output

$ ruby private_class_method.rb 
7
private_class_method.rb:13: private method `bar' called for Foo:Class (NoMethodError)

private_class_methods.rb

class Foo
  class << self
    private

    def bar
      7
    end
  end

  puts bar # prints 7
end

Foo.bar # raises NoMethodError: private method `bar' called for Foo:Class