Java封装商场打折策略(Strategy模式)

有了OO的基础后，开始认真学习设计模式。

首先学习的是Strategy，下面就封装商场打折策略来分析下策略模式是怎样一回事。

商场每逢节假日都会对不同的商品采用不同的打折策略，首先卖苹果的说我的苹果要打折，好的，我们建立Market和Apple类。

/**
 * 
 * @author LingJian
 *
 */
public class Market {

    /**
     * 只对Apple
     * @param apple
     */
    public static void normalSell(Apple apple) {
        System.out.println("未打折价钱:" + apple.getPrice() * apple.getWeight());
    }
    /**
     * 只对Apple
     * @param apple
     */
    public static void discountSell(Apple apple) {
        double weight = apple.getWeight();
        //打折算法
        if(weight < 10) {
            normalSell(apple);
        }else if(weight >= 10 && weight < 50) {
            System.out.println("打八八折价钱:" + apple.getPrice() * apple.getWeight() * 0.88 );
        }else if(weight >= 50) {
            System.out.println("打五折价钱:" + apple.getPrice() * apple.getWeight() * 0.5 );
        }        
    }
}

/**
 * 
 * @author LingJian
 *
 */
public class Apple {
    //重量
    private double weight;
    //单价 实际开发中 涉及金钱等精确计算都是用BigDecimal
    private double price;
        
    public double getWeight() {
        return weight;
    }
    public void setWeight(double weight) {
        this.weight = weight;
    }
    public double getPrice() {
        return price;
    }
    public void setPrice(double price) {
        this.price = price;
    }
    public Apple(double weight, double price) {
        super();
        this.weight = weight;
        this.price = price;
    }


    
}

/**
 * 
 * @author LingJian
 *
 */
public class Test {

    /**
     * @param args
     */
    public static void main(String[] args) {
        //只能对苹果打折 还不能对通用的一类事物打折 而且都是要卖什么就写什么打折算法 其实每类事物打折算法又是不一致的
        Apple apple = new Apple(10.35, 3.6);
        Market.normalSell(apple);
        Market.discountSell(apple);
    }

}

节假日过了两天后，卖香蕉的一看，苹果打折促销后，原先买香蕉的都跑去买苹果了，不行，香蕉也要打折，不然卖不出去就烂了，OK，这个时候，我们再往Market加一个卖香蕉的打折方法和Banana类，问题很好的解决了….后来，卖梨卖橙子卖橘子的都来了，但是由于每个商品打折的算法和策略都不一样，咱们只能先往Market不停的加方法….显然这不是很好的设计，不停的写打折方法就说明我们这个打折方法不够通用，扩展性不强，那这个时候怎么办呢？对，肯定是想办法让咱们的打折方法通用，增强扩展性，那么说到扩展我们肯定得想到OO的核心-多态。

这里，我们创建一个Discountable接口，让需要打折的商品都实现这个接口，并且在重写打折方法，这样对于Market来说，我们就把具体的打折实现方式都交给了具体的对象本身来实现，我们就不用担心算法策略不同而不停的增加方法。

/**
 * 
 * @author LingJian
 *
 */
public class Market {



    /**
     * 对可打折的一类事物进行打折
     * @param apple
     */
    public static void discountSell(Discountable d) {
        d.discountSell();
    }
    
    
}

public class Apple implements Discountable {
    //重量
    private double weight;
    //单价 实际开发中 涉及金钱等精确计算都是用BigDecimal
    private double price;
        
    public double getWeight() {
        return weight;
    }
    public void setWeight(double weight) {
        this.weight = weight;
    }
    public double getPrice() {
        return price;
    }
    public void setPrice(double price) {
        this.price = price;
    }
    public Apple(double weight, double price) {
        super();
        this.weight = weight;
        this.price = price;
    }
    
    @Override
    public void discountSell() {
        //打折算法
        if(weight < 10) {
            System.out.println("Apple未打折价钱:" + weight * price);
        }else if(weight >= 10 && weight < 50) {
            System.out.println("Apple打八八折价钱:" + weight * price * 0.88 );
        }else if(weight >= 50) {
            System.out.println("Apple打五折价钱:" + weight * price * 0.5 );
        }
    }
    
    
}

/**
 * 
 * @author LingJian
 *
 */
public class Banana implements Discountable {
    //重量
    private double weight;
    //单价 实际开发中 涉及金钱等精确计算都是用BigDecimal
    private double price;
    
    public double getWeight() {
        return weight;
    }
    public void setWeight(double weight) {
        this.weight = weight;
    }
    public double getPrice() {
        return price;
    }
    public void setPrice(double price) {
        this.price = price;
    }
    public Banana(double weight, double price) {
        super();
        this.weight = weight;
        this.price = price;
    }
    
