# 法一
def MyClass.a_class_method; end

# 法二
class MyClass
  def self.anther_class_method; end
end

# 法三*
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

# 法一: 打開單件類來擴展
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]
# 法二：Object#extend方法
module MyModule
  def my_method; ‘hello'; end
end
obj = Object.new
#對象擴展
obj.extend MyModule
obj.my_method  # => “hello” 
#類擴展
class MyClass
  extend MyModule
end
MyClass.my_method # => “hello”