protected_and_private_class_methods.rb

class Test
  def self.public_method
    "Hello from public"
  end

  protected

    def self.protected_method
      "Hello from 'supposted' protected method"
    end

  private

    def self.private_method
      "Hello from 'supposted' private method"
    end

  class << self
    protected

    def real_protected_method
      "Hello from real protected method"
    end

    private

    def real_private_method
      "Hello from real private method"
    end
  end
end

# Test.public_method
# Test.protected_method
# Test.private_method
# Test.real_protected_method
# Test.real_private_method

ruby-1.8.7-p249 > Test.public_method
 => "Hello from public"
ruby-1.8.7-p249 > Test.protected_method
 => "Hello from 'supposted' protected method"
ruby-1.8.7-p249 > Test.private_method
 => "Hello from 'supposted' private method"
ruby-1.8.7-p249 > Test.real_protected_method
NoMethodError: protected method `real_protected_method' called for Test:Class
ruby-1.8.7-p249 > Test.real_private_method
NoMethodError: private method `real_private_method' called for Test:Class