private_data.rb

# coding: utf-8
# Private Class Methods in Ruby
# Every object oriented language has a different take on the rules of methods and
# objects and classes, and I cut my teeth on Java, which means that forever and
# ever after I’ll assume everything works like Java. That left me writing code
# like the `Apple` class below that has a private class method, but not really.
# You can still call it.

class Apple
  def self.latin_name
    binomial_name
  end

  private

  def self.binomial_name
    'Malus domestica'
  end
end

Apple.latin_name
Apple.binomial_name
# no error, works fine

# There’s a way to make class methods private in Ruby, you just gotta jump
# through some hoops. Err, I mean use the `class << self` syntax. This oddity
# pushes an instance singleton onto the class effectively creating class methods.

class Orange
  def self.latin_name
    binomial_name
  end

  class << self
    private def binomial_name
      'Citrus × sinensis'
    end
  end
end

Orange.latin_name
Orange.binomial_name
# ERROR!!! you can't call a private method.