Contains the base traits and objects needed to use and extend Scala's collection library.
GuideA detailed guide for using the collections library is available at http://docs.scala-lang.org/overviews/collections/introduction.html. Developers looking to extend the collections library can find a description of its architecture at http://docs.scala-lang.org/overviews/core/architecture-of-scala-collections.html.
Using CollectionsIt is convenient to treat all collections as either a scala.collection.Traversable or scala.collection.Iterable, as these traits define the vast majority of operations on a collection.
Collections can, of course, be treated as specifically as needed, and the library is designed to ensure that the methods that transform collections will return a collection of the same type:
scala> val array = Array(1,2,3,4,5,6)
array: Array[Int] = Array(1, 2, 3, 4, 5, 6)
scala> array map { _.toString }
res0: Array[String] = Array(1, 2, 3, 4, 5, 6)
scala> val list = List(1,2,3,4,5,6)
list: List[Int] = List(1, 2, 3, 4, 5, 6)
scala> list map { _.toString }
res1: List[String] = List(1, 2, 3, 4, 5, 6)Creating Collections
The most common way to create a collection is to use its companion object as a factory. The three most commonly used collections are scala.collection.Seq, scala.collection.immutable.Set, and scala.collection.immutable.Map. They can be used directly as shown below since their companion objects are all available as type aliases in either the scala package or in scala.Predef. New collections are created like this:
scala> val seq = Seq(1,2,3,4,1) seq: Seq[Int] = List(1, 2, 3, 4, 1) scala> val set = Set(1,2,3,4,1) set: scala.collection.immutable.Set[Int] = Set(1, 2, 3, 4) scala> val map = Map(1 -> "one", 2 -> "two", 3 -> "three", 2 -> "too") map: scala.collection.immutable.Map[Int,String] = Map(1 -> one, 2 -> too, 3 -> three)
It is also typical to prefer the scala.collection.immutable collections over those in scala.collection.mutable; the types aliased in the scala.Predef object are the immutable versions.
Also note that the collections library was carefully designed to include several implementations of each of the three basic collection types. These implementations have specific performance characteristics which are described in the guide.
The concrete parallel collections also have specific performance characteristics which are described in the parallel collections guide
Converting to and from Java CollectionsThe scala.collection.JavaConverters object provides a collection of decorators that allow converting between Scala and Java collections using asScala and asJava methods.
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