A Java game designer wishes to store all the game preferences (e.g., player name,

screen size, music volume, etc.) wisthin a custom Preference class.

(i) Assuming each preference is stored as a unique String key mapping to

a String value, give a simple implementation ofs Preference that allows

for effiffifficiently setting or updating preferences and retrieving previously set

ones. Your implementation should defifine an exception that is thrown when

a preference key is requested but not present. [5 marks]

(ii) It is important that only one Preference object exists in a running game.

Show how to apply access modififiers and the Singleton design pattern to

ensure this. Your implementation should lazily instantiate the object. Is it

necessary to make your class final or Cloneable? Explain your answer.

[6 marks]

(c) The designer also implements other Singleton classes in the game and proposes

to create a SingletonBase base class from which all such classes would inherit

the singleton behaviour. By providing example Java code, explain why this is

not viable. [5 marks]

4CST.2014.1.5

4

Object-Oriented Programming

A Lecturer wishes to create a program that lists his students sorted by the number

of practical assignments they have completed. The listing should be greatest number

of assignments fifirst, sub-sorted by name in lexicographical order (A to Z).

A class StudentInfo stores the name and number of assignments completed for a

student. Amongst other methods, it contains a void setCompleted(int n) method

that allows changes to the number of completed assignments.

(a) Provide a defifinition of StudentInfo with an equals() method and a natural

ordering that matches the given requirement. [9 marks]

(b) A TreeSet is used to maintain the StudentInfo objects in appropriate order.

When setCompleted(...) is called on a StudentInfo object it is necessary

to remove the object from the set, change the value and then reinsert it to

ensure the correct ordering. This is to be automated by applying the Observer

design pattern via classes UpdatableTreeSet and SubscribableStudentInfo.

A partial defifinition of UpdatableTreeSet is provided below.

public class UpdatableTreeSet extends

TreeSet {

// To be called just before the StudentInfo object is updated

public void beforeUpdate(SubscribableStudentInfo s) {

remove(s);

}

// To be called just after the StudentInfo object is updated

public void afterUpdate(SubscribableStudentInfo s) {

add(s);

}

}

(i) Extend StudentInfo to create SubscribableStudentInfo such that: mul

tiple UpdatableTreeSet objects can subscribe and unsubscribe to receive

updates from it; and the beforeUpdate(...) and afterUpdate(...)

methods are called appropriately on the subscribed UpdatableTreeSet

objects whenever setCompleted(...) is called. [6 marks]

Explain the purpose of access modififiers in OOP languages. [2 marks]

(ii) Copy and complete the table below to show the access restrictions for the

four access modififiers in Java. [2 marks]

Access Modififier

Defifining class

Class in same package

Subclass in difffferent package

Non-subclass in difffferent package

(b) A Java game designer wishes to store all the game preferences (e.g., player name,

screen size, music volume, etc.) within a custom Preference class.

(i) Assuming each preference is stored as a unique String key mapping to

a String value, give a simple implementation of Preference that allows

for effiffifficiently setting or updating preferences and retrieving previously set

ones. Your implementation should defifine an exception that is thrown when

a preference key is requested but not present. [5 marks]

(ii) It is important that only one Preference object exists in a running game.

Show how to apply access modififiers and the Singleton design pattern to

ensure this. Your implementation should lazily instantiate the object. Is it

necessary to make your class final or Cloneable? Explain your answer.

[6 marks]

(c) The designer also implements other Singleton classes in the game and proposes

to create a SingletonBase base class from which all such classes would inherit

the singleton behaviour. By providing example Java code, explain why this is

not viable. [5 marks]

4CST.2014.1.5

4

Object-Oriented Programming

A Lecturer wishes to create a program that lists his students sorted by the number

of practical assignments they have completed. The listing should be greatest number

of assignments fifirst, sub-sorted by name in lexicographical order (A to Z).

A class StudentInfo stores the name and number of assignments completed for a

student. Amongst other methods, it contains a void setCompleted(int n) method

that allows changes to the number of completed assignments.

(a) Provide a defifinition of StudentInfo with an equals() method and a natural

ordering that matches the given requirement. [9 marks]

(b) A TreeSet is used to maintain the StudentInfo objects in appropriate order.

When setCompleted(...) is called on a StudentInfo object it is necessary

to remove the object from the set, change the value and then reinsert it to

ensure the correct ordering. This is to be automated by applying the Observer

design pattern via classes UpdatableTreeSet and SubscribableStudentInfo.

A partial defifinition of UpdatableTreeSet is provided below.

public class UpdatableTreeSet extends

TreeSet {

// To be called just before the StudentInfo object is updated

public void beforeUpdate(SubscribableStudentInfo s) {

remove(s);

}

// To be called just after the StudentInfo object is updated

public void afterUpdate(SubscribableStudentInfo s) {

add(s);

}

}

(i) Extend StudentInfo to create SubscribableStudentInfo such that: mul

tiple UpdatableTreeSet objects can subscribe and unsubscribe to receive

updates from it; and the beforeUpdate(...) and afterUpdate(...)

methods are called appropriately on the subscribed UpdatableTreeSet

objects whenever setCompleted(...) is called. [6 marks]

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