CAIE 9618 · Paper 4 · Topic 4.1.3

OOP Inheritance
& Polymorphism

Superclass · Subclass · INHERITS · Method Overriding · Polymorphism · Abstract Class

CSZone Cambridge International AS & A Level Computer Science 9618

Inheritance

Subclass Inherits from Superclass

// Superclass (base class)
CLASS Vehicle
PRIVATE Speed : REAL
PRIVATE Fuel : STRING

PUBLIC PROCEDURE NEW(s:REAL, f:STRING)
Speed ← s
Fuel ← f
ENDPROCEDURE

PUBLIC PROCEDURE Move()
OUTPUT "Vehicle moving at ", Speed
ENDPROCEDURE
ENDCLASS

// Subclass (derived class)
CLASS Car INHERITS Vehicle
PRIVATE NumDoors : INTEGER

PUBLIC PROCEDURE NEW(s:REAL, f:STRING, d:INTEGER)
CALL SUPER.NEW(s, f)
NumDoors ← d
ENDPROCEDURE
ENDCLASS

Vehicle (superclass)
Speed, Fuel, Move()

▲

Car (subclass)
+ NumDoors

WHAT IS INHERITED?

All PUBLIC methods and attributes from the superclass

PRIVATE attributes of the superclass are inherited but not directly accessible — must use getters/setters

Subclass can add new attributes and methods

Subclass can override inherited methods

CAIE keyword: INHERITS. Call superclass constructor: CALL SUPER.NEW()

Polymorphism & Method Overriding

Same Name, Different Behaviour

Polymorphism means "many forms". A subclass can override a method from its superclass to provide its own implementation. The correct version is selected at runtime based on the actual object type.

// Superclass method
CLASS Animal
PUBLIC PROCEDURE MakeSound()
OUTPUT "..."
ENDPROCEDURE
ENDCLASS

// Dog overrides MakeSound
CLASS Dog INHERITS Animal
PUBLIC PROCEDURE MakeSound()
OUTPUT "Woof" // override
ENDPROCEDURE
ENDCLASS

// Cat overrides MakeSound
CLASS Cat INHERITS Animal
PUBLIC PROCEDURE MakeSound()
OUTPUT "Meow" // override
ENDPROCEDURE
ENDCLASS

WHY IS THIS USEFUL?

Code that processes a list of Animal objects can call .MakeSound() on each — the correct version runs automatically based on object type

Adding a new animal type (e.g. Bird) requires no changes to existing processing code

Reduces code duplication and improves maintainability

OVERRIDE vs OVERLOAD

Override — subclass redefines a method with the SAME signature from the superclass

Overload — same method name, DIFFERENT parameter list in the SAME class (not in CAIE 9618 pseudocode)

Exam Practice

Cambridge-style questions

Question 1

A class Shape has a method CalculateArea() that returns 0. Two subclasses Rectangle and Circle each override CalculateArea() with their own implementations. Explain the concept being demonstrated and state one benefit. [3]

1

This demonstrates polymorphism — the same method name (CalculateArea) has different implementations in different subclasses.

1

Each subclass overrides the inherited CalculateArea() method to provide its own specific formula (e.g. length × width for Rectangle; π × r² for Circle).

1

Benefit: code that processes a collection of Shape objects can call CalculateArea() on each — the runtime selects the correct version automatically, so no if-else logic is needed. Adding new shape types requires no changes to existing code (open-closed principle).

Common Mistakes

Don't lose easy marks

1

Confusing inheritance and polymorphism — inheritance is about reusing code from a superclass; polymorphism is about the same method name producing different behaviours in different subclasses. They are related but distinct concepts.

2

Forgetting to call CALL SUPER.NEW() in the subclass constructor — in CAIE pseudocode the subclass constructor must explicitly call the superclass constructor to initialise the inherited attributes. Omitting this leaves inherited attributes uninitialised.

3

Saying "the subclass gets a copy of the superclass code" — inheritance is not copying. The subclass IS-A superclass (e.g. Car IS-A Vehicle). The subclass object contains the superclass attributes and can use superclass methods directly — it is a single unified object.

Topic Summary — 4.1.3

What You Need to Know

INHERITANCE

CLASS Sub INHERITS Super. Subclass inherits all PUBLIC methods/attributes. Can add new members and override methods. Call SUPER.NEW() in subclass constructor. IS-A relationship.

POLYMORPHISM

Subclass overrides a superclass method with same signature. Correct version selected at runtime based on object type. Enables extensible code — add new subclasses without changing existing processing code.

KEY TERMS

Superclass/base class = parent. Subclass/derived class = child. Override = redefine same method. INHERITS keyword. SUPER.NEW() to call parent constructor. IS-A vs HAS-A (composition).

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4.2.1

UML & Class Design

Class Diagrams · Associations · Multiplicity

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