Objects (OOP) (OCR A-Level Computer Science): Revision Notes

Objects (OOP)

Overview

In Object-Oriented Programming (OOP), a class is a blueprint that defines the structure and behaviour of objects. An object is an instance of a class, representing a specific entity with attributes (data) and methods (functions) that define its state and behaviour. Understanding how to create and use objects is fundamental to OOP and is essential for writing organised, reusable, and modular code.

Objects in OOP

Defining and Creating Objects

• An object is a specific instance of a class, with its own values for the attributes defined in the class.
• To create an object, you call the class as though it were a function, passing in any necessary data for initialisation.
• This creates a unique instance with its own data, distinct from other instances of the same class.

class Car:
    def __init__(self, make, model):
        self.make = make
        self.model = model

# Creating objects of the Car class
car1 = Car("Toyota", "Corolla")
car2 = Car("Honda", "Civic")

Object Attributes

• Attributes represent the state of an object and are defined within the class.
• Each object has its own set of attribute values, which can vary from one instance to another.

print(car1.make)  # Output: Toyota
print(car2.make)  # Output: Honda

Object Methods

Methods are functions defined within a class that describe the behaviours of objects. They can use and modify the object's attributes.

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def introduce(self):
        print(f"Hi, I'm {self.name} and I'm {self.age} years old.")

# Creating a Person object and calling its method
person1 = Person("Alice", 30)
person1.introduce()  # Output: Hi, I'm Alice and I'm 30 years old.

Modifying Object Attributes

You can change the values of an object's attributes directly or by using methods, allowing objects to update their state during the programme's execution.

person1.age = 31  # Directly modifying an attribute
print(person1.age)  # Output: 31

Interacting with Multiple Objects

Each object operates independently, so you can create multiple instances of a class, each with its own attribute values and behaviours.

person2 = Person("Bob", 25)
person1.introduce()  # Output: Hi, I'm Alice and I'm 31 years old.
person2.introduce()  # Output: Hi, I'm Bob and I'm 25 years old.

Example Programme: Objects in Action

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

    def get_description(self):
        return f"{self.title} by {self.author}, published in {self.year}"

# Creating Book objects
book1 = Book("1984", "George Orwell", 1949)
book2 = Book("To Kill a Mockingbird", "Harper Lee", 1960)

# Accessing attributes and calling methods on objects
print(book1.get_description())  # Output: 1984 by George Orwell, published in 1949
print(book2.get_description())  # Output: To Kill a Mockingbird by Harper Lee, published in 1960

Tracing and Modifying Objects

Tracing Object Attributes:

To trace an object, you can inspect its attribute values at any point in the programme.

print(book1.title)  # Outputs: 1984

Modifying an Object's State:

You can update an object's state by changing its attributes directly or through methods.

book1.year = 1950  # Updates the publication year
print(book1.get_description())  # Output: 1984 by George Orwell, published in 1950

Note Summary