OOP Concepts in Ruby

A class defines attributes (data) and methods (behavior). An object is created from a class using .new.

Basic Class Syntax

class ClassName
end

---

Ruby uses the initialize method as a constructor. It runs automatically when you create an object using .new.

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

Creating Objects

car1 = Vehicle.new("Toyota", "Camry")
car2 = Vehicle.new("Honda", "Accord")

---

In Ruby, a method returns the last evaluated expression, so return is usually optional.

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

  def make
    @make
  end

  def model
    @model
  end
end

---

Instead of accessing instance variables directly, you update values through methods.

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

  def model
    @model
  end

  def change_model(new_model)
    @model = new_model
  end
end

car = Vehicle.new("Toyota", "Camry")
car.change_model("Corolla")
puts car.model

---

Ruby provides built-in helpers to create getter and setter methods.

• attr_reader → getter only
• attr_writer → setter only
• attr_accessor → getter + setter

class Vehicle
  attr_accessor :make, :model

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

car = Vehicle.new("Toyota", "Camry")
car.model = "Corolla"
puts car.model

---

Ruby supports multiple variable types.

Instance Variables (@var)

Belong to a single object.

Class Variables (@@var)

Shared by all objects of a class.

Global Variables ($var)

Accessible everywhere (not recommended for most programs).

---

Class variables are shared across instances. Use class methods to access them.

class Vehicle
  @@count = 0

  def initialize(make, model)
    @make = make
    @model = model
    @@count += 1
  end

  def self.count
    @@count
  end
end

Vehicle.new("Toyota", "Camry")
Vehicle.new("Honda", "Accord")

puts Vehicle.count

---

Inheritance allows a child class to reuse and extend behavior from a parent class.

class Device
  def description
    puts "This is a general device."
  end
end

class Laptop < Device
  def description
    puts "This is a laptop."
  end
end

Laptop.new.description

---

Use super to call the parent method.

class Device
  def description
    puts "This is a general device."
  end
end

class Laptop < Device
  def description
    puts "This is a laptop."
    super
  end
end

Laptop.new.description

---

Ruby doesn’t support multiple inheritance directly, but you can reuse behavior using modules.

Using include (instance methods)

module Features
  def feature_list
    puts "Supports Wi-Fi and Bluetooth."
  end
end

class Laptop
  include Features
end

Laptop.new.feature_list

Using extend (class methods)

module Features
  def feature_list
    puts "This class supports advanced features."
  end
end

class Laptop
  extend Features
end

Laptop.feature_list

---

Ruby controls access at the method level, using:

• public (default)
• private
• protected

Public and Private Example

class Device
  def display_info
    greet
    puts "Device ready."
  end

  private

  def greet
    puts "Hello!"
  end
end

Device.new.display_info

---

Polymorphism means the same method can behave differently depending on the object.

Polymorphism via Inheritance

class Instrument
  def play
    puts "Playing an instrument"
  end
end

class Guitar < Instrument
  def play
    puts "Playing the guitar"
  end
end

class Piano < Instrument
  def play
    puts "Playing the piano"
  end
end

[Instrument.new, Guitar.new, Piano.new].each(&:play)

Polymorphism via Duck Typing

Ruby cares about what an object can do, not what it is.

class Store
  def order(customer)
    customer.payment_type
    customer.discount
  end
end

class RegularCustomer
  def payment_type; puts "Paying with card"; end
  def discount; puts "No discount"; end
end

class PremiumCustomer
  def payment_type; puts "Paying with points"; end
  def discount; puts "10% discount"; end
end

store = Store.new
store.order(RegularCustomer.new)
store.order(PremiumCustomer.new)

---

freeze makes an object immutable.

item = "Laptop"
item.freeze

puts item.frozen?   # true

---

Ruby is a fully object-oriented language where classes, objects, inheritance, modules, and polymorphism are core tools for writing maintainable code. By combining:

• classes and constructors (initialize)
• accessors (attr_accessor)
• inheritance (< and super)
• modules (include and extend)
• access control (public, private, protected)
• polymorphism (inheritance + duck typing)

…you can design clean, scalable Ruby applications.