java - 如何重构代码以遵守规则 ‘open-closed’ ？

Question

UML 位于此处。不同的产品有不同的优惠策略。
将这些商品添加到购物车时，需要根据其具体的优惠策略，调用'checkout()'方法计算'totalPrice'和'loyaltyPoints'。

但是当新产品有新的优惠策略时，我需要再添加一个“else if”。
我认为这打破了开闭原则。那么你能给我一些关于如何重构代码的建议吗？

 public Order checkout() {
    double totalPrice = 0;
    Map<String,Integer> buy2Plus1=new HashMap<>();
    int loyaltyPointsEarned = 0;
    for (Product product : products) {
        double discount = 0;
        if (product.getProductCode().startsWith("DIS_10")) {
            discount = (product.getPrice() * 0.1);
            loyaltyPointsEarned += (product.getPrice() / 10);
        } else if (product.getProductCode().startsWith("DIS_15")) {
            discount = (product.getPrice() * 0.15);
            loyaltyPointsEarned += (product.getPrice() / 15);
        }else if(product.getProductCode().startsWith("DIS_20")){
            discount=(product.getPrice()*0.2);
            loyaltyPointsEarned+=(product.getPrice()/20);
         }else if(product.getProductCode().startsWith("Buy2Plus1")){
            if(buy2Plus1.containsKey(product.getProductCode())){
                buy2Plus1.put(product.getProductCode(), buy2Plus1.get(product.getProductCode())+1);
            }
            else{
                buy2Plus1.put(product.getProductCode(),1);
            }
            if(buy2Plus1.get(product.getProductCode())%3==0){
                discount+=product.getPrice();
                continue;
            }
            loyaltyPointsEarned+=(product.getPrice()/5);
        }else {
            loyaltyPointsEarned += (product.getPrice() / 5);
        }
        totalPrice += product.getPrice() - discount;
    }

    return new Order(totalPrice, loyaltyPointsEarned);
}
          

Answer 1

我会在这里使用工厂模式。首先定义一个接口(interface):

public interface PreferentialStrategy {

  String strategyCode();
  StrategyResult applyStrategy(Product product);
}

StrategyResult 是一个简单的 pojo，持有折扣和忠诚度积分。

每个策略都是一个实现 PreferentialStrategy 的类:

public class Dis10Strategy implements PreferentialStrategy {

  @Override
  public String strategyCode() {
    return "DIS_10";
  }

  @Override
  public StrategyResult applyStrategy(Product product) {
    double discount = product.getPrice() * 0.1;
    int points = (int) product.getPrice() / 10;
    return new StrategyResult(discount, points);
  }
}

与 dis_15、dis_20 等相同。

工厂:

public class StrategyFactory {

  private final List<PreferentialStrategy> strategies;
  private final PreferentialStrategy defaultStrategy = new NoPrefStrategy();

  public StrategyFactory(Collection<PreferentialStrategy> strategies) {
    this.strategies = new ArrayList<>(strategies);
  }
  
  public void registerPreferentialStrategy(PreferentialStrategy strategy) {
    this.strategies.add(strategy);
  }

  public PreferentialStrategy getStrategy(Product product) {
    return this.strategies
            .stream()
            .filter(strategy -> product.getProductCode().startsWith(strategy.strategyCode()))
            .findFirst()
            .orElse(this.defaultStrategy);
  }

  private static final class NoPrefStrategy implements PreferentialStrategy {

    @Override
    public String strategyCode() {
      return "";
    }

    @Override
    public StrategyResult applyStrategy(Product product) {
      return new StrategyResult(0, 0);
    }
  }
}

它获取产品的策略，如果没有，则返回默认策略，没有任何偏好(0折扣和0积分)。每当您需要添加新策略时，只需在工厂中注册即可。

顺便说一句，由于您在这里处理的是金钱，因此您不应该使用 float (它们不精确)，而应该使用 BigDecimal 。

使用示例:

public class TempSecond {

  public static void main(String[] args) {
    List<Product> products = Arrays.asList(new Product(1000, "DIS_10", "Prod1"),
            new Product(1000, "DIS_15_asd", "Prod2"),
            new Product(1100, "qwerty", "Prod3"));
    StrategyFactory factory = new StrategyFactory(Arrays.asList(new Dis10Strategy(), new Dis15Strategy()));
    int loyaltyPointsEarned = 0;
    for (Product product : products) {
      PreferentialStrategy preferentialStrategy = factory.getStrategy(product);
      StrategyResult result = preferentialStrategy.applyStrategy(product);
      double discount = result.getDiscount();
      loyaltyPointsEarned += result.getLoyaltyPoints();
    }
  }
}