scala - 如何在 Scala 中向上转换集合？

Question

我有一些类型：

abstract class TransitionalState
class State extends TransitionalState
class Parallel extends TransitionalState

和他们的包装：

trait TransitionalStateWrapper[T <: TransitionalState]  {
  def state: T
}
trait StateWrapper extends TransitionalStateWrapper[State]
trait ParallelWrapper extends TransitionalStateWrapper[Parallel]

然后我有方法：

 def convert(): List[TransitionalStateWrapper[TransitionalState]] = {
    val list: List[StateWrapper] = List(new StateWrapper {
      def state: State = new State {}
    })
    list
  }

我怎么能list上当List[TransitionalStateWrapper[TransitionalState]]？

我有Error:(18, 5) type mismatch; found : List[StateWrapper] required: List[TransitionalStateWrapper[TransitionalState]] list

标签： scala

Answer 1

您不能向上转换它，因为TransitionalStateWrapper[State]它不是TransitionalStateWrapper[TransitionalState].

TransitionalStateWrapper在类型参数中必须是协变的T。

trait TransitionalStateWrapper[+T <: TransitionalState]  {
  def state: T
}