昔の蛇口と現代の蛇口をオブジェクト指向で表現してみた。勉強会用資料。

faucet.rb

class Water
  attr_reader :temperature, :amount
  
  def initialize(temperature:, amount:)
    @temperature = temperature
    @amount = amount
  end

  def +(other)
    total_amount = self.amount + other.amount
    total_calorie = self.temperature * self.amount + other.temperature * other.amount
    Water.new(temperature: total_calorie.to_f / total_amount, amount: total_amount)
  end
end

class ColdWater < Water
  TEMPERATURE = 10

  def initialize(amount:)
    super(temperature: TEMPERATURE, amount: amount)
  end
end

class HotWater < Water
  TEMPERATURE = 42
  
  def initialize(amount:)
    super(temperature: TEMPERATURE, amount: amount)
  end
end

class OldStyleFaucet
  attr_reader :hot_water_amount, :cold_water_amount
  
  def initialize
    @hot_water_amount = 0
    @cold_water_amount = 0
  end

  def turn_hot_water_faucet(amount:)
    @hot_water_amount = amount
  end

  def turn_cold_water_faucet(amount:)
    @cold_water_amount = amount
  end
  
  def pour
    HotWater.new(amount: hot_water_amount) + ColdWater.new(amount: cold_water_amount)
  end
end

class CurrentStyleFaucet
  attr_accessor :temperature, :amount

  def initialize(temperature:, amount:)
    @temperature = temperature
    @amount = amount
  end
  
  def pour
    HotWater.new(amount: hot_water_amount) + ColdWater.new(amount: cold_water_amount)
  end

  private

  def hot_water_amount
    amount.to_f * (temperature - ColdWater::TEMPERATURE) / (HotWater::TEMPERATURE - ColdWater::TEMPERATURE)
  end

  def cold_water_amount
    amount - hot_water_amount
  end
end

# ==============================================================================
old_style_faucet = OldStyleFaucet.new
old_style_faucet.turn_hot_water_faucet(amount: 80)
old_style_faucet.turn_cold_water_faucet(amount: 20)
old_style_faucet.pour # => #<Water @amount=100, @temperature=35.6>

old_style_faucet.turn_hot_water_faucet(amount: 90)
old_style_faucet.turn_cold_water_faucet(amount: 10)
old_style_faucet.pour # => #<Water @amount=100, @temperature=38.8>

old_style_faucet.turn_hot_water_faucet(amount: 95)
old_style_faucet.turn_cold_water_faucet(amount: 5)
old_style_faucet.pour # => #<Water @amount=100, @temperature=40.4>

# ==============================================================================
current_style_faucet = CurrentStyleFaucet.new(temperature: 40, amount: 100)
current_style_faucet.pour # => #<Water @amount=100.0, @temperature=40.0>

current_style_faucet.send(:hot_water_amount) # => 93.75
current_style_faucet.send(:cold_water_amount) # => 6.25