Refactoring 029 – NULL را با مجموعه جایگزین کنید

public class ShoppingCart {
    private List<Item> items = new ArrayList<>();
    private Coupon coupon = null;

    public void addItem(Item item) {
        this.items.add(item);
    }

    public void redeemCoupon(Coupon coupon) {
        this.coupon = coupon;
    }

    public double total() {
        double total = 0;

        for (Item item : this.items) {
            total += item.getPrice();
        }

        // This a polluted IF and null check
        if (this.coupon != null) {
            total -= this.coupon.getDiscount();
        }

        return total;
    }

    public boolean hasUnsavedChanges() {
        // Explicit null check
        return !this.items.isEmpty() || this.coupon != null;
    }

    public boolean hasCoupon() {        
        return this.coupon != null;
    }
}

public class ShoppingCart {
    private final List<Item> items = new ArrayList<>();

    // This version uses Optionals
    // Not all programming languages support this feature
    private Optional<Coupon> coupon = Optional.empty();

    public void addItem(Item item) {
        items.add(item);
    }

    public void redeemCoupon(Coupon coupon) {
        // You need to understand how optionals work
        this.coupon = Optional.ofNullable(coupon);
    }

    public boolean hasUnsavedChanges() {
        return !items.isEmpty() || !coupon.isPresent();
    }

    public boolean hasCoupon() {
        return coupon.isPresent();
    }
}

public class ShoppingCart {
  private List<Item> items = new ArrayList<>();

  // 1. Identify nullable optional attributes
  // that could be collections
  // 2. Replace single nullable objects with empty collections
  private List<Coupon> coupons = new ArrayList<>();

  public void addItem(Item item) {
      this.items.add(item);
  }

  // Step 4: Work with collection
  // instead of single nullable object
  public void redeemCoupon(Coupon coupon) {
      this.coupons.add(coupon);
  }

  // Step 4: Simplified logic without null checks
  public double total() {
    double total = 0;

    for (Item item : this.items) {
        total += item.getPrice();
    }

    // 3. Remove all null checks 
    // related to these optional attributes        
    for (Coupon coupon : this.coupons) {
        total -= coupon.getDiscount();
    }

    return total;
  }

  // Consistent behavior with empty collections
  public boolean hasUnsavedChanges() {
    // 4. Update methods to work with collections
    // instead of single objects 
    return !this.items.isEmpty() || !this.coupons.isEmpty();
  }

  // 3. Remove all null checks 
  // related to these optional attributes
  // Collection-based check instead of null check
  public boolean hasCoupon() {
    return !this.coupons.isEmpty();
  }
}