Class Creation: Unit Review (Computer Science A Unit 3)

Read the big ideas and the method table, then try the seven questions without notes; each combines two or more topics of Unit 3.

These are original Marlbridge practice questions, not past exam questions, with fictional scenarios. The rubrics are a suggested Marlbridge rubric, not official scoring. No calculator is allowed. Every code fragment was compiled and run, and the stated outputs are the real ones.

Big ideas of the unit

• Abstraction manages complexity. A class gives data a name (attributes) and processes a name (methods); parameters generalise a method, and its internals can change if its signature and behaviour do not (3.1).
• Programs affect people. Reliability means performing as expected under stated conditions; programs can have beneficial, harmful and unintended effects; reused code carries legal conditions (3.2).
• A class has a header, private instance variables, public constructors and methods. private hides, public exposes, and each object has its own copy of each instance variable (3.3).
• Constructors set the initial state. With no constructor Java supplies a no-argument one and defaults 0, 0.0, false, null; a mutable parameter should be copied (3.4).
• Methods are behaviours. A non-void method returns one value; return ends the method; primitive parameters are copies (3.5).
• Object parameters are copies of references. A method can change the object, not the caller’s variable; returned objects are the same objects (3.6).
• static means one per class. Class methods cannot use instance variables without an instance; final means unchangeable (3.7).
• Scope controls names. Locals and parameters exist only in their block and can hide instance variables (3.8); this is the current object (3.9).

Key relationships and methods

Question 1 (multiple choice · mixed)

public class Lamp
{
    private boolean on;
    private int watts;

    public Lamp(boolean on, int watts)
    {
        on = on;
        this.watts = watts;
    }

    public boolean isOn() { return on; }
    public int getWatts() { return watts; }
}

What is printed by Lamp l = new Lamp(true, 40); System.out.println(l.isOn() + " " + l.getWatts());?

• (A) true 40
• (B) false 40
• (C) true 0
• (D) false 0

Question 2 (multiple choice · mixed)

public class Gauge
{
    private int reading;
    public Gauge(int start) { reading = start; }
    public void add(int n) { reading += n; }
    public int getReading() { return reading; }
}

public class Mixer
{
    public int run(Gauge g, int n)
    {
        g.add(n);
        n = n * 2;
        g = new Gauge(100);
        g.add(n);
        return g.getReading();
    }
}

Gauge gg = new Gauge(5);
Mixer mx = new Mixer();
int n = 3;
int r = mx.run(gg, n);
System.out.println(r + " " + gg.getReading() + " " + n);

What is printed?

• (A) 106 8 3
• (B) 106 108 6
• (C) 106 5 3
• (D) 8 8 6

Question 3 (multiple choice · mixed)

public class Member
{
    private static int total = 0;
    private int id;

    public Member()
    {
        total++;
        id = total;
    }

    public int getId() { return id; }
    public static int getTotal() { return total; }
}

Member m1 = new Member();
Member m2 = new Member();
Member m3 = new Member();
System.out.println(m1.getId() + " " + m3.getId() + " " + Member.getTotal());

What is printed?

• (A) 1 1 3
• (B) 3 3 3
• (C) 1 3 3
• (D) 1 3 1

Question 4 (multiple choice · mixed)

public class Flag
{
    private int colour;
    public Flag(int c) { colour = c; }
    public void setColour(int c) { colour = c; }
    public int getColour() { return colour; }
}

public class Banner
{
    private Flag flag;
    public Banner(Flag f) { flag = f; }
    public int shown() { return flag.getColour(); }
}

Flag f = new Flag(1);
Banner bn = new Banner(f);
f.setColour(9);
System.out.println(bn.shown());

What is printed, and what change would make bn.shown() stay 1?

• (A) 1; no change is needed.
• (B) 9; the Banner constructor should store a copy: flag = new Flag(f.getColour());
• (C) 9; Flag should be declared static.
• (D) 1; the Flag constructor should be declared private.

Question 5 (multiple choice · mixed)

A designer writes: “Our Account class keeps a balance and gives the rest of the program deposit and withdraw methods. We can rewrite how the balance is stored next year without changing any code that calls those methods.” Which two Unit 3 ideas does this describe?

• (A) Data abstraction hiding the representation, and procedural abstraction preserving the signature and behaviour
• (B) Class variables and the this keyword
• (C) Default constructors and unintended effects
• (D) Scope and open-source licensing

Question 6 (constructed response · mixed)

A cafe issues vouchers. Write a complete class Voucher from this specification.

• Each voucher has a value in pounds (an int), a code and a flag for whether it has been redeemed.
• The first voucher issued gets code 1001, the second 1002, and so on; the class can report how many vouchers have been issued.
• The constructor takes the value.
• redeem(int bill) returns the amount still to pay. If the voucher has already been redeemed, it returns bill unchanged. Otherwise it marks the voucher redeemed, then returns 0 if the value covers the bill, or bill - value if not.
• worthMoreThan(Voucher other) returns whether this voucher’s value is greater than the other’s.
• getCode() and isRedeemed() return the code and flag.

Then state the output of:

Voucher v1 = new Voucher(20);
Voucher v2 = new Voucher(50);
System.out.println(v1.getCode() + " " + v2.getCode() + " " + Voucher.getIssued());
System.out.println(v1.redeem(35) + " " + v1.redeem(35) + " " + v2.redeem(35) + " " + v2.isRedeemed());
System.out.println(v2.worthMoreThan(v1) + " " + v1.worthMoreThan(v2));

public class Voucher
{
    private static int issued = 0;
    private int value;
    private int code;
    private boolean redeemed;

    public Voucher(int value)
    {
        this.value = value;
        issued++;
        code = 1000 + issued;
        redeemed = false;
    }

    public int redeem(int bill)
    {
        if (redeemed)
        {
            return bill;
        }
        redeemed = true;
        if (value >= bill)
        {
            return 0;
        }
        return bill - value;
    }

    public int getCode() { return code; }

    public boolean isRedeemed() { return redeemed; }

    public static int getIssued() { return issued; }

    public boolean worthMoreThan(Voucher other)
    {
        return this.value > other.value;
    }
}

Question 7 (constructed response · mixed)

A savings app uses this method to say how many whole months are needed to reach a goal when saving a fixed amount each month. The stated conditions are that goal is 0 or more and perMonth is greater than 0.

public static int monthsToSave(int goal, int perMonth)
{
    return (goal + perMonth - 1) / perMonth;
}

(a) Give four test calls that form a variety of conditions within the stated conditions, with the value each returns.

(b) Give one call that is within the stated conditions and returns a wrong value, and explain why.

(c) Describe one social or ethical impact of a savings app that gives users wrong answers.

How did you do?

• Mostly secure: move on to the Unit 4 study guides, starting with Ethical and Social Issues Around Data Collection.
• Missed Questions 1 or 5: reread 3.8, 3.9 and 3.1.
• Missed Questions 2, 3 or 4: reread 3.5, 3.6, 3.7 and 3.4.
• Missed Questions 6 or 7: rewrite a class from a specification, then see 3.2 and the Unit 3 diagnostic.