Scala 2.8 Collections API -- An Overview of the Collections API

Traversable

Iterable

Seq

IndexedSeq

immutable.Vector

mutable.ResizableArray

mutable.ArraySeq

mutable.StringBuilder

(String, Array)

LinearSeq

immutable.List

immutable.Stream

mutable.MutableList

mutable.Queue

mutable.Buffer

mutable.ListBuffer

mutable.ArrayBuffer

Range

Set

SortedSet

immutable.TreeSet

immutable.HashSet

mutable.HashSet

mutable.LinkedHashSet

BitSet

(immutable.EmptySet, immutable.Set1, ..., immutable.Set4)

Map

SortedMap

immutable.TreeMap

immutable.HashMap

mutable.HashMap

mutable.LinkedHashMap

(immutable.EmptyMap, immutable.Map1, ..., immutable.Map4)

The most important collection classes are shown in the figure above. There is quite a bit of commonality shared by all these classes. For instance, every kind of collection can be created by the same uniform syntax, writing the collection class name followed by its elements:

Traversable(1, 2, 3)

Iterable("x", "y", "z")

Map("x" -> 24, "y" -> 25, "z" -> 26)

Set(Color.red, Color.green, Color.blue)

SortedSet("hello", "world")

Buffer(x, y, z)

IndexedSeq(1.0, 2.0)

LinearSeq(a, b, c)

List(1, 2, 3)

HashMap("x" -> 24, "y" -> 25, "z" -> 26)

All collections support the API provided by Traversable, but specialize types wherever this makes sense. For instance the map method in class Traversable returns another Traversable as its result. But this result type is overridden in subclasses. For instance, calling map on a List yields again a List, calling it on a Set yields again a Set and so on.

scala> List(1, 2, 3) map (_ + 1)

res0: List[Int] = List(2, 3, 4)

scala> Set(1, 2, 3) map (_ * 2)

res0: Set[Int] = Set(2, 4, 6)

Most of the classes in the collections hierarchy exist in three variants: root, mutable, and immutable. The only exception is the Buffer trait which only exists as a mutable collection.