Example for "Interface" and "Concrete" class in Ruby

auto_test.rb

require 'test_helper'

# Add the `described_class` so it can be used in the AutoInterfaceTest
class ActiveSupport::TestCase
  def described_class
    self.class.to_s.gsub(/Test$/, '').constantize
  end
end

# In the interface, the example Auto - Car is dummy, because it should be
# an instance, and instance method, but as you asked for class methods
# it's the same idea
module Auto
  # Do not use Ruby's NotImplementedError, that is reserved for platform methods
  # create your own inside the module
  class NotImplementedError < StandardError; end

  def self.included(base)
    base.extend(ClassMethods)
  end

  # methods to be implemented
  module ClassMethods
    def start
      raise NotImplementedError.new('Must implement start')
    end

    def stop
      raise NotImplementedError.new('Must implement stop')
    end
  end
end

# Esse é o teste da interface, testa que ela força a implementação do start/stop
class AutoTest < ActiveSupport::TestCase
  class ImplementAuto
    # it extends `ClassMethods` on base
    # so you can add instance and class methods
    include Auto
  end

  test "raise error for start" do
    err = assert_raises(Auto::NotImplementedError) { ImplementAuto.start }
    assert_match('Must implement start', err.message)
  end

  test "raise error for stop" do
    err = assert_raises(Auto::NotImplementedError) { ImplementAuto.start }
    assert_match('Must implement start', err.message)
  end
end

# This module tests the interface and you need to add on your test
# so it tests the implemented methods, same as rspec's shared_examples
module AutoInterfaceTest
  def test_must_respond_to_start
    described_class.send(:stop)
  end

  def test_must_respond_to_stop
    described_class.send(:stop)
  end
end

# The 'Concrete' class
class Car
  extend Auto

  class << self
    # If you comment the methods below, CarTest will fail
    def start
      true
    end

    def stop
      true
    end
  end
end

# The of the "concrete" class, ensure the interface is implemented
class CarTest < ActiveSupport::TestCase
  include AutoInterfaceTest
end