OxfordAQA A-Level Computer Science: Object-oriented and additional programming (9645) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

This set tests section 3.9 Object-oriented and additional programming (3.9.1 to 3.9.4) of the OxfordAQA International AS and A-level Computer Science (9645) specification, Version 1.1, for International AS exams May/June 2025 onwards and International A-level exams May/June 2026 onwards. The section is International A-level only; Unit 3: Advanced Programming assesses it on screen, in C#, Python or VB.Net. Answers use Python; any of the three languages earns the same credit here.

If the ideas are new, work through the study guide or the shorter revision notes before attempting these. Other topics in this course are on the hub, /boards/oxfordaqa/a-level/computer-science/, and outcomes can be ticked off on the checklist, /checklists/oxfordaqa/a-level/computer-science/. A free diagnostic will point you to weaker areas.

Questions

1. State what is meant by instantiation and by a constructor. [2]

2. A gliding club uses this class.

class Glider:
    def __init__(self, reg, wingspan_m):
        self.reg = reg
        self.wingspan_m = wingspan_m
        self.flights = 0

    def log_flight(self):
        self.flights = self.flights + 1

g1 = Glider("G-KTRL", 15)

(a) From the code, identify one example of each: a class, an attribute, a method, an object. [4] (b) g1.log_flight() is called twice. State the value of g1.flights. [1]

3. Explain three reasons why the object-oriented paradigm is used. [3]

4. A WaterTank class has a public attribute capacity_litres, set by a constructor parameter, and level_litres, which starts at 0. Its method fill(litres) adds water. If the water would overflow, the tank is filled to capacity and the method returns the litres that did not fit; otherwise it returns 0.

(a) Write the class. [5] (b) A tank with capacity 500 is created. State the values returned by fill(320) and then fill(260). [2]

5. A RoomBooking class must store the number of guests, which must be from 1 to 6 and starts at 1.

(a) Write the attribute declaration in the constructor, a getter and a setter that uses encapsulation appropriately. [4] (b) Explain why the guest count should be private. [2]

6. Describe how Python represents public, protected and private members. [3]

7. A shipping program has this base class.

class Crate:
    def __init__(self, ref, weight_kg):
        self.ref = ref
        self._weight_kg = weight_kg

    def postage(self):
        return 4.00 + 1.50 * self._weight_kg

A ChilledCrate is a crate with an extra attribute min_temp. Its postage is the normal postage plus 6.25.

(a) Write the ChilledCrate class. [4] (b) Calculate the postage for a 5 kg chilled crate. [1] (c) Explain why _weight_kg is protected rather than private. [2]

8. Study this class diagram.

 Instrument
 # serial : String
 - hire_count : Integer
 + hire()
 + daily_rate() : Real
        △
        |
 Piano
 + tuned : Boolean
 + daily_rate() : Real

(a) Name the base class. [1] (b) State which attribute of Instrument a Piano method can use directly, and why the other cannot be used. [2] (c) State what daily_rate() appearing in the Piano box shows. [1]

9. For each pair, state whether inheritance or association fits better.

(a) Hatchback and Car [1] (b) Tournament and Referee [1] (c) Explain why association is a weaker relationship than inheritance. [2]

10. orders.txt holds one order per line as item,quantity, for example:

hinges,12
screws,250
brackets,6
washers,10
bolts,9

(a) Write a program that writes the name of every item with a quantity of 10 or more to a new file bulk.txt, one per line, then outputs how many were written. [6] (b) State the output for the file above. [1]

11. Study this subroutine.

def zig(n):
    if n <= 0:
        return ""
    if n % 2 == 0:
        return zig(n // 2) + "E"
    return zig(n - 1) + "O"

(a) Identify the base case. [1] (b) State how many recursive cases zig has. [1] (c) Trace zig(11) and state the value returned. [3] (d) Explain why zig always terminates for a positive whole number. [1]

12. heights.txt holds one whole number per line: 152, 167, 149, 171, 160.

(a) Write a subroutine read_heights(filename) that returns the values as a list of integers. [3] (b) Write a recursive function largest(values, n) that returns the largest of the first n values. It must not use a loop or a built-in maximum function. [4] (c) State the value returned by largest(values, 3) and the number of calls made, including the first. [2]

Answers

1. Instantiation is creating an object from a class [1]. A constructor is called when an object is instantiated and initialises it to a given state [1]. [2] Examiner insight: “a constructor creates the object” alone tends to miss the credit; the point the specification stresses is that it sets the starting state.

2. (a) Class: Glider [1]. Attribute: reg, wingspan_m or flights [1]. Method: log_flight or __init__ [1]. Object: g1 [1]. (b) 2 [1] Examiner insight: writing Glider as the object, or "G-KTRL" as the object, confuses the class or an argument with the instance; the object is the variable that refers to it.

3. Any three, one mark each. Each real thing is modelled as one object, so code mirrors the problem [1]. Encapsulation means data changes only through methods, so errors stay local [1]. Inheritance lets subclasses reuse tested code [1]. Also creditworthy: classes can be written, tested and changed independently, easing maintenance. Examiner insight: “it is easier” earns little on its own; each reason needs the mechanism (encapsulation, inheritance, independent classes) that makes it true.

