Instance Methods vs. Class Methods

Instance Methods

Instance methods keep track of behaviors of objects, i.e., what it is able to do.

class Obake
  def speak
    puts "けけけけ"
  end
end

obake = Obake.new
obake.speak  # => けけけけ

The instantiated object from the class Obake is able to speak by mean of the speak instance method.

Instance methods can only be called on the instance of a class.

Class Methods

In contrast, class methods are methods that can be called on the class itself, without the need to instantiate objects beforehand.

class Obake
  def self.country_of_origin
    puts "#{self} are mainly found in Japan"
  end

  def speak
    puts "けけけけ"
  end
end

Obake.country_of_origin  # => Obake are mainly found in Japan

Class methods are useful when we don’t need to deal with states. A classic example is to count the number of time a class has been instantialized.

class Obake
  @@number_of_obake = 0

  def initialize
    @@number_of_obake += 1
  end

  def self.total_number_of_obake
    @@number_of_obake
  end

  def self.country_of_origin
    puts "#{self} are mainly found in Japan"
  end

  def speak
    puts "けけけけ"
  end
end

puts Obake.total_number_of_obake # => 0

obake1 = Obake.new
obake2 = Obake.new
obake3 = Obake.new

puts Obake.total_number_of_obake # => 3