AQA A-Level Computer Science: Programming Paradigms — 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 available free from your board.

Related: Fundamentals of programming study guide | Fundamentals of programming revision notes

These questions cover section 4.1.2 Programming paradigms of the AQA AS and A-level Computer Science specification (7517), Version 1.6: 4.1.2.1 the procedural and object-oriented paradigms, 4.1.2.2 procedural-oriented programming (structured design and hierarchy charts) and 4.1.2.3 object-oriented programming (classes, objects, instantiation, encapsulation, inheritance, aggregation, composition, polymorphism, overriding, design principles and class diagrams). Code is written in Python-style pseudocode; answer in the language your school uses. For data types, selection, iteration and subroutines, use the Fundamentals of programming practice questions.

What does 4.1.2 require?

Section	You must be able to
4.1.2.1	Describe the characteristics of the procedural and object-oriented paradigms
4.1.2.2	Explain the structured approach, draw and use hierarchy charts, and explain the approach’s advantages
4.1.2.3	Explain class, object, instantiation, encapsulation, inheritance, aggregation, composition, polymorphism and overriding; explain why OOP is used and the three design principles; write OOP code with abstract, virtual and static methods and public, private and protected specifiers; draw and interpret class diagrams

Questions

1. Describe two characteristics of the procedural programming paradigm. [2]

2. A program to run a school library is to be designed using a structured approach.

(a) Explain what is meant by the structured approach to program design. [2] (b) The top-level task “Library system” is broken down into “Manage members”, “Manage loans” and “Produce reports”. “Manage loans” is broken down into “Issue book”, “Return book” and “Calculate fine”. Describe how this would be shown on a hierarchy chart. [3] (c) Explain three advantages of the structured approach. [3]

3. Define the following terms: (a) class; (b) object; (c) instantiation. [3]

4. Study this class definition.

CLASS BankAccount
    PRIVATE balance
    PRIVATE accountNumber
    PUBLIC PROCEDURE New(number)
        accountNumber = number
        balance = 0
    ENDPROCEDURE
    PUBLIC PROCEDURE Deposit(amount)
        IF amount > 0 THEN
            balance = balance + amount
        ENDIF
    ENDPROCEDURE
    PUBLIC FUNCTION GetBalance()
        RETURN balance
    ENDFUNCTION
ENDCLASS

(a) Identify the constructor. [1] (b) Explain how this class shows encapsulation, and why this is useful. [3] (c) Write a statement that creates an object of this class with account number 4471 and assigns a reference to it to a variable called acc. [1] (d) After the statements acc.Deposit(50), acc.Deposit(-20) and acc.Deposit(30), state the value returned by acc.GetBalance(). [1]

5. SavingsAccount inherits from BankAccount. It adds a private attribute interestRate and a public method AddInterest().

(a) Explain what is meant by inheritance. [2] (b) balance is declared private in BankAccount. Explain why AddInterest() cannot change balance directly, and state the specifier that would allow it. [2] (c) Draw (or describe) the class diagram for the two classes, showing the inheritance arrow and access specifiers. [3]

6. A game has a class Shape with a method Area(). Classes Circle and Rectangle inherit from Shape, and each has its own Area() method.

(a) Explain what is meant by overriding. [1] (b) A list contains a Circle with radius 2 and a Rectangle 3 by 5. The program loops through the list calling Area() on each. Explain how this demonstrates polymorphism, and calculate the value returned for each object (use pi = 3.14). [4] (c) Shape.Area() has no body and must be overridden. State the type of method Area() is in Shape, and the consequence for creating Shape objects. [2]

7. Explain the difference between a virtual method and a static method. [2]

8. A Car object contains four Wheel objects that are created when the Car is created and destroyed with it. A Garage object holds references to Car objects that exist independently.

(a) Identify which relationship is composition and which is aggregation. Explain your answer. [3] (b) State how each relationship is shown on a UML class diagram. [2]

9. Explain what is meant by each object-oriented design principle: (a) encapsulate what varies; (b) favour composition over inheritance; (c) program to interfaces, not implementation. [3]

10. Explain three reasons why the object-oriented paradigm is used for large software projects. [3]

11. In a class diagram, an attribute is written as # speed : Integer and a method as + Accelerate(amount : Integer). State what # and + mean. [2]

Answers

1. Any two [1 each]: the program is a sequence of instructions executed in order; it is broken into procedures / functions (subroutines) that are called; data and the code that acts on it are kept separate; it uses selection, iteration and sequence (structured constructs); variables can be local or global.

