Java和Kotlin中委托关系的比较

Question

我试图理解继承与委派的斗争，以及如何实现这两者。我的问题是为什么我要用Kotlin的方式用by关键字实现委派？我觉得太过分了。

让我给你举个例子。我们有一个可以点餐的应用程序。这款应用名为DeliveryQueen。

下面是Java示例：

public interface FoodDeliverable {
    void deliverSomeFood();
}

public class PizzaCastle implements FoodDeliverable {
    @Override
    public void deliverSomeFood() {
        System.out.println("deliver the tastiest pizza in the world");
    }
}

public class BurgerCastle implements FoodDeliverable {
    @Override
    public void deliverSomeFood() {
        System.out.println("deliver the tastiest burger in the world");
    }
}

public class DeliveryQueen {
    private FoodDeliverable orderedFood;

    void chooseFood(FoodDeliverable food) {
        this.orderedFood = food;
    }

    void orderSomeFood() {
        this.orderedFood.deliverSomeFood();
    }
}

public class Main {

    public static void main(String[] args) {
        System.out.println("Hello and welcome ");

        DeliveryQueen dq = new DeliveryQueen();

        dq.chooseFood(new BurgerCastle());
        dq.orderSomeFood();
    }
}

Kotlin示例#1没有明确使用by关键字：

interface FoodDeliverable {
    fun deliverSomeFood()
}

class PizzaCastle: FoodDeliverable {
    override fun deliverSomeFood() {
        print("deliver the tastiest pizza in the world")
    }
}

class BurgerCastle: FoodDeliverable {
    override fun deliverSomeFood() {
        print("deliver the tastiest burger in the world")
    }
}

class DeliveryQueen {
    private var orderedFood: FoodDeliverable = PizzaCastle()

    fun chooseFood(food: FoodDeliverable) {
        orderedFood = food
    }

    fun orderSomeFood() {
        orderedFood.deliverSomeFood()
    }
}

fun main() {
    println("Hello")

    val dq = DeliveryQueen()
    dq.chooseFood(BurgerCastle())
    dq.orderSomeFood()
}

interface FoodDeliverable {
    fun deliverSomeFood()
}

class PizzaCastle: FoodDeliverable {
    override fun deliverSomeFood() {
        print("deliver the tastiest pizza in the world")
    }
}

class BurgerCastle: FoodDeliverable {
    override fun deliverSomeFood() {
        print("deliver the tastiest burger in the world")
    }
}

class DeliveryQueen(var orderedFood: FoodDeliverable): FoodDeliverable by orderedFood {
    //private var orderedFood: FoodDeliverable = PizzaCastle()

    fun chooseFood(food: FoodDeliverable) {
        orderedFood = food
    }

    fun orderSomeFood() {
        orderedFood.deliverSomeFood()
    }
}

fun main() {
    println("Hello")

    val dq = DeliveryQueen(PizzaCastle())
    dq.chooseFood(BurgerCastle())
    dq.orderSomeFood()
}

Answer 1

在Kotlin示例#1中，DeliveryQueen不是FoodDeliverable，因为它没有实现接口，所以它不是一个正确的组合实现。要修复Kotlin示例#1，您必须实现fooddeliverable，这将迫使您重写它的方法：

class DeliveryQueen: FoodDeliverable  {
    private var orderedFood: FoodDeliverable = PizzaCastle()

    override fun deliverSomeFood() {
        orderedFood.deliverSomeFood()
    }

    fun chooseFood(food: FoodDeliverable) {
        orderedFood = food
    }

    fun orderSomeFood() {
        orderedFood.deliverSomeFood()
    }

}

了解如何实现DeliversomeFood()，尽管它所做的只是将调用传递给委托OrderedFood？这就是Kotlin使用by进行委托的地方，如Kotlin示例#2所示，默认情况下，您获得了实现，并且只在需要添加额外逻辑的地方使用override方法。当您的接口有更多的方法时，好处会更明显，在这种情况下，使用by委派可以为您节省大量样板。