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?

Chris Martin · Accepted Answer · 2014-11-03 07:26:28Z

7

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

For example, Traversable:

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.

edited Nov 3, 2014 at 7:26

answered Nov 3, 2014 at 7:09

Chris Martin

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4 Comments

ssj Over a year ago

but there is not apply method in the companion object of Traversable?

Chris Martin Over a year ago

There is. Look at the second link. It's the second item listed in the "Value Members" section.

ssj Over a year ago

oh, I see. The apply method is extends from TraversableFactory.

Chris Martin Over a year ago

If you click on apply, 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.

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