Java OCA OCP Practice Question 1191

Question

Given: 

interface Printable<E> { void print(E x); }

abstract class Shape {}

interface TwoDimension<E extends Shape> extends Printable<E> {}

interface ThreeDimension<E extends Plant> extends Printable<E> {}

abstract class Plant {}

class Grass extends Plant {}

class Cube extends Shape implements ThreeDimension<Cube> {
  public void print(Cube x) {}
}

class Square extends Shape implements TwoDimension<Cube> {
  public void print(Cube x) {}
}

Which of the following changes (taken separately) would allow this code to compile?

Choose all that apply. 

A.  Change the TwoDimension interface to

    interface TwoDimension<E extends Plant> extends Printable<E> {}

B.  Change the ThreeDimension interface to

    interface ThreeDimension<E extends Shape> extends Printable<E> {}

C.  Change the Cube class to

    class Cube extends Shape implements ThreeDimension<Plant> {
        public void print(Grass x) {}
    }/*from   ww w .  ja va 2s  .  c o m*/

D.  Change the Cube class to

    class Cube extends Plant implements TwoDimension<Square> {
       public void print(Square x) {}
    }

E.  Change the Square class to

    class Square extends Shape implements ThreeDimension<Grass> {
       public void print(Grass x) {}
    }

F.   No changes are necessary



B is correct.

Note

The problem with the original code is that Cube tries to implement ThreeDimension<Cube> and ThreeDimension declares that its type parameter E can be any type that extends Plant.

Since a Cube is not a Plant, ThreeDimension<Cube> makes no sense-the type Cube is outside the allowed range of ThreeDimension's parameter E.

Only solutions that either alter the definition of a Cube or alter the definition of ThreeDimension will be able to fix this.

So A, E, and F are eliminated.

B works-changing the definition of an ThreeDimension to allow it to eat Cube solves the problem.

C doesn't work because an ThreeDimension<Plant> must have a print(Plant) method, not print (Grass).

And D doesn't work, because in D we made Cube extend Plant-now the Square class breaks because its print(Cube) method no longer fulfills the contract of TwoDimension.




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