Object Creation and Storage (Instantiation): Practice Questions (Computer Science A 1.13)

These are original Marlbridge practice questions, not past exam questions. All code is invented for these questions and was compiled and run before publication. The rubrics are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. No calculator is allowed on the exam, so work everything by hand. Code fragments are assumed to be inside a main method. Most questions use this class.

class Box {
    private int width;
    private int height;

    public Box() {
        width = 1;
        height = 1;
    }

    public Box(int side) {
        width = side;
        height = side;
    }

    public Box(int w, int h) {
        width = w;
        height = h;
    }

    public int area() {
        return width * height;
    }
}

Question 1 (multiple choice · foundation)

Which statement about a constructor is true?

• (A) It has the return type void
• (B) It has the same name as the class and is called with new to create an object
• (C) It is called with the class name and the dot operator, like a class method
• (D) It can only be written once in a class

Question 2 (multiple choice · foundation)

Which statement creates a Box with width 3 and height 5 and stores a reference to it in the variable c?

• (A) Box c = Box(3, 5);
• (B) Box c = new Box(5, 3);
• (C) Box c = new Box(3, 5);
• (D) new Box c(3, 5);

Question 3 (multiple choice · core)

What is the signature of the constructor public Box(int w, int h)?

• (A) Box(int, int)
• (B) Box(w, h)
• (C) public Box(int w, int h)
• (D) void Box(int, int)

Question 4 (multiple choice · core)

A class has the constructors Box(), Box(int) and Box(int, int). Which of the following statements would not compile?

• (A) Box d = new Box(2, 3, 4);
• (B) Box a = new Box();
• (C) Box b = new Box(4);
• (D) Box c = new Box(2, 3);

Question 5 (multiple choice · core)

What is printed by the following code segment?

Box b = new Box(4);
System.out.println(b.area());

• (A) 4
• (B) 8
• (C) 16
• (D) 1

Question 6 (multiple choice · core)

Consider the following declaration.

Box d = null;

Which statement is correct?

• (A) d refers to a Box whose width and height are both 0
• (B) The statement does not compile, because a Box variable cannot hold null
• (C) d holds the int 0
• (D) d refers to no object

Question 7 (multiple choice · core)

A class Wallet is defined as follows. What is printed by the code segment after it?

class Wallet {
    private int coins;

    public Wallet(int start) {
        start = start + 5;
        coins = start;
    }

    public int getCoins() {
        return coins;
    }
}

int n = 10;
Wallet w = new Wallet(n);
System.out.println(n + " " + w.getCoins());

• (A) 10 10
• (B) 10 15
• (C) 15 15
• (D) 15 10

Question 8 (writing statements · core)

Using the Box class, write statements that:

(a) create a Box 5 wide and 2 high and store the reference in a variable shelf;

(b) create a square Box with side 6 and store the reference in tile;

(c) declare a Box variable spare that currently refers to no object;

(d) state the values of shelf.area() and tile.area().

Box shelf = new Box(5, 2);
Box tile = new Box(6);
Box spare = null;

Question 9 (identifying constructors · core)

For each statement, say which constructor is called, and give the width and height of the new object. If the statement does not compile, say why.

(a) new Box() (b) new Box(8) (c) new Box(2, 9) (d) new Box(2, 9, 4) (e) new Box(1.5)

Question 10 (order of execution · stretch)

Trace the program and state the output.

class Logger {
    public Logger() {
        System.out.println("L0");
    }

    public Logger(int n) {
        System.out.println("L1:" + n);
    }
}

public class Run {
    public static void main(String[] args) {
        System.out.println("start");
        Logger a = new Logger();
        Logger b = new Logger(7);
        System.out.println("end");
    }
}

start
L0
L1:7
end

How did you do?

• Questions 1 to 4: re-read “Making an object” and “Constructor signatures and overloading” in the study guide.
• Questions 5, 7 and 10: trace the constructor call, then return to the line after it.
• Questions 6 and 8: revisit “Creating objects and storing references”.
• Question 9: count and type-check the arguments against each signature.

Then try the checklist.