Generic Types extending Abstract implementing Interface

[Sandra Bachan]

Chapter 7, Sierra/Bates, Quiz Question # 6

import java.util.*;

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(Sheep x) {}
} 
class Wolf extends Animal implements Carnivore<Sheep> {
	public void munch(Sheep x) {}
}

class Eat{
	public static void main(String[] args){
	}
}

Which of the following changes (taken separately) would allow this code to compile? (Choose all that apply.)

A. Change the Carnivore interface to

interface Carnivore<E extends Plant> extends Hungry<E> {}

B. Change the Herbivore interface to

interface Herbivore<E extends Animal> extends Hungry<E> {}

C. Change the Sheep class to

class Sheep extends Animal implements Herbivore<Plant> {
   public void munch(Grass x) {}
}

D. Change the Sheep class to

class Sheep extends Plant implements Carnivore<Wolf> {
   public void munch(Wolf x) {}
}

E. Change the Wolf class to

class Wolf extends Animal implements Herbivore<Grass> {
   public void munch(Grass x) {}
}

F. No changes are necessary

I can understand why B is the answer. However, I cannot understand the explanation for D

And D doesn’t work, because in D we made Sheep extend Plant, now the Wolf class breaks because its munch(Sheep) method no longer fulfills the contract of Carnivore.

How is Wolf class broken?

[Sandra Bachan]

The following code, which incorporates D does not compile:

import java.util.*;
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 Plant implements Carnivore<Wolf> {
	public void munch(Wolf x) {}
} 
class Wolf extends Animal implements Carnivore<Sheep> {
	public void munch(Sheep x) {}
}
class Eat{
	public static void main(String[] args){
	}
}

Compiler outputs:

Eat.java:11: type parameter Sheep is not within its bound
class Wolf extends Animal implements Carnivore<Sheep> {
^
1 error

Please give a hint, thanks!

[Faisal A Khan]

Hi Sandra,

By changing Sheep to extend Plant from Animal you have broken the interface contract of Carnivore when used in Wolf. How? By passing it a Sheep which doesn't extend an Animal anymore.

Please let me know if this helps.

[Sandra Bachan]

@ Faisal: Now I get it. I changed Sheep to extend Animal and now it compiles.