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

puts Apple.latin_name
puts 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

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

H/T Mike Chau