具有隐式转换的Scala映射

Question

I have a Scala case class and a corresponding Java class. I've declared an implicit conversion from Scala class to Java class. Now I have a Scala collection of Scala classes, and I want to convert them to Scala collection of Java classes. Is it possible with the implicit conversion? Here's what I've tried, naively perhaps:

case class PositionScala(latitude: Double, longitude: Double)
object PositionScala {
  implicit def toJava(p: PositionScala): Position = new Position(p.latitude, p.longitude) // Position is the Java class
}
...
val p: List[Position] = routeScala.shape.map(p => p) 
                     // routeScala.shape is List[PositionScala]

But it doesn't compile with the type mismatch error. Of course I can call the conversion function explicitly, but it defeats the purpose of using implicit converter. Is there a way to do what I want?

Answer 1

You need to put the type annotation in a different place: routeScala.shape.map(p => p: Position) . But I'd say this is less clear than just writing routeScala.shape.map(PositionScala.toJava) .

Answer 2

您可以通过将转换函数'toJava'传递给map函数来实现：

val p: List[Position] = routeScala.shape.map(toJava)

Answer 3

Additionally to @alexey-romanov 's answer I should make a stand for scala type resolution.

map method for scala collections have pretty complex behaviour. It tries to derive resulting collection\\element type basing on visible CanBuildFrom implicit, so it can't say definitely what the result type of map argument should be. Because of that it can't apply implicit conversion here.

If for example you had

val posOpt: Option[PositionScala] = ???

and tried to apply

val posJava: Option[Position] = posOpt.map(p => p) 

It would found your conversion with no problem.