Why can traits in scala.collection create an instance?

Question

I am trying the following code in Scala, which is coming from An Overview of the Collections API.

import collection._
scala> Traversable(1, 2, 3)
res5: Traversable[Int] = List(1, 2, 3)
scala> Iterable("x", "y", "z")
res6: Iterable[String] = List(x, y, z)
scala> Map("x" -> 24, "y" -> 25, "z" -> 26)
res7: scala.collection.Map[String,Int] = Map(x -> 24, y -> 25, z -> 26)
scala> SortedSet("hello", "world")
res9: scala.collection.SortedSet[String] = TreeSet(hello, world)
scala> IndexedSeq(1.0, 2.0)
res11: IndexedSeq[Double] = Vector(1.0, 2.0)

The result shows that those trait can all call its apply method to create an instance of its implementation. But after looking for scala.collection.package object, I found nothing. I think there must be somewhere that binds those trait to its subclass and imports to my program. Can someone explain where it is?

Answer 1

You're calling apply on the trait's companion object, not on the trait.

For example, Traversable:

• The trait

• The object

If you click on apply in the companion object's scaladoc, you can see that the Traversable object inherits its apply method from GenericCompanion, which has a link to the source so you can see how it's implemented.