策略模式

用于封装系列的算法，客户端可以自由选择其中一种算法，考虑以下情形：现需要开发一个网上书店，该书店为了更好地促销，经常需要对图书进行打折促销，程序需要考虑各种打折促销的计算方法：

package com.fcy.model;

interface DiscountStrategy{
	double getDiscount(double originPrice);
}
class VipDiscount implements DiscountStrategy {
	public double getDiscount(double originPrice){
		System.out.println("使用vip折扣...");
		return originPrice*0.5;
	}
}
class OldDiscount implements DiscountStrategy{
	public double getDiscount(double originPrice){
		System.out.println("使用旧书折扣...");
		return originPrice*0.7;
	}
}
class DiscountContext{
	private DiscountStrategy strategy;
	public DiscountContext(DiscountStrategy strategy){
		this.strategy=strategy;
	}
	public double getDiscountPrice(double price){
		if(strategy==null){
			strategy=new OldDiscount();
		}
		return this.strategy.getDiscount(price);
	}
	public void changeDiscount(DiscountStrategy strategy){
		this.strategy=strategy;
	}
}
public class Strategy {
	public static void main(String[] args){
		DiscountContext dc=new DiscountContext(null);
		double price1=79;
		System.out.println("79元图书默认打折后的价格是："+dc.getDiscountPrice(price1));
		dc.changeDiscount(new VipDiscount());
		double price2=89;
		System.out.println("89元的书对vip用户价格是："+dc.getDiscountPrice(price2));
	}
}

运行结果 ：