设计模式 -- 装饰者模式

 装饰者模式动态地将责任附加到对象上。若要扩展功能，装饰者提供了比继承更加有弹性的替代方案。

对于我自己的理解：装饰者模式就是一个在继承层面上的递归。

对于装饰者模式而言，有两个很重要的概念：component 与 decorator；

component （组件）：被装饰的主体

decorator（装饰者）：用于装饰主体的配件

下面我们来用一个咖啡馆的订单系统作为实例。

咖啡馆需求：根据客户的订单，生成价格。而客户可选择一个单品咖啡，并且可以在这个选择的单品咖啡上添加相应装饰（牛奶、奶泡、茶....etc）

根据上面的需求，我们应用装饰者模式去实现。

类图：

注意：标准的装饰者模式是没有 coffeeComponent 这一层的~ 这里是为了实现更加便捷、清晰而添加上去的。

代码实现：

AbstractCoffeeComponent ：component，被装饰的组件

package decorator.component;

/**
 * Created by chenjiayin on 2017/3/18.
 */
public abstract class AbstractCoffeeComponent
{
    public String description;
    private double price;

    public String getDescription()
    {
        return description + ":" + this.getPrice();
    }

    public void setDescription(String description)
    {
        this.description = description;
    }

    public double getPrice()
    {
        return price;
    }

    public void setPrice(double price)
    {
        this.price = price;
    }

    public abstract double cost();
}

CoffeeComponent ：抽取这一层，cost()里面的价格就是它本身的价格，因为点单品只会点一个单品的。。。两个单品混合的咖啡不在本次需求的考虑范围内

package decorator.component;

/**
 * Created by chenjiayin on 2017/3/18.
 */
public class CoffeeComponent extends AbstractCoffeeComponent
{
    @Override
    public double cost()
    {
        return super.getPrice();
    }
}

单品咖啡，Decaf

package decorator.component;

/**
 * Created by chenjiayin on 2017/3/18.
 */
public class Decaf extends CoffeeComponent
{
    public Decaf()
    {
        super.setDescription("Decaf");
        super.setPrice(30);
    }
}

单品咖啡，Expresso

package decorator.component;

/**
 * Created by chenjiayin on 2017/3/18.
 */
public class Expresso extends CoffeeComponent
{
    public Expresso()
    {
        super.setDescription("Expresso");
        super.setPrice(31);
    }
}

单品咖啡，LongBlack

package decorator.component;

/**
 * Created by chenjiayin on 2017/3/18.
 */
public class LongBlack extends CoffeeComponent
{
    public LongBlack()
    {
        super.setDescription("LongBlack");
        super.setPrice(32);
    }
}

单品咖啡，ShortBlack

package decorator.component;

/**
 * Created by chenjiayin on 2017/3/18.
 */
public class ShortBlack extends CoffeeComponent
{
    public ShortBlack()
    {
        super.setDescription("ShortBlack");
        super.setPrice(33);
    }
}

好了，上面，我们已经将我们需要的组件都已经定义好了，下面，我们开始定义装饰者了。

AbstractCoffeeDecorator：装饰者的抽象类，继承了组件，并且以组件作为它的一个属性，因为需要装饰它。

package decorator.decorator;

import decorator.component.AbstractCoffeeComponent;

/**
 * Created by chenjiayin on 2017/3/18.
 */
public abstract class AbstractCoffeeDecorator extends AbstractCoffeeComponent
{
    private AbstractCoffeeComponent coffeeComponent;

    public AbstractCoffeeDecorator(AbstractCoffeeComponent coffeeComponent)
    {
        this.coffeeComponent = coffeeComponent;
    }

    @Override
    public double cost()
    {
        return super.getPrice() + coffeeComponent.cost();
    }

    @Override
    public String getDescription()
    {
        return super.getDescription() + "+" + coffeeComponent.getDescription();
    }
}

装饰者，Milk

package decorator.decorator;

import decorator.component.AbstractCoffeeComponent;

/**
 * Created by chenjiayin on 2017/3/18.
 */
public class Milk extends AbstractCoffeeDecorator
{
    public Milk(AbstractCoffeeComponent coffeeComponent)
    {
        super(coffeeComponent);
        super.setDescription("Milk");
        super.setPrice(5);
    }
}

装饰者，Whip

package decorator.decorator;

import decorator.component.AbstractCoffeeComponent;

/**
 * Created by chenjiayin on 2017/3/18.
 */
public class Whip extends AbstractCoffeeDecorator
{
    public Whip(AbstractCoffeeComponent coffeeComponent)
    {
        super(coffeeComponent);
        super.setDescription("Whip");
        super.setPrice(3);
    }
}

装饰者，Tea

package decorator.decorator;

import decorator.component.AbstractCoffeeComponent;

/**
 * Created by chenjiayin on 2017/3/18.
 */
public class Tea extends AbstractCoffeeDecorator
{
    public Tea(AbstractCoffeeComponent coffeeComponent)
    {
        super(coffeeComponent);
        super.setDescription("Tea");
        super.setPrice(10);
    }
}

我们的装饰者，与我们的组件都已经定义好了，我们来看一下成果。

测试类，DecoratorMain

package decorator;

import decorator.component.AbstractCoffeeComponent;
import decorator.component.Decaf;
import decorator.component.Expresso;
import decorator.decorator.Milk;
import decorator.decorator.Tea;
import decorator.decorator.Whip;

/**
 * Created by chenjiayin on 2017/3/18.
 */
public class DecoratorMain
{
    public static void main(String[] args)
    {
        AbstractCoffeeComponent fatCatCoffeeOrder;
        fatCatCoffeeOrder = new Expresso();
        fatCatCoffeeOrder = new Milk(fatCatCoffeeOrder);
        System.out.println("order desc:" + fatCatCoffeeOrder.getDescription());
        System.out.println("order price:" + fatCatCoffeeOrder.cost());


        AbstractCoffeeComponent thinCatCoffeeOrder;
        thinCatCoffeeOrder = new Decaf();
        thinCatCoffeeOrder = new Whip(thinCatCoffeeOrder);
        thinCatCoffeeOrder = new Whip(thinCatCoffeeOrder);
        thinCatCoffeeOrder = new Tea(thinCatCoffeeOrder);
        System.out.println("order desc:" + thinCatCoffeeOrder.getDescription());
        System.out.println("order price:" + thinCatCoffeeOrder.cost());

    }
}

输出结果：

order desc:Milk:5.0+Expresso:31.0
order price:36.0
order desc:Tea:10.0+Whip:3.0+Whip:3.0+Decaf:30.0
order price:46.0