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In this section, we will delve into class variables and methods in Ruby. Understanding these concepts is crucial for mastering object-oriented programming in Ruby.

Class Variables

Class variables are variables that are shared among all instances of a class. They are prefixed with @@ and are used to store information that is common to all objects of a class.

Key Points:

Class variables are shared across all instances of a class.
They are defined using @@ followed by the variable name.
Class variables can be accessed and modified by class methods and instance methods.

Example:

class Car
  @@number_of_cars = 0

  def initialize(model)
    @model = model
    @@number_of_cars += 1
  end

  def self.total_cars
    @@number_of_cars
  end
end

car1 = Car.new("Toyota")
car2 = Car.new("Honda")

puts Car.total_cars  # Output: 2

Explanation:

@@number_of_cars is a class variable that keeps track of the number of Car instances.
The initialize method increments @@number_of_cars each time a new Car object is created.
self.total_cars is a class method that returns the value of @@number_of_cars.

Class Methods

Class methods are methods that are called on the class itself, rather than on instances of the class. They are defined using self. followed by the method name.

Key Points:

Class methods are called on the class itself, not on instances.
They are defined using self. before the method name.
Class methods can access class variables and other class methods.

Example:

class Car
  @@number_of_cars = 0

  def initialize(model)
    @model = model
    @@number_of_cars += 1
  end

  def self.total_cars
    @@number_of_cars
  end

  def self.create_cars(models)
    models.each { |model| Car.new(model) }
  end
end

Car.create_cars(["Toyota", "Honda", "Ford"])
puts Car.total_cars  # Output: 3

Explanation:

self.create_cars is a class method that takes an array of models and creates a new Car object for each model.
Car.create_cars(["Toyota", "Honda", "Ford"]) creates three Car objects.
Car.total_cars returns the total number of Car objects created, which is 3.

Practical Exercises

Exercise 1: Implementing Class Variables and Methods

Task: Create a Library class that keeps track of the total number of books and allows adding new books.

Solution:

class Library
  @@total_books = 0

  def initialize(name)
    @name = name
    @books = []
  end

  def add_book(book)
    @books << book
    @@total_books += 1
  end

  def self.total_books
    @@total_books
  end
end

library1 = Library.new("City Library")
library2 = Library.new("County Library")

library1.add_book("1984")
library2.add_book("Brave New World")
library2.add_book("Fahrenheit 451")

puts Library.total_books  # Output: 3

Explanation:

@@total_books is a class variable that keeps track of the total number of books across all libraries.
add_book is an instance method that adds a book to the library and increments @@total_books.
self.total_books is a class method that returns the total number of books.

Common Mistakes:

Forgetting to use self. for class methods: Ensure you prefix class methods with self..
Misusing class variables: Remember that class variables are shared across all instances, so changes in one instance affect all others.

Conclusion

In this section, we covered class variables and methods in Ruby. We learned how to define and use class variables to store information shared among all instances of a class. We also explored class methods, which are called on the class itself and can access class variables. Understanding these concepts is essential for effective object-oriented programming in Ruby. In the next section, we will explore inheritance, which allows us to create hierarchical class structures.

Class Variables and Methods

Class Variables

Key Points:

Example:

Explanation:

Class Methods

Key Points:

Example:

Explanation:

Practical Exercises

Exercise 1: Implementing Class Variables and Methods

Explanation:

Common Mistakes:

Conclusion

Ruby Programming Course

Module 1: Introduction to Ruby

Module 2: Basic Ruby Concepts

Module 3: Working with Collections

Module 4: Object-Oriented Programming in Ruby

Module 5: Advanced Ruby Concepts

Module 6: Ruby on Rails Introduction

Module 7: Testing in Ruby

Module 8: Ruby Best Practices

Module 9: Final Project