我正在学习越来越多关于Scala和那个漂亮的游戏框架。 但是有一些事情让我烦恼，我无法开始工作。 
 
 例如，我喜欢将Generics用于某种集合。 但是我需要将它们存储在JSON中的数据库中。 有这个很酷的自动转换的东西，但它不适用于泛型，我没有尝试过： - / 
 
 好的，具体而言，代码优先： 
 
case class InventorySlot(id: Long, item: Option[Item])

object InventorySlot {
  implicit val fmt = Json.format[InventorySlot]
}


case class Inventory[T <: Item](slots: Vector[InventorySlot]) {
  def length = slots.length

  def items: Vector[T] = slots.map(slot => slot.item).flatten.asInstanceOf[Vector[T]]

  def item(id: Long): Option[T] = {
    slots.find(_.id == id) match {
      case Some(slot: InventorySlot) =>
        Some(slot.item.asInstanceOf[T])
      case None =>
        Logger.warn(s"slot with id $id not found")
        None
    }
  }
}

object Inventory {
  implicit val fmt = Json.format[Inventory]
}
 
 Item是可以放入该库存的不同项目的基本抽象类。 没关系。 但有时我想要一个库存，只适用于ItemType A，我们称之为AItem 。 所以我想用这样的东西创建我的库存： val myInventory = Inventory[AItem]("content of vector here") ，当我调用myInventory.item(2) ，我想要在插槽2中获取该项目，并且它应该是AItem类型的对象，而不仅仅是Item 。 （这就是我在这里使用泛型的原因） 
 
 所以问题 
 
 显然， Inventory的隐式格式不起作用。 Item ，以及所有特殊项目，我可以在下面发布它的代码， InventorySlot应该工作。 
 
 编译时的错误是： 
 
Error:(34, 34) Play 2 Compiler: 
 C:\depot\mars\mainline\server\app\models\Test.scala:34: class Inventory takes type parameters
   implicit val fmt = Json.format[Inventory]
                                  ^
 
 我试着明确写出读写，比如 
 
implicit val fmt = (
  (__ \ "slots").format[Vector[InventorySlot]]
  )(Inventory.apply, unlift(Inventory.unapply))
 
 甚至没有在我的IDE中工作，我找不到问题。 我很困惑。 我不知道我的错误在哪里，或者我做错了什么，或者我错过了什么。 
 
 任何帮助将不胜感激。 
 
 我很无奈，我甚至考虑过为所有可能的库存类型做一个类，比如 
 
case class AItemInventory(protected var slots: Vector[InventorySlot]) extends Inventory[AItem](slots)

object AItemInventory {
  implicit val fmt = Json.format[AItemInventory]
}
 
 哪个有效。 没问题，一切都很好。 所以......我不明白。 如果它看起来完全相同，只是硬编码，为什么这个工作？ 
 
 附录 
 
 项目格式化程序，工作原理： 
 
implicit val itemFormat = new Format[Item] {
  override def reads(json: JsValue): JsResult[Item] = {
    (json \ "itemType").as[ItemType] match {
      case ItemType.AITEM => fmtAItem.reads(json)
    }
  }

  override def writes(item: Item): JsValue = item match {
    case subItem: AItem => fmtAItem.writes(subItem)
    case _ => JsNumber(item.itemType.id)
  }
}

I am learning more and more about Scala and that nice playframework. But there are some things that bother me and that I can't get to work.
 
I like using Generics for some kind of collections, for example. But I need those to be stored in our database, in JSON. There is this cool auto conversion thing, but it does not work for generics, in no way I have tried :-/
 
Okay, to be concrete, code first:
 
case class InventorySlot(id: Long, item: Option[Item])

object InventorySlot {
  implicit val fmt = Json.format[InventorySlot]
}


case class Inventory[T <: Item](slots: Vector[InventorySlot]) {
  def length = slots.length

  def items: Vector[T] = slots.map(slot => slot.item).flatten.asInstanceOf[Vector[T]]

  def item(id: Long): Option[T] = {
    slots.find(_.id == id) match {
      case Some(slot: InventorySlot) =>
        Some(slot.item.asInstanceOf[T])
      case None =>
        Logger.warn(s"slot with id $id not found")
        None
    }
  }
}

object Inventory {
  implicit val fmt = Json.format[Inventory]
}
 
Item is a basic abstract class of different items that can be put in that inventory. It doesn't matter. But sometimes I want to have an inventory, that just works for ItemType A, lets call it AItem. So I want to create my inventory with something like this: val myInventory = Inventory[AItem]("content of vector here") and when I call myInventory.item(2), then I want to get the item in slot 2, and it should be an object of type AItem, not just Item. (That's the reason why I am using generics here)
 
So the problem
 
The implicit format for Inventory does not work, obviously. Item does, also with all special items, I can post the code for it below, and InventorySlot should work as well.
 
The error when compiling is:
 
Error:(34, 34) Play 2 Compiler: 
 C:\depot\mars\mainline\server\app\models\Test.scala:34: class Inventory takes type parameters
   implicit val fmt = Json.format[Inventory]
                                  ^
 
I tried to write the read and write explicitly, like
 
implicit val fmt = (
  (__ \ "slots").format[Vector[InventorySlot]]
  )(Inventory.apply, unlift(Inventory.unapply))
 
wich is not even working in my IDE, and I can't find the problem. I am confused. I don't know where my error lies, or if I am doing something wrong, or if I just miss something.
 
Any help will be appreciated.
 
I am so helpless, I even have considered doing a class for all possible inventory types, like
 
case class AItemInventory(protected var slots: Vector[InventorySlot]) extends Inventory[AItem](slots)

object AItemInventory {
  implicit val fmt = Json.format[AItemInventory]
}
 
wich works. No problems, everything fine. So... I don't understand. Why is this working if it seems to be exactly the same, just hardcoded?
 
Appendix
 
The item formatter, wich works:
 
implicit val itemFormat = new Format[Item] {
  override def reads(json: JsValue): JsResult[Item] = {
    (json \ "itemType").as[ItemType] match {
      case ItemType.AITEM => fmtAItem.reads(json)
    }
  }

  override def writes(item: Item): JsValue = item match {
    case subItem: AItem => fmtAItem.writes(subItem)
    case _ => JsNumber(item.itemType.id)
  }
}

原文：https://stackoverflow.com/questions/31135563