    @Override
    public void discountSell() {
        //打折算法
        if(weight < 5) {
            System.out.println("Banana未打折价钱:" + weight * price);
        }else if(weight >= 5 && weight < 10) {
            System.out.println("Banana打八八折价钱:" + weight * price * 0.88 );
        }else if(weight >= 10) {
            System.out.println("Banana打五折价钱:" + weight * price * 0.5 );
        }        
    }

}

/**
 * 
 * @author LingJian
 *
 */
public interface Discountable {
    public void discountSell();
}

/**
 * 
 * @author LingJian
 *
 */
public class Test {

    /**
     * @param args
     */
    public static void main(String[] args) {
        //可对打折一类事物进行打折啦 先是Apple
        Discountable d = new Apple(10.35, 3.6);
//        Discountable d = new Banana(10.35, 1.6);
        Market.discountSell(d);
    }

}

OK，这一次咱们采用的都是固定的按购买量打折，但是打折的策略算法是有可能变化的，不一定每次节假日都是按购买量打折，所以咱们的打折策略不能写死了，得能够灵活的变化，怎么办呢？当然，还是多态，我们再创建一个Discountor接口，而具体的打折策略都交给具体的实现类来实现，再在需要打折的商品类中，让其持有Discountor接口的实现子类，这样不管每次打折策略怎么变化，我们只需要让打折商品持有不同的实现对象即可灵活的应对变化，这就是策略(Strategy)模式。

/**
 * 
 * @author LingJian
 *
 */
public class Apple implements Discountable {
    //重量
    private double weight;
    //单价 实际开发中 涉及金钱等精确计算都是用BigDecimal
    private double price;
    //按购买量打折
//    private Discountor d = new AppleWeightDiscountor();
    //按购买总价打折
    private Discountor d = new ApplePriceDiscountor();
    
    public double getWeight() {
        return weight;
    }
    public void setWeight(double weight) {
        this.weight = weight;
    }
    public double getPrice() {
        return price;
    }
    public void setPrice(double price) {
        this.price = price;
    }
    public Apple(double weight, double price) {
        super();
        this.weight = weight;
        this.price = price;
    }
    
    @Override
    public void discountSell() {
        d.discount(this);
    }
    
    
}

/**
 * 
 * @author LingJian
 *
 */
public interface Discountor {
    public void discount(Discountable d);
}

/**
 * 
 * @author LingJian
 *
 */
public class AppleWeightDiscountor implements Discountor {

    /**
     * 按购买量打折
     */
    @Override
    public void discount(Discountable d) {
        Apple apple = (Apple)d;
        if(apple.getWeight() < 10) {
            System.out.println("Apple未打折价钱:" + apple.getWeight() * apple.getPrice());
        }else if(apple.getWeight() >= 10 && apple.getWeight() < 50) {
            System.out.println("Apple打八八折价钱:" + apple.getWeight() * apple.getPrice() * 0.88 );
        }else if(apple.getWeight() >= 50) {
            System.out.println("Apple打五折价钱:" + apple.getWeight() * apple.getPrice() * 0.5 );
        }        
    }

}

/**
 * 
 * @author LingJian
 *
 */
public class ApplePriceDiscountor implements Discountor {

    /**
     * 购买满10元立减1角
     * 购买满20元立减1元
     * 购买满30元立减5元
     */
    @Override
    public void discount(Discountable d) {
        Apple apple = (Apple)d;
        if(apple.getWeight() * apple.getPrice() < 10) {
            System.out.println("Apple未打折价钱:" + apple.getWeight() * apple.getPrice());
        }else if(apple.getWeight() * apple.getPrice() >= 10 && apple.getWeight() * apple.getPrice() < 20) {
            System.out.println("Apple购买满10元立减1角:" + (apple.getWeight() * apple.getPrice() - 0.1) );
        }else if(apple.getWeight() * apple.getPrice() >= 20 && apple.getWeight() * apple.getPrice() < 30) {
            System.out.println("Apple购买满20元立减1元:" + (apple.getWeight() * apple.getPrice() - 1) );
        }else if(apple.getWeight() * apple.getPrice() >= 30) {
            System.out.println("Apple购买满30元立减5元:" + (apple.getWeight() * apple.getPrice() - 5) );
        }        
    }

}

Test类无需改变。

简单的描绘下封装商场打折策略的设计图。如下所示：

                       

Market负责给Discountable的商品打折，而具体的可打折的商品的打折的算法则交给Discountor的实现子类来具体实现。