多型(polymorphism)

多型是物件導向語嚴重要的特性之一，介紹多型前先來介紹下列的名詞：

• overload 多載
  相同函數名稱，不同參數類型、個數，依據傳入不同的參數，呼叫相對應的函數
• override 覆載
  相同函數名稱，覆寫方法內容，當子類別繼承覆類別時，可以覆寫父類別的方法內容

由於英文很像的關係常常讓我們搞錯差別，我們來看看下面的例子

overload多載

python中其實是使用預設參數來呈現多載，我們看看下面的例子

override 覆載

當子類別繼承父類別時，覆寫父類別的方法

class Car:
    def __init__(self, wheel_number, door_number, power):
        self.wheel_number = wheel_number
        self.door_number = door_number
        self.power = power

    def accelerate(self):
        print("Normal 0~100 km/hr acceleration: 10 seconds.")


class EletricCar(Car):
    def __init__(self, wheel_number, door_number, power, brand):
        super(EletricCar, self).__init__(wheel_number, door_number, power)
        self.brand = brand

    def accelerate(self):
        print(f"{self.brand} 0~100 km/hr acceleration: 4 seconds.")

c = EletricCar(4,5,"Motor","Tesla")
c.accelerate()

多型

接續上面的例子，我們再定義一個跑車子類別叫SportCar，然後覆寫父類別的accelerate()方法

class EletricCar(Car):
    def __init__(self, wheel_number, door_number, power, brand):
        super(EletricCar, self).__init__(wheel_number, door_number, power)
        self.brand = brand

    def accelerate(self):
        print(f"{self.brand} 0~100 km/hr acceleration: 4 seconds.")

class SportCar(Car):
    def __init__(self, wheel_number, door_number, power, brand):
        super(SportCar, self).__init__(wheel_number, door_number, power)
        self.brand = brand

    def accelerate(self):
        print(f"{self.brand} 0~100 km/hr acceleration: 2.8 seconds.")

t = EletricCar(4,5,"Motor","Tesla")
s = SportCar(4,2,"Engine","Lamborghini")
t.accelerate()
s.accelerate()

從上述例子中可以看到，當子類別繼承父類別，而子類別覆寫了父類別的方法，繼承替父類別衍伸出許多型態的子類別，不同的子類別呼叫相同名稱的方法，產生不同的實作，就是多型的概念，所以說有了繼承才有多型