爲什麼BitSet要求將顯式強制轉換視爲Set [Int]的實例？

Question

在scaladoc中，BitSet定義爲延伸Set[Int]。所以我想用BitSet如在Set[Int]實例會的工作，但我得到一個類型不匹配：

Welcome to Scala version 2.9.1.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_29). 

scala> import collection.BitSet 
import collection.BitSet 

scala> val b: Set[Int] = BitSet() 
<console>:8: error: type mismatch; 
found : scala.collection.BitSet 
required: Set[Int] 
     val b: Set[Int] = BitSet() 
          ^

然而鑄造的工作原理：

scala> val b: Set[Int] = BitSet().asInstanceOf[Set[Int]] 
b: Set[Int] = BitSet() 

那麼，爲什麼我需要明確地投下BitSet到Set[Int]而Set[Int]是Set[Int]的超級類型？

Answer 1

事實證明，你的Set實際上是scala.collection.immutable.Set。所以，你可以但

val b0: Set[Int] = collection.immutable.BitSet() 
val b1: collection.Set[Int] = collection.BitSet() 
val b2: collection.immutable.Set[Int] = collection.immutable.BitSet() 
val b3: collection.mutable.Set[Int] = collection.mutable.BitSet() 
val b4: collection.Set[Int] = collection.immutable.BitSet() 
val b5: collection.Set[Int] = collection.mutable.BitSet() 

沒有任何

val x1: collection.immutable.Set[Int] = collection.BitSet() 
val x2: collection.immutable.Set[Int] = collection.mutable.BitSet() 
val x3: collection.mutable.Set[Int] = collection.BitSet() 
val x4: collection.mutable.Set[Int] = collection.immutable.BitSet() 

，它原來是爲Set默認的導入給你x2。進口collection.immutable.BitSet或進口collection.Set（覆蓋collection.immutable.Set）。