If I import scala.collection._ and create a queue:
import scala.collection._
var queue = new Queue[Component]();
I get the following error:
error: not found: type Queue
However, if I also add
import scala.collection.mutable.Queue
The error will disappear.
Why is this happening?
Shouldn't scala.collection._ contain scala.collection.mutable.Queue?
You have to know how the Scala collections library is structured. It splits collections based on whether they are mutable or immutable.
Queue lives in the scala.collection.mutable package and scala.collection.immutable package. You have to specify which one you want e.g.
scala> import scala.collection.mutable._
import scala.collection.mutable._
scala> var q = new Queue[Int]()
q: scala.collection.mutable.Queue[Int] = Queue()
scala> import scala.collection.immutable._
import scala.collection.immutable._
scala> var q = Queue[Int]()
q: scala.collection.immutable.Queue[Int] = Queue()
After import scala.collection._ you can use mutable.Queue; you could write Queue if there was a scala.collection.Queue (or one of scala.Queue, java.lang.Queue, and scala.Predef.Queue, since all of their members are imported by default), but there isn't.
It should be easy to see why it works this way: otherwise, the compiler (or anyone reading your code) would have no idea where they should look for the type: do you want scala.collection.Queue, scala.collection.mutable.Queue, or scala.collection.some.subpackage.from.a.library.Queue?