2. (a) The problem is broken down (top-down / stepwise refinement) into smaller sub-problems / modules [1], each solved by a subroutine using sequence, selection and iteration, with clear interfaces (parameters) [1]. (b) “Library system” is a single box at the top [1]; “Manage members”, “Manage loans” and “Produce reports” are boxes on the next level, joined by lines to the top box [1]; “Issue book”, “Return book” and “Calculate fine” are on the level below, joined only to “Manage loans” [1]. (c) Any three [1 each]: modules can be written and tested independently, so errors are easier to find; tasks can be shared between programmers; modules can be reused in other programs; programs are easier to read, understand and maintain; a change to one module is less likely to affect others.

3. (a) A class is a template / blueprint that defines the attributes (properties) and methods common to a type of object [1]. (b) An object is an instance of a class, with its own values for the attributes [1]. (c) Instantiation is creating an object from a class, using a constructor [1].

4. (a) New(number) [1]. (b) The attributes balance and accountNumber are private [1], so they can only be accessed through the public methods Deposit and GetBalance [1]; this protects the data from invalid changes (for example Deposit rejects negative amounts) / hides implementation, so the class can change internally without affecting other code [1]. (c) acc = NEW BankAccount(4471) (accept language-equivalent, for example acc = BankAccount(4471)) [1]. (d) 0 + 50 = 50; -20 is rejected; 50 + 30 = 80 [1].

5. (a) A subclass (child) takes on the attributes and methods of a superclass (parent) [1], and can add its own attributes and methods or override inherited ones [1]. (b) Private members can be accessed only within the class that declares them, not by subclasses [1]; declaring balance as protected would allow the subclass to access it [1]. (c) Two class boxes, each with name, attributes and methods sections [1]; an arrow with an unfilled (hollow) triangle head pointing from SavingsAccount to BankAccount [1]; private attributes shown with - (- balance, - accountNumber, - interestRate) and public methods with + (+ Deposit(amount), + GetBalance(), + AddInterest()) [1].

6. (a) A subclass provides its own version of a method inherited from its superclass, with the same name (and parameters) [1]. (b) The same method call Area() is used for every object, but each object responds with its own class’s version [1]; the correct version is chosen at run time according to the object’s class [1]. Circle: 3.14 x 2 x 2 = 12.56 [1]. Rectangle: 3 x 5 = 15 [1]. (c) An abstract method [1]; Shape becomes an abstract class, so Shape objects cannot be instantiated (only objects of its subclasses) [1].

7. A virtual method has an implementation in the superclass that a subclass may override [1]. A static method belongs to the class itself rather than any object, so it can be called without instantiating an object (and cannot access an object’s attributes) [1].

8. (a) Car–Wheel is composition [1], because the wheels are created and destroyed with the car / cannot exist without it [1]. Garage–Car is aggregation, because the cars exist independently of the garage [1]. (b) Composition: a line with a black (filled) diamond at the containing class (Car) [1]. Aggregation: a line with a white (hollow) diamond at the containing class (Garage) [1].

9. (a) Identify the parts of a program likely to change and keep them separate (encapsulated) from the parts that stay the same, so changes affect less code [1]. (b) Build classes by containing objects of other classes rather than by deep inheritance hierarchies, which is more flexible and avoids tight coupling to a parent class [1]. (c) Code should depend on the methods an object offers (its interface), not on how a particular class implements them, so implementations can be swapped [1].

10. Any three [1 each]: classes model real-world entities, making design easier to understand; encapsulation protects data and reduces unintended side effects; inheritance and composition allow code reuse; classes can be developed and tested independently by different programmers; programs are easier to maintain and extend, as new subclasses can be added without changing existing code (polymorphism).

11. # means protected: accessible in the class and its subclasses [1]. + means public: accessible from outside the class [1].

Where marks are usually lost

• Defining a class and an object as the same thing.
• Saying private attributes are “hidden from the user” without saying they are only accessible through methods.
• Confusing the black diamond (composition) and white diamond (aggregation).
• Saying a subclass can access private attributes of its superclass.
• Describing polymorphism without saying the same method call gives different behaviour depending on the object.
• Drawing hierarchy chart lines between boxes on the same level.

Frequently asked questions

Will I have to program to an interface in the exam? No. The specification says students will not be explicitly tested on programming to interfaces, or required to program to interfaces in any practical exam, though they should be aware of the principle.

What is the difference between overriding and polymorphism? Overriding is a subclass redefining an inherited method. Polymorphism is the ability to call that method on objects of different classes and get each class’s own behaviour.

Which class diagram features are examined? Single inheritance, composition (black diamond), aggregation (white diamond), and public (+), private (-) and protected (#) specifiers.

Official syllabus

AQA AS and A-level Computer Science (7516/7517) specification, Version 1.6 (31 July 2023), section 4.1 Fundamentals of programming, 4.1.2 Programming paradigms (4.1.2.1 to 4.1.2.3).