[Solved] CS2030s Problem Set #5

1. In Java, a Set is a Collection that does not contain duplicate elements (this is in contrast to a List which does allow duplicates). You are given the Point class below:

public class Point { private final int x; private final int y;

public Point(int x, int y) { this.x = x; this.y = y;

}

@Override

public String toString() { return ( + this.x + , + this.y + );

}

}

• What is the output of the following program fragment executed in jshell?

List<Point> points = new ArrayList<>() points.add(new Point(1, 1)) points.add(new Point(1, 1)) points.indexOf(new Point(1, 1))

• By defining an appropriate overriding equals method, demonstrate how the indexOf method can now give the correct behaviour.
• What is the output of the following program fragment executed in jshell?

Point p = new Point(1, 1);

Point q = new Point(1, 1); p.equals(q)

Set<Point> set = new HashSet<>()

set.add(p) set.add(q) set

• Notice that although equals(q) returns true, the two points are considered distinct by HashSet. How do we ensure that only one point is maintained in the set?

Hint: Refer to the definition of the equals method in Object class

2. The Java Collection<E> interface extends the Iterable<E> interface with the following abstract method declared.

Iterator<E> iterator();

• Using the methods in the Iterator class, demonstrate how iteration is performed on a List, e.g.

List<Point> list = new ArrayList<>(); list.add(new Point(1, 1)); list.add(new Point(2, 2));

• How is the use of an Iterator object, different from the following

for (Point p : list) {

System.out.println(p);

}

3. What is the output of the following program fragment? Explain.

class A { static void f() throws Exception { try { throw new Exception(); } finally {

System.out.print(1);

}

}

static void g() throws Exception { System.out.print(2); f();

System.out.print(3);

}

public static void main(String[] args) { try {

g();

} catch (Exception e) {

System.out.print(4);

}

}

}

4. You are given two classes MCQ and TFQ that implements a question-answer system:

• MCQ: multiple-choice questions comprising answers: A B C D E
• TFQ: true/false questions comprising answers: T F

class MCQ { String question; char answer;

public MCQ(String question) { this.question = question;

}

void getAnswer() {

System.out.print(question + );

answer = (new Scanner(System.in)).next().charAt(0);

if (answer < A || answer > E) { throw new InvalidMCQException(Invalid MCQ answer); }

}

}

class TFQ { String question; char answer;

public TFQ(String question) { this.question = question;

}

void getAnswer() {

System.out.print(question + );

answer = (new Scanner(System.in)).next().charAt(0);

if (answer != T && answer != F) { throw new InvalidTFQException(Invalid TFQ answer); }

}

}

In particular, an invalid answer to any of the questions will cause an exception (either InvalidMCQException or InvalidTFQException) to be thrown.

class InvalidMCQException extends IllegalArgumentException { public InvalidMCQException(String mesg) { super(mesg);

}

}

class InvalidTFQException extends IllegalArgumentException { public InvalidTFQException(String mesg) { super(mesg);

}

}

By employing the various object-oriented design principles, design a more general question-answer class QA that can take the place of both MCQ and TFQ types of questions (and possibly more in future, each with their own type of exceptions).