Object-Oriented Programming

Object-Oriented Programming (OOP) is a programming paradigm that organises code around objects — entities that combine data (attributes) and behaviour (methods). OOP models real-world entities, making complex systems easier to design and maintain.

Classes and Objects

Class

A class is a blueprint or template that defines the attributes and methods that all objects of that type will have. A class defines the structure but does not represent a specific instance.

Object

An object is a specific instance of a class. Multiple objects can be created from the same class, each with their own unique attribute values.

// AQA pseudocode class definition
CLASS Animal
    PRIVATE name : STRING
    PRIVATE species : STRING
    PRIVATE age : INTEGER

    PUBLIC PROCEDURE NEW(n : STRING, s : STRING, a : INTEGER)
        name ← n
        species ← s
        age ← a
    ENDPROCEDURE

    PUBLIC FUNCTION getName() RETURNS STRING
        RETURN name
    ENDFUNCTION

    PUBLIC PROCEDURE setAge(a : INTEGER)
        age ← a
    ENDPROCEDURE

    PUBLIC PROCEDURE speak()
        OUTPUT name & " makes a sound."
    ENDPROCEDURE
ENDCLASS

// Creating objects (instances)
myDog ← NEW Animal("Rex", "Dog", 3)
myCat ← NEW Animal("Luna", "Cat", 2)
OUTPUT myDog.getName()   // Output: Rex

Attributes and Methods

Encapsulation

Encapsulation is one of the four pillars of OOP. It is the bundling of data (attributes) and methods that operate on that data within a single class, while restricting direct access to the internal data from outside the class.

Access Modifiers

Getters and Setters

Since attributes are typically PRIVATE, access is provided through getter and setter methods:

• Getter (accessor) — a PUBLIC method that returns the value of a private attribute (e.g. getName())
• Setter (mutator) — a PUBLIC method that sets/updates the value of a private attribute, often with validation (e.g. setAge())

PUBLIC FUNCTION getAge() RETURNS INTEGER
    RETURN age
ENDFUNCTION

PUBLIC PROCEDURE setAge(a : INTEGER)
    IF a >= 0 THEN
        age ← a
    ENDIF
ENDPROCEDURE

Benefits of Encapsulation

• Data hiding / information hiding — internal implementation is hidden from external code
• Control over data — setters can validate data before updating attributes
• Reduced coupling — code outside the class doesn't depend on internal implementation details
• Easier maintenance — internal implementation can change without affecting external code (as long as the interface remains the same)

Instantiation

Instantiation is the process of creating an object from a class using the constructor (NEW). Each object is an independent instance with its own copy of the attributes.