More readable interface in Ruby

more_readable_interface.rb

# ----------------------------------------------------------------------
#
# I think Java style interface is unreadable in Ruby.
# Because must methods and normal methods are mixed.
#
# Ruby で Java の interface のような実装をすると、
# 普通のメソッドと実装必須なメソッドが混ざってわかりにくい。
#
class Car
  def engine
    raise NotImplementedError, 'Please implement `engine` method.'
  end

  def start
    "#{engine.class.name} is started!"
  end
end

class AwesomeEngine
end

class AwesomeCar < Car
  def engine
    @engine ||= AwesomeEngine.new
  end

  def awesome_feature
    'hoge'
  end
end

awesome_car = AwesomeCar.new
awesome_car.start
#=> "AwesomeEngine is started!"

class AwfulCar < Car
end

awful_car = AwfulCar.new
awful_car.start
#=> NotImplementedError: Please implement `engine` method.

# ----------------------------------------------------------------------
#
# I'd like to make a suggestion.
# Explicit the must method using the class method. It is more readable.
# (I don't know the name of this technique... Can anybody teach me?)
#
# そこで提案。
# クラスメソッドで必須な処理を明示化するとたぶんきっと分かりやすくなる。
# （この技の名前がわからないので誰か教えてください）
#
class Car
  class << self
    def engine(engine_class)
      @engine_class = engine_class
    end
  end

  def engine
    return @engine if @engine

    engine_class = self.class.instance_variable_get(:@engine_class)
    raise NotImplementedError, 'Please call `engine` method.' unless engine_class

    @engine = engine_class.new
  end

  def start
    "#{engine.class.name} is started!"
   end
end

class AwesomeEngine
end

class AwesomeCar < Car
  engine AwesomeEngine

  def awesome_feature
    'hoge'
  end
end

awesome_car = AwesomeCar.new
awesome_car.start
#=> "AwesomeEngine is started!"

class AwfulCar < Car
end

awful_car = AwfulCar.new
awful_car.start
#=> NotImplementedError: Please call `engine` method.