因爲ArrayBuffer [Node]不是ArrayBuffer [AbstraceNode]的子類型。 它與Variances有關。 在scala中A是B的一個子類型並不意味着S [A]是S [B]的一個子類型。 它可能導致S [A] S [B]或S [B] S [A]亞型,或者什麼也不是。
例如:
scala> class A
defined class A
scala> class B extends A
defined class B
scala> class S1[+T]
defined class S1
scala> val s11: S1[A] = null.asInstanceOf[S1[B]]
s11: S1[A] = null
scala> val s12: S1[B] = null.asInstanceOf[S1[A]]
<console>:8: error: type mismatch;
found : S1[A]
required: S1[B]
val s12: S1[B] = null.asInstanceOf[S1[A]]
^
scala> class S2[-T]
defined class S2
scala> val s21: S2[A] = null.asInstanceOf[S2[B]]
<console>:8: error: type mismatch;
found : S2[B]
required: S2[A]
val s21: S2[A] = null.asInstanceOf[S2[B]]
^
scala> val s22: S2[B] = null.asInstanceOf[S2[A]]
s22: S2[B] = null
scala> class S3[T]
defined class S3
scala> val s31: S3[A] = null.asInstanceOf[S3[B]]
<console>:8: error: type mismatch;
found : S3[B]
required: S3[A]
Note: B <: A, but class S3 is invariant in type T.
You may wish to define T as +T instead. (SLS 4.5)
val s31: S3[A] = null.asInstanceOf[S3[B]]
^
scala> val s32: S3[B] = null.asInstanceOf[S3[A]]
<console>:8: error: type mismatch;
found : S3[A]
required: S3[B]
Note: A >: B, but class S3 is invariant in type T.
You may wish to define T as -T instead. (SLS 4.5)
val s32: S3[B] = null.asInstanceOf[S3[A]]
編輯:
class AbstractNode
class Node extends AbstractNode {
def update() {
// update something here
}
}
class AbstractGraph[T <: AbstractNode] {
val allNodes = ArrayBuffer[T]()
}
class Graph extends AbstractGraph[Node] {
def updateAll() {
allNodes.foreach(_.update())
}
}
謝謝,它幫助!但仍然存在問題。更改`class AbstractNode(val graph:AbstractGraph){graph.allNodes + = this}`。現在彈出另一個奇怪的類型錯誤。有任何想法嗎? – TautrimasPajarskas 2011-01-20 13:09:48
嗯,似乎通過`graph.allNodes + = this.asInstanceOf [AbstractNode.this.graph.Node]`鑄造`幫助。雖然,我不知道這是否是解決問題的正確方法。 – TautrimasPajarskas 2011-01-20 13:40:04