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