Rubyオブジェクト指向プログラミングにおけるクラスメソッドとクラスエクステンション

# method one
def MyClass.a_class_method; end


# Method 2
class MyClass
  def self.other_class_method; end
end


# Method 3*
class MyClass
  class << self
    def yet_another_class_method; end
  end
end

module MyModule
  def my_method; 'hello'; end
end

class MyClass
  class < self
    include MyModule
  end
end

MyClass.my_method

# Method 1: Open a single class to extend
module MyModule
  def my_method; 'hello'; end
end

obj = Object.new
class << obj
  include MyModule
end

obj.my_method # => "hello"
obj.singleton_methods # => [:my_method]
# Method 2: Object#extend method
module MyModule
  def my_method; 'hello'; end
end

obj = Object.new
# Object extensions
obj.extend MyModule
obj.my_method # => "hello" 
# class extension
class MyClass
  extend MyModule
end

MyClass.my_method # => "hello"