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

Use this diagnostic to find which Unit 3 topics to revisit: one question per topic.

These are original Marlbridge practice questions with fictional scenarios, not past exam questions. It is not calibrated and gives no predicted score.

How to take it. Allow 30 minutes; answer everything before opening any answer. No calculator. Assume every code fragment compiles unless a question says otherwise. Every code fragment was compiled and run, and the stated outputs are the real ones.

Question 1 (multiple choice · 3.1)

The same four lines of code appear in three methods of a class. A programmer moves them into one private method with a parameter and calls it from the three places. What is the best description of the benefit?

(A) There is one place to fix or improve the code, and the method’s name says what the code does.
(B) The instance variables become class variables.
(C) The methods no longer need return types.
(D) The class no longer needs a constructor.

Answer and explanation

Answer: (A). Procedural abstraction names a process so it can be reused and changed in one place, and a parameter generalises it.

(B) is unrelated: moving code into a method does not change what kind of variable anything is.
(C) is false: the three methods keep their return types.
(D) is false: constructors are unaffected.

If you missed this: 3.1 study guide.

Question 2 (multiple choice · 3.2)

A fictional method taxDue returns the wrong amount for one valid income band but never crashes or shows an error message. Which statement is best?

(A) The method is unreliable, because it does not perform as expected under its stated conditions.
(B) The method is reliable, because it never crashes.
(C) The method is reliable, because wrong answers only matter when the program is slow.
(D) The method is unreliable only if it is open-source code.

Answer and explanation

Answer: (A). Reliability means performing as expected under stated conditions without failure. A wrong result for a valid input is a failure, even when nothing crashes.

(B) confuses “does not crash” with “correct”.
(C) mixes up speed and correctness.
(D) is irrelevant: licensing does not decide reliability.

If you missed this: 3.2 study guide.

Question 3 (multiple choice · 3.3)

public class Bad
{
    public static void main(String[] args)
    {
        Locker l = new Locker();
        l.code = 5;
    }
}

class Locker
{
    private int code;
}

What happens when this is compiled?

(A) It compiles and sets code to 5.
(B) It does not compile, because code has private access in Locker.
(C) It compiles but throws an exception when run.
(D) It does not compile, because Locker has no constructor.

Answer and explanation

Answer: (B). A private instance variable cannot be used from another class. The compiler reports code has private access in Locker, which was confirmed by compiling.

(A) would need code to be public.
(C) imagines a run-time error; this is a compile-time error.
(D) is false: a class with no constructor still gets a default one.

If you missed this: 3.3 study guide.

Question 4 (multiple choice · 3.4)

public class Gadget
{
    private int uses;
    private String name;
    private boolean armed;

    public int getUses() { return uses; }
    public String getName() { return name; }
    public boolean isArmed() { return armed; }
}

What is printed by Gadget g = new Gadget(); System.out.println(g.getUses() + " " + g.getName() + " " + g.isArmed());?

(A) 0 "" false
(B) 0 null false
(C) 0 null true
(D) The code does not compile because the class has no constructor.

Answer and explanation

Answer: (B). With no constructor, Java supplies a default one and the instance variables take default values: 0, null and false. The compiled program printed 0 null false.

(A) treats the default of a String as the empty string.
(C) makes the boolean default true.
(D) is false: the default constructor exists.

If you missed this: 3.4 study guide.

Question 5 (multiple choice · 3.5)

Piggy stores coins. save(int n) executes n = n * 2; coins += n;. take(int n) returns false and changes nothing if n > coins; otherwise it subtracts n and returns true.

Piggy p = new Piggy(10);
p.save(4);
System.out.println(p.take(30) + " " + p.getCoins());
System.out.println(p.take(15) + " " + p.getCoins());

What is printed?

(A) false 14 then true -1
(B) false 18 then true 3
(C) true 0 then false 18
(D) false 10 then true 3

Answer and explanation

Answer: (B). save(4) doubles the local n to 8 and adds it: 18 coins. take(30) fails (30 > 18) and leaves 18. take(15) succeeds and leaves 3. Output: false 18, then true 3.

