6-4-3.rb

class Bicycle
  attr_reader :size, :chain, :tire_size

  def initialize(args={})
    @size      = args[:size]
    @chain     = args[:chain] || default_chain
    @tire_size = args[:tire_size] || default_tire_size
  end

  def default_chain
    '10-speed'
  end
end

class MountainBike < Bicycle
  attr_reader :front_shock, :rear_shock

  def initialize(args)
    @front_shock = args[:front_shock]
    @rear_shock  = args[:rear_shock]
    super(args)
  end

  def spares
    super.merge(rear_shock: rear_shock)
  end

  def default_tire_size
    '23'
  end
end

class RoadBike < Bicycle
  attr_reader :tape_color

  def initialize(args)
    @tape_color  = args[:tape_color]
    super(args) # RoadBikeはsuperを必ず呼ばなければならなくなった
  end

  def spares
    {
      chain: '10-speed',
      tire_size: '23', # milimeters
      tape_color: tape_color,
    }
  end

  def default_tire_size
    '2.1'
  end
end


road_bike = RoadBike.new(size: 'M', tape_color: 'red')
p road_bike.tire_size
p road_bike.chain

mountain_bike = MountainBike.new(size: 'S', front_shock: 'Manitou', rear_shock: 'Fox')
p mountain_bike.tire_size
p mountain_bike.chain