行为型模式--策略模式(Strategy)

一：定义:

Strategy:Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

二：引入

假设现在要设计一个贩卖各类书籍的电子商务网站的购物车（Shopping Cat）系统。一个最简单的情况就是把所有货品的单价乘上数量，但是实际情况肯定比这要复杂。

比如:

• 本网站可能对所有的儿童类图书实行每本一元的折扣；
• 对计算机类图书提供每本7％的促销折扣，而对电子类图书有3％的折扣；
• 对其余的图书没有折扣。
• 未来可能还会有新的打折策略。

由于有这样复杂的折扣算法，使得价格计算问题需要系统地解决。

            方案一：业务逻辑放在各具体子类

/**/ /*
*各子类实现销售价格算法
*/
public   abstract   class  Book  ... {
    private double price;
    private String name;
    
    public String getName() ...{
        return name;
    }
    public void setName(String name) ...{
        this.name = name;
    }
    public double getPrice() ...{
        return price;
    }
    public void setPrice(double price) ...{
        this.price = price;
    }

    public abstract double getSalePrice() ;
    
}

public   class  CsBook  extends  Book ... {
      public CsBook(String name,double price)
      ...{
        this.setName(name);
        this.setPrice(price);
      }
      
      public double getSalePrice()
      ...{
          return this.getPrice()*0.93;
      }
}

public   class  ChildrenBook  extends  Book  ... {
    
    public ChildrenBook(String name, double price) ...{
        this.setName(name);
        this.setPrice(price);
    }

    public double getSalePrice() ...{
        return this.getPrice() - 1;
    }
}

public   class  Client  ... {

    public static void main(String args[])
    ...{
        Book csBook1=new CsBook("Think in java",45);
        Book childrenBook1=new ChildrenBook("Hello",20);
        
        System.out.println(csBook1.getName()+":"+csBook1.getSalePrice());
        System.out.println(childrenBook1.getName()+":"+childrenBook1.getSalePrice());
    }
}

问题：每个子类必须都各自实现打折算法，即使打折算法相同。所以code reuse不好

方案二：

// 把打折策略代码提到父类来实现code reuse
public   abstract   class  Book  ... {
    private double price;
    private String name;
    
    public String getName() ...{
        return name;
    }
    public void setName(String name) ...{
        this.name = name;
    }
    public double getPrice() ...{
        return price;
    }
    public void setPrice(double price) ...{
        this.price = price;
    }

//    销售价格
    public static double toSalePrice(Book book)
    ...{
        if (book instanceof ChildrenBook)
        ...{
            return book.getPrice()-1;
        }
        else if (book instanceof CsBook)
        ...{
            return book.getPrice()*0.93;
        }
        return 0;
    }
}


public   class  Client  ... {

    
    public static void main(String args[])
    ...{
        Book csBook1=new CsBook("Think in java",45);
        Book childrenBook1=new ChildrenBook("Hello",20);
        System.out.println(csBook1.getName()+":"+Book.toSalePrice(csBook1));
        System.out.println(childrenBook1.getName()+":"+Book.toSalePrice(childrenBook1));
    }
}

 toSalePrice方法是比较容易change的地方，如果策略复杂用if判断比较乱，并且策略修改或增加时需改变原代码。

方案三：策略模式

code reuse时最好用合成(HAS-A)而不用(IS-A)，更加灵活。 

public   abstract   class  Book  ... {
    private double price;
    private String name;
    private DiscountStrategy discountStrategy;//折扣策略
    
    public String getName() ...{
        return name;
    }
    public void setName(String name) ...{
        this.name = name;
    }
    public double getPrice() ...{
        return price;
    }
    public void setPrice(double price) ...{
        this.price = price;
    }
    
    public DiscountStrategy getDiscountStrategy() ...{
        return discountStrategy;
    }
    public void setDiscountStrategy(DiscountStrategy discountStrategy) ...{
        this.discountStrategy = discountStrategy;
    }
    public  double getSalePrice() 
    ...{
        return discountStrategy.getSalePrice(price);
    }
    
}

public   class  CsBook  extends  Book ... {
    public CsBook(String name,double price)
    ...{
      this.setName(name);
      this.setPrice(price);
    }
}

public   abstract   class  DiscountStrategy  ... {
    public  abstract double getSalePrice(double orgPrice) ;
}

/**/ /*
 * 按折扣率打折
 */
public    class  PercentDiscountStrategy  extends  DiscountStrategy  ... {

    private double percent;//折扣率
    public PercentDiscountStrategy(double percent)
    ...{
        this.percent=percent;
    }
    public double getPercent() ...{
        return percent;
    }
    public void setPercent(double percent) ...{
        this.percent = percent;
    }

    public   double getSalePrice(double orgPrice) 
    ...{
        return orgPrice*percent;
    }
}


public   class  Client ... {

    public static void main(String args[])
    ...{
        Book csBook1=new CsBook("Think in java",45);
        csBook1.setDiscountStrategy(new NoDiscountStrategy());
        System.out.println(csBook1.getName()+":"+csBook1.getSalePrice());
        csBook1.setDiscountStrategy(new PercentDiscountStrategy(0.95));
        System.out.println(csBook1.getName()+":"+csBook1.getSalePrice());
        Book childrenBook1=new ChildrenBook("Hello",20);
        
        childrenBook1.setDiscountStrategy(new PercentDiscountStrategy(0.9));
        System.out.println(childrenBook1.getName()+":"+childrenBook1.getSalePrice());
    }
}

三：结构

四：实际应用

1. AWT中的 LayoutManager

五：适用情形

Use the Strategy pattern when

• many related classes differ only in their behavior. Strategies provide a way to configure a class with one of many behaviors.

   

• you need different variants of an algorithm. For example, you might define algorithms reflecting different space/time trade-offs. Strategies can be used when these variants are implemented as a class hierarchy of algorithms .

   

• an algorithm uses data that clients shouldn't know about. Use the Strategy pattern to avoid exposing complex, algorithm-specific data structures.

   

• a class defines many behaviors, and these appear as multiple conditional statements in its operations. Instead of many conditionals, move related conditional branches into their own Strategy class.

参考文献：
1:阎宏，《Java与模式》，电子工业出版社
2:Eric Freeman & Elisabeth Freeman，《Head First Design Pattern》，O'REILLY
3:GOF ,《designpatterns-elements.of.reuseable.object-oriented.software》

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