Scala子類無法識別父類中的通用映射？

Question

我有一個代碼段，其看起來如下：

abstract class MultipleOutputWriter { 
    protected def writers: collection.mutable.Map[Any, OutputStream] 
    def write(basePath: String, value: Any) 
    def close = writers.values.foreach(_.close) 
} 
class LocalMultipleOutputWriter extends MultipleOutputWriter { 
    protected val writers = collection.mutable.Map[String, FileOutputStream]() 
    def write(key: String, value: Any) = { 
    //some implementation 
    } 
} 

然而在編譯時它會拋出父類和派生類writers之間的類型不匹配。爲什麼會發生？ scala編譯器不檢查映射參數是否是子類型？

Answer 1

Scala的地圖是在密鑰類型不變，從而Map[String, _]沒有類型關係Map[Any, _]

Map documentation

trait Map[A, +B]

注意，沒有對A沒有方差標記，因此它是不變的。

你可以將其參數化：

abstract class MultipleOutputWriter[K, V <: OutputStream] { 
    protected def writers: collection.mutable.Map[K, V] 
    def write(basePath: String, value: Any) 
    def close = writers.values.foreach(_.close) 
} 
class LocalMultipleOutputWriter extends MultipleOutputWriter[String, FileOutputStream] { 
    protected val writers = collection.mutable.Map[String, FileOutputStream]() 
    def write(key: String, value: Any) = { 
    //some implementation 
    } 
}