4. (a)

class WaterTank:
    def __init__(self, capacity_litres):
        self.capacity_litres = capacity_litres
        self.level_litres = 0

    def fill(self, litres):
        space = self.capacity_litres - self.level_litres
        if litres > space:
            self.level_litres = self.capacity_litres
            return litres - space
        self.level_litres = self.level_litres + litres
        return 0

Class with constructor taking a parameter [1]; capacity set from parameter and level set to 0 [1]; space left worked out and compared [1]; overflow branch fills to capacity and returns the excess [1]; otherwise level increased and 0 returned [1]. (b) fill(320) returns 0 [1]; fill(260) returns 80, because only 180 litres fit [1]. Examiner insight: a constructor that sets level_litres from a parameter, rather than to 0, does not give the state the question describes.

5. (a)

    def __init__(self, room):
        self.room = room
        self.__guests = 1

    def get_guests(self):
        return self.__guests

    def set_guests(self, n):
        if 1 <= n <= 6:
            self.__guests = n
            return True
        return False

Private attribute starting at 1 [1]; getter returns it [1]; setter checks 1 to 6 [1]; stores only a valid value and signals the result [1]. (b) Other classes cannot set an invalid value such as 9 directly; every change goes through the validating setter [1]. How the count is stored is hidden, so it can change without affecting other classes [1]. Examiner insight: a setter that stores the value without checking it shows syntax but not appropriate encapsulation.

6. Public: no leading underscore, e.g. room [1]. Protected: one leading underscore, _room, a convention only [1]. Private: two leading underscores, __room, which Python name-mangles to _ClassName__room [1]. Examiner insight: swapping the single and double underscore is a common slip; check which one the interpreter actually enforces.

7. (a)

class ChilledCrate(Crate):
    def __init__(self, ref, weight_kg, min_temp):
        super().__init__(ref, weight_kg)
        self.min_temp = min_temp

    def postage(self):
        return super().postage() + 6.25

Inherits from Crate [1]; calls the base constructor [1]; adds min_temp [1]; overrides postage, adding 6.25 to the base result [1]. (b) 4.00 + 1.50 × 5 + 6.25 = 17.75 [1] (c) Protected lets ChilledCrate methods use _weight_kg [1] while objects of unrelated classes still cannot access it [1]. Examiner insight: copying the base formula into the subclass works, but reusing the base method through super() shows that you understand inheritance and overriding.

8. (a) Instrument [1] (b) serial, because # marks it protected [1]; hire_count is private (-), so only Instrument can use it [1]. (c) Piano overrides daily_rate() [1]. Examiner insight: name the symbol and the modifier together; “it has a hash” without “protected” is incomplete.

9. (a) Inheritance: a hatchback is a more specialised kind of car [1]. (b) Association: a tournament uses referees, but neither is a kind of the other [1]. (c) Inheritance makes the subclass a version of the base class, inheriting its members [1]; association only lets one object make use of another, and both exist independently [1]. Examiner insight: answering with the “is a” or “uses” test, in a full sentence, is clearer than naming the relationship alone.

10. (a)

count = 0
with open("orders.txt", "r") as src, open("bulk.txt", "w") as out:
    for line in src:
        item, qty = line.strip().split(",")
        if int(qty) >= 10:
            out.write(item + "\n")
            count = count + 1
print(count)

Opens orders.txt for reading [1]; opens bulk.txt for writing [1]; loops through lines and splits each at the comma [1]; converts the quantity and tests ≥ 10 [1]; writes the item name with a newline [1]; counts and outputs the total [1]. (b) 3 (hinges, screws, washers) [1] Examiner insight: comparing the text “10” with the number 10, or using > instead of ≥, drops washers and loses the condition mark.

11. (a) n <= 0, which returns an empty string [1]. (b) Two (even and odd) [1]. (c) Calls: 11 → 10 → 5 → 4 → 2 → 1 → 0 [1]. Returning: “” then “O” (from 1), “E” (2), “E” (4), “O” (5), “E” (10), “O” (11) are added in turn [1]. Result “OEEOEO” [1]. (d) Each recursive case makes n smaller (n − 1 or n // 2), so it must reach 0 [1]. Examiner insight: a trace that lists the calls but never shows values being returned back up usually earns only the first mark.

12. (a)

def read_heights(filename):
    values = []
    with open(filename, "r") as f:
        for line in f:
            values.append(int(line.strip()))
    return values

Opens the file for reading [1]; strips and converts each line, adding it to the list [1]; returns the list [1]. (b)

def largest(values, n):
    if n == 1:
        return values[0]
    rest = largest(values, n - 1)
    if values[n - 1] > rest:
        return values[n - 1]
    return rest

Base case n == 1 returns the first value [1]; recursive call on n - 1 [1]; compares values[n - 1] with that result [1]; returns the larger [1]. (c) 167 [1]; 3 calls (n = 3, 2, 1) [1]. Examiner insight: the recursive call must be on a smaller n; calling largest(values, n) again never reaches the base case.

Where marks are usually lost

• Calling the class the object, or the constructor’s arguments the attributes.
• Constructors that leave some attributes unset.
• Setters with no validation.
• A subclass that never calls the base constructor.
• Private base attributes used in subclass methods.
• Overriding under a new name, which adds a method instead.
• Missing virtual/override (C#) or Overridable/Overrides (VB.Net).
• Reading numbers from a file without stripping and converting.
• "w" used where "a" was needed.
• Recursive traces with no return phase.

Next steps

• Revision notes
• Study guide
• Course hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

OxfordAQA International AS and A-level Computer Science (9645) specification, Version 1.1, for International AS exams May/June 2025 onwards and International A-level exams May/June 2026 onwards, published by OxfordAQA. Section 3.9 Object-oriented and additional programming.