简介

UML图

应用场景

• Java AWT中的LayoutManager（布局管理器）
• 系统中存在多个类，它们的区别仅在于它们的行为
• 不希望暴露复杂与算法有关的数据结构
• Java中Comparator 接口常用的 compare()方法

示例

1. 商品

public class Product {

    /**
     * 产品价格
     */
    private double price;

    /**
     * 产品名称
     */
    private String productName;

    public Product(double price, String productName) {
        this.price = price;
        this.productName = productName;
    }

    public double getPrice() {
        return price;
    }

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

    public String getProductName() {
        return productName;
    }

    public void setProductName(String productName) {
        this.productName = productName;
    }
}

2. 策略：活动优惠策略

public interface Strategy {

    /**
     * 计算商品价格
     *
     * @param product
     * @return
     */
    double computePrice(Product product);
}

没有活动：

public class NormalActivity implements Strategy {

    @Override
    public double computePrice(Product product) {
        return product.getPrice();
    }
}

打折活动：

public class DiscountActivity implements Strategy {

    private double discount;

    public DiscountActivity(double discount) {
        this.discount = discount;
    }

    @Override
    public double computePrice(Product product) {
        return product.getPrice() * discount;
    }
}

代金券活动：

public class VoucherActivity implements Strategy {

    /**
     * 代金券
     */
    private double voucher;


    public VoucherActivity(double voucher) {
        this.voucher = voucher;
    }

    @Override
    public double computePrice(Product product) {
        return product.getPrice() > voucher ? product.getPrice() - voucher : 0;
    }
}

3. 活动上下文：选择适当的活动促销策略

public class PromotionContext {

    /**
     * 策略
     */
    private Strategy strategy;


    public PromotionContext(Strategy strategy) {
        this.strategy = strategy;
    }

    public double executeStrategy(Product product) {
        return this.strategy.computePrice(product);
    }
}

4. 运行

public class Main {

    public static void main(String[] args) {

        // 创建商品
        Product phone = new Product(1999, "手机");

        // 促销策略
        PromotionContext promotionContext;

        // 原价
        promotionContext = new PromotionContext(new NormalActivity());
        System.out.println("原价：" + promotionContext.executeStrategy(phone));

        // 8折优惠
        promotionContext = new PromotionContext(new DiscountActivity(0.8));
        System.out.println("优惠价格：" + promotionContext.executeStrategy(phone));

        // 2000代金券
        promotionContext = new PromotionContext(new VoucherActivity(2000));
        System.out.println("券后价格：" + promotionContext.executeStrategy(phone));

    }
}

总结

• 优点：

  • 满足开闭原则，当增加具体策略时，不需要修改上下文代码
  • 避免使用过多的条件判断，过多的条件判断语句不利于系统可维护性，扩展性和灵活性

• 缺点

  • 策略类可能过多可能导致膨胀，复用性较小