Object-Oriented Programming

Procedural programming structures code as a sequence of procedures that operate on data. Data and the code that manipulates it are kept separate.

Object-oriented programming (OOP) bundles data (attributes) and the procedures that operate on it (methods) together into objects. Programs are modelled as interactions between objects.

Aspect	Procedural	Object-Oriented
Organisation	Procedures and data separate	Data and methods bundled in objects
Reuse	Via procedure calls	Via inheritance and composition
State	Global or passed as parameters	Encapsulated inside objects
Best for	Simple, sequential tasks	Complex systems with many interacting entities

A class is a blueprint that defines the structure and behaviour of a category of objects.

An object is a specific instance of a class — it has its own copies of the attributes defined in the class.

CLASS Animal
    PRIVATE name : STRING
    PRIVATE sound : STRING

    PUBLIC PROCEDURE new(aName, aSound)
        name ← aName
        sound ← aSound
    ENDPROCEDURE

    PUBLIC FUNCTION speak()
        RETURN name + " says " + sound
    ENDFUNCTION
ENDCLASS

dog ← NEW Animal("Dog", "Woof")
cat ← NEW Animal("Cat", "Meow")
OUTPUT dog.speak()    // Dog says Woof
OUTPUT cat.speak()    // Cat says Meow

dog and cat are two distinct instances of the Animal class. Each has its own name and sound attribute values.

Encapsulation is the bundling of data (attributes) and the methods that operate on it inside a class, with controlled external access.

• Private attributes/methods: accessible only from within the class
• Public attributes/methods: accessible from outside the class

Encapsulation protects data from uncontrolled modification. External code accesses private data only through getter (accessor) and setter (mutator) methods:

CLASS BankAccount
    PRIVATE balance : REAL

    PUBLIC PROCEDURE new(initialBalance)
        balance ← initialBalance
    ENDPROCEDURE

    PUBLIC FUNCTION getBalance()
        RETURN balance
    ENDFUNCTION

    PUBLIC PROCEDURE deposit(amount)
        IF amount > 0 THEN
            balance ← balance + amount
        ENDIF
    ENDPROCEDURE
ENDCLASS

Encapsulation prevents external code from setting balance to a negative value directly. The deposit method enforces the business rule that deposits must be positive.

Inheritance allows a class (the subclass or child class) to inherit the attributes and methods of another class (the superclass or parent class), and add or override them.

CLASS Shape
    PROTECTED colour : STRING

    PUBLIC FUNCTION getColour()
        RETURN colour
    ENDFUNCTION

    PUBLIC FUNCTION area()
        RETURN 0        // Default — overridden in subclasses
    ENDFUNCTION
ENDCLASS

CLASS Rectangle INHERITS Shape
    PRIVATE width : REAL
    PRIVATE height : REAL

    PUBLIC PROCEDURE new(w, h, c)
        width ← w
        height ← h
        colour ← c
    ENDPROCEDURE

    PUBLIC FUNCTION area()
        RETURN width * height    // Overrides Shape.area()
    ENDFUNCTION
ENDCLASS

r ← NEW Rectangle(5, 3, "red")
OUTPUT r.area()         // 15
OUTPUT r.getColour()    // red (inherited from Shape)

The Rectangle class inherits getColour() from Shape and overrides area() with its own implementation.

Polymorphism means "many forms" — a method with the same name can behave differently depending on the object it is called on.

CLASS Circle INHERITS Shape
    PRIVATE radius : REAL
    ...
    PUBLIC FUNCTION area()
        RETURN 3.14159 * radius * radius  // Different from Rectangle.area()
    ENDFUNCTION
ENDCLASS

Both Rectangle and Circle have an area() method, but each computes it differently. Code that holds a list of Shape objects can call area() on each without knowing whether it is a Rectangle or Circle.

Class diagram notation shows the structure of classes and their relationships:

          ┌──────────────┐
          │    Shape     │
          │──────────────│
          │ - colour     │
          │──────────────│
          │ + getColour()│
          │ + area()     │
          └──────┬───────┘
                 │ inherits
        ┌────────┴─────────┐
        ▼                  ▼
┌───────────────┐  ┌───────────────┐
│   Rectangle   │  │    Circle     │
│───────────────│  │───────────────│
│ - width       │  │ - radius      │
│ - height      │  │───────────────│
│───────────────│  │ + area()      │
│ + area()      │  └───────────────┘
└───────────────┘

• - prefix: private attribute/method
• + prefix: public attribute/method
• Arrow from subclass to superclass: inheritance relationship

OOP is widely used for complex software systems because:

Advantage	Explanation
Encapsulation	Hides internal implementation; reduces unintended interference between components
Inheritance	Reuse code across related classes; extend existing classes without modifying them
Polymorphism	Write generic code that works with any subclass; easier to add new types
Modularity	Each class is an independent unit; easier to develop, test, and maintain
Reusability	Classes can be imported into new projects as libraries

1. Confusing a class and an object

A class is the blueprint; an object is an instance created from the blueprint. You can have many objects from one class, each with different attribute values.

2. Forgetting NEW when creating an object

dog ← Animal("Dog") is wrong in AQA pseudocode. Instantiation requires NEW: dog ← NEW Animal("Dog").

3. Treating PROTECTED and PRIVATE as identical

PRIVATE attributes are accessible only within the declaring class. PROTECTED attributes are accessible within the declaring class and any subclass. A PRIVATE attribute in a superclass is not directly accessible in a subclass — the subclass must use getters.

4. Describing polymorphism incorrectly

Polymorphism is not simply "different classes having methods with the same name." It is that the same method call on objects of different types (sharing a superclass) executes different code depending on the actual type of the object at runtime.