Constructors: Practice Questions (Computer Science A 3.4)

These are original Marlbridge practice questions, not past exam questions. All scenarios are fictional. Mark points are a suggested Marlbridge rubric, not an official scoring guideline. No calculator is allowed. The classes Player, Defaults, Seat, BookingAlias and BookingCopy are the ones printed in the study guide. All code was compiled and run.

Question 1 (multiple choice · foundation)

What is the main purpose of a constructor?

• (A) To set the initial state of a new object
• (B) To delete an object when it is no longer needed
• (C) To return the value of an instance variable
• (D) To compare two objects of the same class

Question 2 (multiple choice · foundation)

A class declares private int count; private double average; private boolean ready; private String label; and has no constructor. Its getters return each value. What is printed by the code below?

Defaults d = new Defaults();
System.out.println(d.getCount() + " " + d.getAverage() + " " + d.isReady() + " " + d.getLabel());

• (A) 0 0 false ""
• (B) 0 0.0 false null
• (C) 0 0.0 true null
• (D) The code does not compile because there is no constructor.

Question 3 (multiple choice · core)

A class Player has these two constructors:

public Player(String playerName)
public Player(String playerName, int startingLives)

Which statement does not compile?

• (A) Player a = new Player("Asha");
• (B) Player b = new Player("Ben", 5);
• (C) Player c = new Player();
• (D) Player d = new Player("Cy", 2);

Question 4 (multiple choice · core)

With the Player class from the study guide (the one-parameter constructor sets lives to 3), what is printed?

Player p1 = new Player("Asha");
Player p2 = new Player("Ben", 5);
System.out.println(p1.toLine() + " " + p2.toLine());

• (A) Asha:5 Ben:5
• (B) Asha:3 Ben:3
• (C) Asha:0 Ben:5
• (D) Asha:3 Ben:5

Question 5 (multiple choice · core)

Using Seat, BookingAlias and BookingCopy from the study guide, what is printed?

Seat original = new Seat(4, 12);
BookingAlias a = new BookingAlias(original);
BookingCopy c = new BookingCopy(original);
original.setNumber(99);
System.out.println(a.seatNumber() + " " + c.seatNumber());

• (A) 12 12
• (B) 99 99
• (C) 12 99
• (D) 99 12

Question 6 (multiple choice · core)

A programmer writes this class and expects new Cup(250, "red") to work.

public class Cup
{
    private int millilitres;
    private String colour;

    public void Cup(int ml, String c)
    {
        millilitres = ml;
        colour = c;
    }
}

Which statement is correct?

• (A) It works; void is optional in a constructor header.
• (B) It does not compile at the call, because void makes Cup an ordinary method and the class has only the default constructor.
• (C) It compiles, but millilitres and colour stay at their defaults.
• (D) It does not compile because a constructor cannot have parameters.

Question 7 (constructed response · core)

Write a class Ticket with instance variables event (a String), price (a double) and used (a boolean).

• One constructor takes the event name and the price.
• A second constructor takes only the event name and sets the price to 10.0.
• In both, used starts as false.
• A method describe() returns the event, price and used flag separated by single spaces.

Give the output for new Ticket("Fair", 12.5).describe() and new Ticket("Gala").describe().

public class Ticket
{
    private String event;
    private double price;
    private boolean used;

    public Ticket(String eventName, double ticketPrice)
    {
        event = eventName;
        price = ticketPrice;
        used = false;
    }

    public Ticket(String eventName)
    {
        event = eventName;
        price = 10.0;
        used = false;
    }

    public String describe()
    {
        return event + " " + price + " " + used;
    }
}

Question 8 (constructed response · core)

A class Roster is meant to hold a Seat given to its constructor, and its code must prevent the original Seat from changing the roster’s seat later.

public class Roster
{
    private Seat seat;

    public Roster(Seat s)
    {
        seat = s;
    }

    public int seatNumber()
    {
        return seat.getNumber();
    }
}

(a) Explain what is wrong with the constructor.

(b) Rewrite the constructor so that it works as intended.

(c) State what seatNumber() returns after Seat x = new Seat(2, 7); Roster r = new Roster(x); x.setNumber(40); for the corrected class.

public Roster(Seat s)
{
    seat = new Seat(s.getRow(), s.getNumber());
}

Question 9 (constructed response · stretch)

Describe, in order, what happens when Player p = new Player("Dee", 4); executes, and explain why p does not hold the object itself.

How did you do?

• Questions 1 to 6 right and the rubrics mostly met: move on to the Methods: How to Write Them study guide.
• Missed Questions 2 or 3: reread “The default constructor and default values” in the study guide.
• Missed Questions 4 or 6: reread “Constructor parameters” and the misconceptions list.
• Missed Questions 5 or 8: redo Worked example 2, then tick the topic checklist.