IS-A / HAS-A relationships. Chapter 2 Question 14 page 180

[Glen Iris]

You're designing a new online board game in which Floozels are a type of Jammers, Jammers can have Quizels, Quizels are a type of Klakker, and Floozels can have severl Floozets. Which of the following fragments represent this design? (choose all that apply)

The correct answers are given as A and C.

A.

import java.util.*
interface Klakker{}
class Jammer {Set<Quizel> q;}
class Quizel implements Klakker{}
public class Floozel extends Jammer {List<Floozet> f;}
interface Floozet{}

C.

import java.util.*
class Floozet{}
class Quizel implements Klakker{}
class Jammer {List<Quizel> q;}
interface Klakker{}
class Floozel extends Jammer{ List<Floozet> f;}

Given that Floozels can have several Floozets, this implies that Floozet is an instance variable. A shows it as an interface and C shows it as an instance variable.

Looking forward to your help as always :-)

[Joseph Arnold]

Floozel can have several Floozets mean there can be several instances of Floozets in floozel. Well, both options illustrate the same. Whether Floozet is a class type or an interface type does not matter. We must only ensure that there is some appropriate collection of floozets in a floozel.

[Helen Ma]

According to Effective Java by Josh Bloch, we should use interface to define object type. This is the only usage of interface. Therefore, while a class is to define type, an interface is also used for defining type.
In K&B book, use interface as if a 100% abstract class. 100% abstract class is the abstract class that only contains abstract methods. Interface contains all abstract method, too.

So, while we use class/abstract class to define type, we can also use interface to define type.

Correct me if I am wrong.