(A) adds 4 instead of 8 and ignores the failed take.
(C) reverses the outcomes.
(D) forgets that save changed the coins.

If you missed this: 3.5 study guide.

Question 6 (multiple choice · 3.6)

public class Cup
{
    private int level;
    public Cup(int start) { level = start; }
    public void fill() { level += 2; }
    public int getLevel() { return level; }

    public void top(Cup c)
    {
        c.fill();
        c = new Cup(0);
        c.fill();
    }
}

Cup c = new Cup(1);
Cup d = new Cup(4);
c.top(d);
System.out.println(c.getLevel() + " " + d.getLevel());

What is printed?

(A) 1 4
(B) 3 6
(C) 1 6
(D) 1 8

Answer and explanation

Answer: (C). The parameter refers to the same cup as d, so c.fill() inside top raises d from 4 to 6. Then the parameter is reassigned to a new cup, which has no effect on the caller’s d. The cup called c in main is not touched: 1. The compiled program printed 1 6.

(A) forgets that the method can change the object.
(B) fills the cup the method was called on instead of the argument.
(D) assumes the new cup’s fill() also affects d.

If you missed this: 3.6 study guide.

Question 7 (short answer · 3.7)

public class Sticker
{
    private static int made = 0;
    private int serial;

    public Sticker()
    {
        made++;
        serial = made * 10;
    }

    public int getSerial() { return serial; }
    public static int getMade() { return made; }
}

Sticker s1 = new Sticker();
Sticker s2 = new Sticker();
Sticker s3 = new Sticker();
System.out.println(s1.getSerial() + " " + s2.getSerial() + " " + s3.getSerial() + " " + Sticker.getMade());

(a) State the output.

(b) Explain why the three stickers have different serial numbers although the same constructor ran each time.

Answer and explanation

(a) 10 20 30 3.

(b) made is a class variable, so there is one copy shared by all stickers. Each constructor call increments it before using it, so the three stickers see 1, 2 and 3 and store 10, 20 and 30 in their own instance variable serial.

Suggested Marlbridge rubric (3 marks): 1 for the correct output; 1 for saying made is shared (static); 1 for saying serial is separate in each object.

If you missed this: 3.7 study guide.

Question 8 (multiple choice · 3.8)

public class Dial
{
    private int level = 1;

    public void set(int level)
    {
        level = level;
    }

    public int getLevel() { return level; }
}

What is printed by Dial di = new Dial(); di.set(7); System.out.println(di.getLevel());?

(A) 7
(B) 0
(C) 1
(D) The code does not compile.

Answer and explanation

Answer: (C). The parameter level hides the instance variable, so level = level; assigns the parameter to itself. The instance variable stays 1. The compiled program printed 1.

(A) is what the programmer intended.
(B) forgets that the variable was initialised to 1.
(D) is false: the code compiles with no warning.

If you missed this: 3.8 study guide.

Question 9 (short answer · 3.9)

A Duo object has a private Duo instance variable partner and a method getPartner().

(a) Write a method pairWith(Duo other) that makes this object and other each other’s partner, using this.

(b) State the output of the code below.

Duo a = new Duo();
Duo b = new Duo();
a.pairWith(b);
System.out.println((a.getPartner() == b) + " " + (b.getPartner() == a) + " " + (a.getPartner() == a));

Answer and explanation

(a)

public void pairWith(Duo other)
{
    this.partner = other;
    other.partner = this;
}

(The second line is allowed because other is a Duo and the code is inside Duo.)

(b) true true false. a’s partner is b; b’s partner is the object this stood for, which is a; and a’s partner is not a. This was confirmed by running the code.

Suggested Marlbridge rubric (4 marks): 1 for this.partner = other; 1 for other.partner = this; 1 for the first two true values; 1 for false at the end.

If you missed this: 3.9 study guide.

What next?

Missed Question 1 or 2: review 3.1 and 3.2 (design, abstraction and impact).
Missed Question 3, 4 or 5: review 3.3, 3.4 and 3.5 (class anatomy, constructors and methods).
Missed Question 6, 7, 8 or 9: review 3.6, 3.7, 3.8 and 3.9.
Secure on all nine? Try the Unit 3 review, which combines topics.