http://programmers.stackexchange.com/questions/302723/passing-fields-instance-variables-by-arguementsparameters-inside-an-object/302725#302725

car_getters.rb

class Car

  def initialize(steering_wheel, engine)
    @wheel = steering_wheel
    @engine = engine
  end

  def service!
    clean_steering_wheel and repair_engine
  end

  private 

  attr_reader :steering_wheel, :engine

  def clean_steering_wheel
    steering_wheel.do_stuff!
    # more stuff
  end

  def repair_engine
    engine.do_stuff!
    # more stuff
  end
end

car_parameters.rb

class Car

  def initialize(steering_wheel, engine)
    @wheel = steering_wheel
    @engine = engine
  end

  def service!
    clean_steering_wheel(wheel) and repair_engine(engine)
  end

  private 

  attr_reader :steering_wheel, :engine

  def clean_steering_wheel(wheel)
    steering_wheel.do_stuff!
    # more stuff
  end

  def repair_engine(engine)
    engine.do_stuff!
    # more stuff
  end
end