2 also works, then your compiler sees "b" as an object of class "B". Now you can say "b.fb ()". But actually, “b” is the object “A” (remember that you assigned the object of class “A” to “b”). in class "A" there is no function fb (). your application crashes!