upcasting: parent and child at different packages
Dear all,
I'm studying to pass SCJP and I found an example that seems to break the casting rules, of course I might be wrong, that's why I ask your help. See the following example (please don't mind synthax errors):
--Animal.java
package A;
public class Animal{
void eat(){ } // default access, visible only inside package A
}
--Dog.java
package B;
import A.Animal;
public class Dog extends Animal{ //inheritance OK, Animal is public
// it cannot override eat, not heritable (default access)
// it does not implement a eat method for itself
}
--Test.java
package A;
import B.Dog;
public class Test{
public static void main(String [ ] args){
Animal animal = new Dog(); // no casting needed
Dog dog = new Dog();
dog.eat(); // it will be a compile time error, no such method
animal.eat(); // (*)
}
}
--
(*)
At compile time no problem, becasue:
- variable animal is of type Animal which has a method eat()
- class Test is inside the same package of class Animal so method eat is visible for Test
- method eat is not abstract so it does not need to be implemented by Dog
But animal is a Dog object and as the casting is going up the inheritance tree (Dog IS-A Animal) there shouldn't be any restrictions wrt calling Animal methods or accessing its members. But as method eat is not heritable by Dog there will be a runtime exception, Dog does not know how to eat. So it's not allways safe to make upcast, I thought restrictions applied only when we have downcasting.
On the other hand, my certification book (I won't tell the name) says the following: " (...) Dog IS-A Animal, which means that anything an Animal can do, a Dog can do. A Dog can do more, of course, but the point is anyone with an Animal reference can safely call Animal methods on a Dog instance. (...)"
So I'm very very confused :o(
Thanks in advance for your help.
-Thyago Consort
