装饰者模式(decorator pattern): 动态地将责任附加到对象上

意图：为对象动态添加功能

类图

实现

设计不同种类的饮料，饮料可以添加配料，比如可以添加牛奶，并且支持动态添加新配料。每增加一种配料，该饮料的价格就会增加，要求计算一种饮料的价格。

项目结构

$ tree
.
├── Beverage.java                       # 接口
├── Client.java                         # 客户端调用方
└── impl
    ├── CondimentDecorator.java         # 装饰器抽象类
    ├── DarkRoast.java                  # Beverage的实现类
    ├── HouseBlend.java                 # Beverage的实现类
    ├── Milk.java                       # 装饰器抽象类的实现类
    └── Mocha.java                      # 装饰器抽象类的实现类

具体代码

public interface Beverage {
    double cost();
}

public class DarkRoast implements Beverage {
    @Override
    public double cost() {
        return 1;
    }
}

public class HouseBlend implements Beverage {
    @Override
    public double cost() {
        return 1;
    }
}

public abstract class CondimentDecorator implements Beverage {
    protected Beverage beverage;
}

public class Milk extends CondimentDecorator {

    public Milk(Beverage beverage) {
        this.beverage = beverage;
    }

    @Override
    public double cost() {
        return 1 + beverage.cost();
    }
}

public class Mocha extends CondimentDecorator {

    public Mocha(Beverage beverage) {
        this.beverage = beverage;
    }

    @Override
    public double cost() {
        return 1 + beverage.cost();
    }
}

public class Client {
    public static void main(String[] args) {
        Beverage beverage = new HouseBlend();
        beverage = new Mocha(beverage);
        beverage = new Milk(beverage);
        System.out.println(beverage.cost());
        // 3.0
    }
}

参考文章

1. Java 全栈知识体系：结构型 - 装饰(Decorator)