Classes and Objects

Before diving into advanced OOP concepts, let’s start with the fundamentals: understanding what classes are and how to write them.

What is a Class?

A class is a blueprint or template for creating objects. It defines:

• Attributes (data/variables)
• Methods (functions that operate on the data)

An object (or instance) is a specific realization of a class - a concrete example created from the blueprint.

Real-World Analogy

Think of a class like a cookie cutter:

• The class is the cookie cutter (the template)
• The objects are the cookies (specific instances created from the template)

All cookies made from the same cutter have the same shape, but each cookie is a separate object.

Basic Class Structure

Here’s the simplest class definition:

The Constructor

The constructor is a special method called when you create a new instance of a class.

Understanding self and this

Remember: self (Python) and this (Java)

• Python: self refers to the current instance of the class. It’s the first parameter of instance methods (though you don’t pass it explicitly when calling the method).
• Java: this refers to the current instance of the class. It’s used to distinguish instance variables from parameters.

Key points:

• self/this allows methods to access and modify instance attributes
• In Python, self is always the first parameter in instance methods
• In Java, this is used implicitly or explicitly to refer to the current object

Instance Attributes

Instance attributes are variables that belong to a specific instance. Each object has its own copy of these attributes.

Instance Methods

Instance methods are functions defined inside a class that operate on instance data.

Complete Example: User Class

Let’s put it all together with a complete example:

classDiagram
    class User {
        -username: str
        -email: str
        -age: int
        -is_active: bool
        +__init__(username, email, age)
        +get_info() str
        +deactivate() str
        +activate() str
        +update_email(new_email) str
    }
    
    note for User "Class: Blueprint\nInstance: Specific user"

Visual Representation

Key Takeaways

Remember

• Class = Blueprint/template for creating objects
• Object/Instance = Specific example created from a class
• Constructor = Special method called when creating an instance (__init__ in Python, constructor in Java)
• self/this = Reference to the current instance
• Instance attributes = Variables that belong to a specific instance
• Instance methods = Functions that operate on instance data
• Each instance is independent - modifying one doesn’t affect others
• Use classes to organize related data and behavior together

Next Steps

Now that you understand the basics of classes, you’re ready to explore:

• Enums - Named constants and type-safe enumerations
• Interfaces - Contracts that classes can implement
• Encapsulation - Protecting data with access modifiers
• Abstraction - Hiding complexity and exposing essential features
• Inheritance - Creating class hierarchies
• Polymorphism - Using objects interchangeably

Remember: Classes are a way to model real-world entities in code, bundling together related data (attributes) and behavior (methods) into a single, reusable unit.