kathy siera book self test question6 of generics

[Arjun Srivastava]

 
interface hungry<E>{ void munch(E x); }
interface carnivore<E extends animal> extends hungry<E> { }
interface herbivore<E extends plant> extends hungry<E> { }
abstract class plant{}
class grass extends plant{}
abstract class animal {}
class sheep extends animal implements herbivore<sheep> {
public void munch(grass x) { }
}
class wolf extends animal implements carnivore<sheep>
        {
public void munch(sheep x)
        {}
}
public class M {
    public static void main(String[] args) {
    }
}

what changes would allow this code to compile?
B change herbivore to

interface herbivore <E extends animal> extends humgry<E>

C change sheep class to

class sheep extends animal implements herbivore<plant> {
              public void munch(grass x) { }
     }

D

change sheep class to 
               class sheep extends plant implements carnivore<wolf> {
               public void munch(wolf x) { }
               }

E change wolf class to

 class wolf extends animal implements herbivore<grass> {
              public void munch(grass x) { }
                }

Answer is B given in book.

yes it is right, i agree, B is correct and C is wrong.

but if i try D and E also instead of B, the code also compiled successfully.

[Dieter Quickfend]

You should read the book better. The code snippet you gave for D is not consistent with that in the book. In the book, sheep extends Plant, which solves sheep's problem, but doesn't compile because it breaks wolf's contract (carnivore)

and E will only compile if the class already compiles. It doesn't solve any problem with sheep.

[Arjun Srivastava]

Dieter Quickfend wrote:In the book, sheep extends Plant, which solves sheep's problem, but doesn't compile because it breaks wolf's contract (carnivore)

hi thanks, i have edited my post.
i get through with E fine.ok

when i use D, i get
type parameter sheep is not within its bound

how sheep breaks wolf contract please elaborate if you can.

[Dieter Quickfend]

Wolf is a Carnivore and eats Sheep.
If Sheep IS-A plant, Wolf must be a Herbivore, because Sheep is no longer an Animal (it IS-A Plant).