Scala子类无法识别父类中的通用映射？

Question

我有一个代码段，其看起来如下：

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 
    } 
} 

然而在编译时它会抛出父类和派生类writers之间的类型不匹配。为什么会发生？ scala编译器不检查映射参数是否是子类型？

Answer 1

Scala的地图是在密钥类型不变，从而Map[String, _]没有类型关系Map[Any, _]

Map documentation

trait Map[A, +B]

注意，没有对A没有方差标记，因此它是不变的。

你可以将其参数化：

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 
    } 
}