Object Oriented Techniques (OCR A-Level Computer Science): Revision Notes

Object Oriented Techniques

Overview

Object-Oriented Programming (OOP) is a programming paradigm that organises software design around objects, which represent real-world entities. OOP provides a modular approach to software development, making it easier to build, maintain, and scale complex systems. Understanding OOP concepts like classes, objects, methods, encapsulation, inheritance, and polymorphism is crucial for writing efficient and reusable code.

Classes and Objects

Class:

• A blueprint for creating objects.
• Defines the properties (attributes) and behaviours (methods) of an object.

class Car:
    def __init__(self, brand, model):
        self.brand = brand  # Attribute
        self.model = model  # Attribute

    def display_info(self):  # Method
        print(f"{self.brand} {self.model}")

Object:

An instance of a class.

my_car = Car("Toyota", "Corolla")
my_car.display_info()  # Outputs: Toyota Corolla

Attributes (Properties)

Attributes store the data or state of an object.

• Public Attributes: Can be accessed directly from outside the class.
• Private Attributes: Prefixed with _ or __ (e.g., _balance). They cannot be accessed directly outside the class.

class Account:
    def __init__(self, balance):
        self.__balance = balance  # Private attribute

Methods

Methods are functions defined within a class that describe the behaviours of an object.

• Instance Methods: Operate on instance attributes.
• Static Methods: Do not depend on instance attributes; defined using @staticmethod.
• Class Methods: Operate on class-level attributes; defined using @classmethod.

Encapsulation

Definition:

Encapsulation is the principle of restricting direct access to an object's attributes and methods to protect its integrity.

Access Modifiers:

1. Public: Accessible from anywhere.
2. Private: Accessible only within the class.

Getters and Setters:

Used to access and modify private attributes.

class Account:
    def __init__(self, balance):
        self.__balance = balance

    def get_balance(self):  # Getter
        return self.__balance

    def set_balance(self, amount):  # Setter
        if amount >= 0:
            self.__balance = amount

Inheritance

Definition:

• Inheritance allows a class (subclass) to inherit attributes and methods from another class (superclass or parent class).
• Promotes code reuse and establishes a hierarchical relationship between classes.

class Animal:
    def speak(self):
        print("Animal sound")

class Dog(Animal):  # Dog is a subclass of Animal
    def speak(self):
        print("Bark")

Superclasses and Subclasses:

• Superclass (Parent Class): The class from which other classes inherit.
• Subclass (Child Class): The class that inherits from the superclass.

Using the super() Function:

Allows access to the superclass's methods and attributes.

class Dog(Animal):
    def __init__(self, name):
        super().__init__()
        self.name = name

Polymorphism

Definition:

• Polymorphism allows objects of different classes to be treated as objects of a common superclass.
• Methods can behave differently depending on the object calling them.

class Animal:
    def speak(self):
        print("Animal sound")

class Dog(Animal):
    def speak(self):
        print("Bark")

class Cat(Animal):
    def speak(self):
        print("Meow")

animals = [Dog(), Cat()]
for animal in animals:
    animal.speak()

Bark
Meow

Interpreting Class Diagrams

Class diagrams visually represent the structure of a class, including its attributes, methods, and relationships with other classes.

+---------------------+
|      Car            |
+---------------------+
| - brand: String     |
| - model: String     |
+---------------------+
| + display_info()    |
+---------------------+

Real-World Scenario: Library System

Problem:

Design a system for managing books in a library.

Solution Using OOP:

Classes:

• Book: Represents individual books.
• Member: Represents library members.
• Library: Manages the collection of books and members. Code Implementation:

class Book:
    def __init__(self, title, author):
        self.title = title
        self.author = author

    def display(self):
        print(f"{self.title} by {self.author}")

class Library:
    def __init__(self):
        self.books = []

    def add_book(self, book):
        self.books.append(book)

    def show_books(self):
        for book in self.books:
            book.display()

# Using the system
lib = Library()
book1 = Book("1984", "George Orwell")
book2 = Book("Brave New World", "Aldous Huxley")

lib.add_book(book1)
lib.add_book(book2)
lib.show_books()

Benefits of Object-Oriented Programming

1. Modularity: Programmes can be divided into self-contained objects.
2. Reusability: Classes can be reused across different programmes.
3. Maintainability: Easier to update and debug specific parts of a programme.
4. Scalability: OOP makes it easier to manage and expand large projects.

Note Summary