Question

I tried to make some sort of convenient class (below) to hold folder and get file by using filename (string). This work as expect but one thing I don’t understand is map part Map(folder.listFiles map {file => file.getName -> file}:_*).

I place :_* there to prevent some kind of type incompatible but I don’t know what does it really do. Also, what is _* and could I replace is with anything more specific ?

thanks

class FolderAsMap (val folderName:String){
  val folder = new File(folderName)
  private val filesAsMap: Map[String, File] = Map(folder.listFiles map 
                                          {file => file.getName -> file}:_*)

  def get(fileName:String): Option[File] = {
    filesAsMap.get(fileName)
  }
}

Answer 1

: _* is correct. Alternatively, you can use toMap:

folder.listFiles map {file => file.getName -> file}.toMap

Map(...) is method apply in object Map: def apply [A, B] (elems: (A, B)*): Map[A, B]. It has a repeated parameter. It is expected to be called with multiple parameters. The : _* is used to signal you are passing all the parameters as just one Seq argument.

It avoids ambiguities. In java, (where equivalent varargs are arrays instead of Seqs) there is a possible ambiguity, if a method f(Object... args) and you call it with f(someArray), it could mean that args has just one item, with is someArray (so f receives an array of just one element, which its someArray), or args is someArray and f receives someArray directly). Java choose the second version. In scala, with a richer type system and Seq rather than Array the ambiguity may arise much more often, and the rule is that you always have to write : _* when passing all arguments as one, even when no ambiguity is possible, as in here, rather than a complex rule to tell when there is an actual ambiguity.