Decorator模式链方法如何调用？

Question

我读了一个Decorator设计模式的例子.我知道这种设计模式动态地修改了一个特定实例的行为.我在下面给出的例子也是可以理解的.我不明白的一点是,当我调用c.getCost()牛奶对象时,它返回1.5.只有Simplecoffee的getCost()回报1,但是从哪里呢c.getCost奶回报1.5吗？

任何人都可以解释Milk类和Simplecoffee类之间的链接,以及getCost()在使用milk对象调用时如何执行方法流？该getCost()方法如何返回1.5？

//Decorator Design Pattern
interface Coffee {
    public double getCost();

    public String getIngredients();
}

class Simplecoffee implements Coffee {
    public double getCost() {
        return 1;
    }

    public String getIngredients() {
        return "Coffee";
    }
}

abstract class CoffeeDecorator implements Coffee {
    protected Coffee decoratedcoffee;
    protected String ingredientseparator = ":";

    public CoffeeDecorator(Coffee decoratedcoffee) {

        this.decoratedcoffee = decoratedcoffee;
    }

    public double getCost() {
        return decoratedcoffee.getCost();
    }

    public String getIngredients() {
        return decoratedcoffee.getIngredients();
    }
}

class Milk extends CoffeeDecorator {

    public Milk(Coffee decoratedcoffee) {

        super(decoratedcoffee);
        System.out.println("Milk Constructor");
    }

    public double getCost() {
        return super.getCost() + 0.5;
    }

    public String getIngredients() {
        return super.getIngredients() + ingredientseparator + "milk";
    }
}

public class Decorator {    
    public static void main(String[] ar) {
        System.out.println("calling simplecoffee in main");
        Coffee c = new Simplecoffee();
        System.out.println("Cost:" + c.getCost());
        c = new Milk(c);
        System.out.println("Cost:" + c.getCost());
    }    
}

Answer 1

getCost()方法如何返回1.5？

该getCost()方法Milk首先调用引用的getCost方法.因此,这将调用返回1 的方法(读取:运行时多态性).接下来,该方法添加此调用的返回值(即1),从而将结果作为decoratedcoffeeSimpleCoffeegetCostSimpleCoffeegetCostMilk0.51.5

这是Decorator模式的重点.的装饰被用于通过缠绕一个新对象到现有的链产生的方法调用的链.