Interface segregation principle: A client should never be forced to implement an interface that it doesn't use or clients shouldn't be forced to depend on methods they do not use.

isp.rb

# Violates ISP
class Airplane
  def load_luggage
  end

  def load_fuel
  end

  def take_off
  end

  def land
  end
end

class Controller
  def run_checklist
    @airplane.load_luggage
    @airplane.load_fuel
  end

  def assign_turn
  end
end

class Pilot
  def flight
    @airplane.take_off
  end

  def end_flight
    @airplane.land
  end
end

# Enforces ISP
class Airplane
  def take_off
  end

  def land
  end
end

class AirplaneDispatcher
  def load_luggage
  end

  def load_fuel
  end
end

class Controller
  def run_checklist
    @airplane_dispatcher.load_luggage
    @airplane_dispatcher.load_fuel
  end

  def assign_turn
  end
end

class Pilot
  def flight
    @airplane.take_off
  end

  def end_flight
    @airplane.land
  end
end