Class Variables and Methods: Practice Questions (Computer Science A 3.7)

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 Member, Tally and Shop are printed in the study guide. All code was compiled and run.

Question 1 (multiple choice · foundation)

What is true of a class variable declared with static?

(A) Each object of the class has its own copy.
(B) All objects of the class share a single copy.
(C) It cannot change after it is first set.
(D) It can only be used inside constructors.

Answer and explanation

Answer: (B). A class variable belongs to the class, with one shared copy.

(A) describes an instance variable.
(C) describes final.
(D) is false: methods can use it too.

Question 2 (multiple choice · core)

Using Member from the study guide, what is printed?

Member a = new Member("Ana");
Member b = new Member("Bo");
Member c = new Member("Cy");
System.out.println(c.getId() + " " + Member.getCount());

Assume these are the first Member objects created in the program.

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

Answer and explanation

Answer: (B). The ids go 1, 2, 3, so c has id 3. nextId is then 4 and getCount() returns nextId - 1, which is 3. The compiled program printed 3 3.

(A) gives the first id to c.
(C) reports nextId (4) instead of nextId - 1.
(D) swaps both values.

Question 3 (multiple choice · core)

Which method in a class that has the instance variable private int id; does not compile?

(A) public int getId() { return id; }
(B) public static int total() { return count; } where count is a private static int
(C) public static int bad() { return id; }
(D) public static boolean same(Item a, Item b) { return a.id == b.id; } where the method is inside class Item and id is Item’s instance variable

Answer and explanation

Answer: (C). A class method has no instance, so it cannot use the instance variable id. The compiler reports non-static variable id cannot be referenced from a static context.

(A) is an ordinary instance method using its own object’s id.
(B) uses a class variable, which class methods may use.
(D) uses the instance variables of objects passed in, inside the class, which is allowed.

Question 4 (multiple choice · core)

Using Tally from the study guide (each hit() increases the object’s own count and the shared total; Tally.reset() sets the shared total to 0), what is printed?

Tally p = new Tally();
Tally q = new Tally();
p.hit();
p.hit();
q.hit();
Tally.reset();
q.hit();
System.out.println(p.getMine() + " " + q.getMine() + " " + Tally.getAll());

(A) 2 2 1
(B) 2 1 0
(C) 0 0 0
(D) 2 2 5

Answer and explanation

Answer: (A). p counted 2; q counted 1 and then 1 more, giving 2. The shared total went 3, then 0 after the reset, then 1. The program printed 2 2 1.

(B) forgets the last q.hit().
(C) assumes reset clears the per-object counts as well.
(D) adds the objects’ counts and ignores the reset.

Question 5 (multiple choice · core)

A class contains public static final int LIMIT = 10;. A method in the same class contains the statement LIMIT = 20;. What happens?

(A) LIMIT becomes 20 for all objects.
(B) LIMIT becomes 20 only for the object that executed the statement.
(C) The code does not compile, because a final variable cannot be changed.
(D) The statement is ignored at run time.

Answer and explanation

Answer: (C). A final variable cannot be modified. The compiler reports cannot assign a value to static final variable LIMIT.

(A) and (B) imagine the assignment succeeds.
(D) imagines a run-time rule; the error is found at compile time.

Question 6 (multiple choice · core)

Shop declares public static final double TAX_RATE = 0.25; and public static double withTax(double price) { return price * (1 + TAX_RATE); }. Which statement from another class prints the value 100.0?

(A) System.out.println(withTax(80.0));
(B) System.out.println(Shop.withTax(80.0));
(C) System.out.println(Shop.TAX_RATE * 80.0);
(D) System.out.println(new Shop().TAX_RATE + 80.0);

Answer and explanation

Answer: (B). A public class method is called with the class name. 80.0 * 1.25 is 100.0; the program printed 100.0.

(A) omits the class name, which is not allowed outside Shop.
(C) prints 20.0 (0.25 × 80.0).
(D) prints 80.25.

Question 7 (constructed response · core)

Write a class Order for a shop.

Each order has an order number. The first order created is numbered 100, the next 101, and so on.
The class can report how many orders have been placed so far, without needing an object.
A method getOrderNumber() returns an order’s number.

State the output of the code that creates three orders x, y, z and prints their numbers and the total count.

Answer and explanation

public class Order
{
    private static int ordersPlaced = 0;
    private int orderNumber;

    public Order()
    {
        orderNumber = 100 + ordersPlaced;
        ordersPlaced++;
    }

    public int getOrderNumber()
    {
        return orderNumber;
    }

    public static int getOrdersPlaced()
    {
        return ordersPlaced;
    }
}

Output for x.getOrderNumber() + " " + y.getOrderNumber() + " " + z.getOrderNumber() + " " + Order.getOrdersPlaced(): 100 101 102 3. This was compiled and run.

Suggested Marlbridge rubric (6 marks).

Mark	Requirement
1	`private static int` counter initialised to 0
1	Private instance variable for the order number
1	Constructor assigns the number from the counter (100 plus the number of earlier orders)
1	Constructor increments the shared counter
1	`getOrdersPlaced` is a `public static` method returning the counter
1	Output `100 101 102 3`

Question 8 (constructed response · core)

The class below does not compile.

public class Bad
{
    private int id = 5;
    private static int total = 0;
    public static final int LIMIT = 10;

    public int getId() { return id; }

    public static int first()  { return id; }
    public static int second() { return getId(); }
    public static int third()  { LIMIT = 20; return total; }
}

(a) For each of first, second and third, explain the error.

(b) Suggest a correction for each.

Answer and explanation

(a) first uses an instance variable in a class method. second calls an instance method from a class method. third assigns to a final variable. (The use of total in third is fine.)

(b) first: change it to an instance method, or pass an object, public static int first(Bad b) { return b.id; }. second: make it an instance method, or call b.getId() on a parameter. third: delete LIMIT = 20; (or remove final if the value genuinely has to change).

The three compiler messages were confirmed by compiling.

Suggested Marlbridge rubric (6 marks). One mark for the correct explanation and one for an acceptable correction, for each of the three methods.

Question 9 (constructed response · stretch)

A programmer says, “I made nextId an instance variable and it always gives every member the id 1.”

(a) Explain why this happens.

(b) State the fix.

Answer and explanation

(a) An instance variable has a separate copy in every object, so each new object starts with its own nextId set to 1, and incrementing it affects only that object.

(b) Declare nextId as private static int nextId = 1;.

(c) With static there is one nextId for the whole class, so each constructor call reads the current value and increments the shared one. id remains an instance variable, so each object keeps the value it was given.

Suggested Marlbridge rubric (4 marks).

Mark	Requirement
1	Part (a): each object has its own copy, restarting at 1
1	Part (b): `static`
1	Part (c): one shared counter advances across all constructions
1	Part (c): `id` stays an instance variable so each object keeps its own value

How did you do?

Questions 1 to 6 right and the rubrics mostly met: move on to the Scope and Access study guide.
Missed Questions 1 or 2: reread “Class variables” in the study guide.
Missed Questions 3, 5 or 8: reread “Class methods” and Worked example 2, Part C.
Missed Questions 4, 7 or 9: redo Worked examples 1 and 2, then tick the topic checklist.