Master Ruby Object-Oriented Programming: Create Classes and Methods Effectively

This tutorial will guide you through creating classes, instantiating objects, and defining instance and class methods in Ruby. You’ll learn how to structure your code using object-oriented programming principles and apply these concepts to real-world scenarios.

Beginner30 minutes

Step 1: Understanding Classes and Objects

In Ruby, a class is a blueprint for creating objects. An object is an instance of a class. Classes define the properties (attributes) and behaviors (methods) that objects will have.

- Class: A template for objects (e.g., Car).

- Object: An instance of a class (e.g., my_car = Car.new).

# Define a simple class
class Car
end

# Create an object (instance) of the Car class
my_car = Car.new

Practice Exercise

Create a class called Book. Then, instantiate an object named my_book from this class.

Show Solution

class Book
end

my_book = Book.new

Step 2: Adding Attributes to a Class

Attributes are the properties of an object. In Ruby, you can define attributes using instance variables (@variable). These variables are accessible within the instance methods of the class.

- Use the initialize method to set attributes when creating an object.

class Car
  def initialize(make, model)
    @make = make
    @model = model
  end
end

my_car = Car.new('Toyota', 'Corolla')

Practice Exercise

Add attributes title and author to the Book class. Instantiate an object with the title '1984' and author 'George Orwell'.

Show Solution

class Book
  def initialize(title, author)
    @title = title
    @author = author
  end
end

my_book = Book.new('1984', 'George Orwell')

Step 3: Defining Instance Methods

Instance methods define the behavior of an object. They can access and modify the object's attributes.

- Use def to define a method inside the class.

class Car
  def initialize(make, model)
    @make = make
    @model = model
  end

  def display_info
    puts "Make: #{@make}, Model: #{@model}"
  end
end

my_car = Car.new('Toyota', 'Corolla')
my_car.display_info

Practice Exercise

Add an instance method display_info to the Book class that prints the title and author of the book.

Show Solution

class Book
  def initialize(title, author)
    @title = title
    @author = author
  end

  def display_info
    puts "Title: #{@title}, Author: #{@author}"
  end
end

my_book = Book.new('1984', 'George Orwell')
my_book.display_info

Step 4: Using Class Methods

Class methods are methods that belong to the class itself, not to individual objects. They are defined using self.method_name.

- Class methods are often used for utility functions or operations that don't depend on instance data.

class Car
  def self.description
    puts 'This is a Car class'
  end
end

Car.description

Practice Exercise

Add a class method total_books to the Book class that keeps track of the total number of books created. Hint: Use a class variable @@total_books.

Show Solution

class Book
  @@total_books = 0

  def initialize(title, author)
    @title = title
    @author = author
    @@total_books += 1
  end

  def self.total_books
    @@total_books
  end
end

Book.new('1984', 'George Orwell')
Book.new('To Kill a Mockingbird', 'Harper Lee')
puts "Total books: #{Book.total_books}"

Step 5: Using Accessor Methods

Accessor methods allow you to read and write instance variables from outside the class. Ruby provides shortcuts: attr_reader, attr_writer, and attr_accessor.

- attr_reader: Read-only access.

- attr_writer: Write-only access.

- attr_accessor: Read and write access.

class Car
  attr_accessor :make, :model

  def initialize(make, model)
    @make = make
    @model = model
  end
end

my_car = Car.new('Toyota', 'Corolla')
puts my_car.make
my_car.model = 'Camry'
puts my_car.model

Practice Exercise

Add attr_accessor for title and author to the Book class. Then, create an object and modify its title.

Show Solution

class Book
  attr_accessor :title, :author

  def initialize(title, author)
    @title = title
    @author = author
  end
end

my_book = Book.new('1984', 'George Orwell')
my_book.title = 'Animal Farm'
puts my_book.title

Step 6: Inheritance in Ruby

Inheritance allows one class to inherit attributes and methods from another class. The child class (subclass) inherits from the parent class (superclass).

- Use < to define inheritance.

class Vehicle
  attr_accessor :make, :model

  def initialize(make, model)
    @make = make
    @model = model
  end
end

class Car < Vehicle
  def display_info
    puts "Make: #{@make}, Model: #{@model}"
  end
end

my_car = Car.new('Toyota', 'Corolla')
my_car.display_info

Practice Exercise

Create a subclass EBook that inherits from Book. Add an attribute file_size and a method display_file_size.

Show Solution

class Book
  attr_accessor :title, :author

  def initialize(title, author)
    @title = title
    @author = author
  end
end

class EBook < Book
  attr_accessor :file_size

  def display_file_size
    puts "File Size: #{@file_size} MB"
  end
end

my_ebook = EBook.new('1984', 'George Orwell')
my_ebook.file_size = 5
my_ebook.display_file_size

Step 7: Real-World Application: Building a Library System

Let's apply everything we've learned to build a simple library system. The system will include classes for Book, Library, and Member.

- Book: Represents a book with attributes like title and author.

- Library: Manages a collection of books and allows borrowing/returning.

- Member: Represents a library member who can borrow books.

class Book
  attr_accessor :title, :author

  def initialize(title, author)
    @title = title
    @author = author
  end
end

class Library
  def initialize
    @books = []
  end

  def add_book(book)
    @books << book
  end

  def list_books
    @books.each { |book| puts "Title: #{book.title}, Author: #{book.author}" }
  end
end

class Member
  def initialize(name)
    @name = name
    @borrowed_books = []
  end

  def borrow_book(library, book_title)
    book = library.books.find { |b| b.title == book_title }
    if book
      @borrowed_books << book
      puts "#{@name} borrowed #{book.title}"
    else
      puts "Book not found."
    end
  end
end

Practice Exercise

Extend the library system by adding a method return_book to the Member class and a method remove_book to the Library class. Test the system by borrowing and returning a book.

Show Solution

class Member
  def return_book(library, book_title)
    book = @borrowed_books.find { |b| b.title == book_title }
    if book
      @borrowed_books.delete(book)
      library.add_book(book)
      puts "#{@name} returned #{book.title}"
    else
      puts "Book not found in borrowed list."
    end
  end
end

class Library
  def remove_book(book_title)
    @books.delete_if { |b| b.title == book_title }
  end
end

# Test the system
library = Library.new
book1 = Book.new('1984', 'George Orwell')
library.add_book(book1)

member = Member.new('Alice')
member.borrow_book(library, '1984')
member.return_book(library, '1984')