Design Pattern(4)-Decrator Pattern

装饰者模式，比前面几种design pattern都复杂，一般情况下并不应该用它。它是为了尽量不修改原有类、接口的情况下为其动态添加功能。但是如果允许修改原有类，我觉得还是应该修改原有类，以达到更好的设计。用了装饰者模式，复杂度增加，不便于后续维护，好处是不需要修改现有的接口啊，不需要动其他使用该类的程序。

下面以一个麦当劳的例子，去麦当劳点一个果汁juice，你需要动态地实现加冰块、糖甚至其他东西。价格也随之变化，相信大家都做过鸡蛋饼或者武大郎烧饼啥的。麦当劳接口和类是原有的组件，我们不希望修改它。
McDonald接口：

public interface McDonald {    double orderJuice();}

ConcreteMcDonald类：

public class ConcreteMcDonald implements McDonald {    private double price;    public ConcreteMcDonald(double primaryPrice){        this.price=primaryPrice;    }    @Override    public double orderJuice() {        System.out.println("you have ordered a juice");        return price;    }}

Decorator抽象类：

public abstract class Decorator implements McDonald{    McDonald mcDonald;    public Decorator(McDonald mcDonald){        this.mcDonald=mcDonald;    }}

IceblocksDecrator类：

public class IceblocksDecrator extends Decorator {    public IceblocksDecrator(McDonald mcDonald){        super(mcDonald);    }    @Override    public double orderJuice() {        double p=mcDonald.orderJuice()+1;        System.out.println("you have added some ice blocks");        return p;    }}

SugarDecrator类：

public class SugarDecrator extends Decorator {    public SugarDecrator(McDonald mcDonald){        super(mcDonald);    }    @Override    public double orderJuice() {        double p=mcDonald.orderJuice()+3;        System.out.println("you have added some sugar");        return p;    }}

Client类：

public class Client {    public static void main(String[] args){        McDonald mcDonald=new ConcreteMcDonald(10);        mcDonald=new IceblocksDecrator(mcDonald);        mcDonald=new SugarDecrator(mcDonald);        System.out.print(mcDonald.orderJuice());    }}

输出：
you have ordered a juice
you have added some ice blocks
you have added some sugar
12.0