实体也可以做加减？

class Order{
  Long id;
  String number;
  <Set>Item items;
  public Order add(Order order){
    if(!order.number.equals(this.number)){
       throw new OrderException("not same order operation");
    }
    Order momentOrder=new Order(new HashSet<Item>());
    for(Item item:this.items){
        for(Item orderItem:order.items){
            if(item.product.equals(orderItem.product)){
              if(item.count+orderItem.count<0){throw new OrderException("count error");}
              momentOrder.items.add(new Item(item.product,item.count+orderItem.count));
            }
        }
    }
    return momentOrder;
  }
}
class Item{
  int count;
  //equals hashcode
}