Choosing a Collections Class

Tips

If you’re coming to Scala from Java, forget the Java collections classes. Using them will just slow you down.
Vector and ArrayBuffer are your “go to” sequence types.

Immutable vs Mutable

The collections types can be classified as immutable (unchangeable) and mutable:

Immutable type are available by default (Vector, List, and the immutable Map and Set)
- Immutable is preferred (always use immutable, unless there’s a good reason not to)
Mutable types have to be imported to use them

Sequences are Indexed or Linear

Sequence types can also be classified as indexed or linear:

Indexed
- Fast, random access to elements
- Vector, Seq, ArrayBuffer
Linear
- i.e., a linked list (List)
- Random access is slow

The “Go To” (common) data types

Vector and ArrayBuffer are the preferred sequence types.
List is also okay for small, immutable sequences (or when you are prepending elements and accessing the head and tail components).

Immutable     |  Mutable
----------------------------
List, Vector  |  ArrayBuffer
Map           |  Map
Set           |  Set

Common types

Vector is an immutable, indexed sequence
List is an immutable, linear sequence
ArrayBuffer is a mutable, indexed sequence
We use other types like Map, Set, Range, and tuples all the time.

Other:

There is an Array, which wraps the Java array, but we don’t really use it directly (use ArrayBuffer instead)

When returning types from functions:

Seq implies immutable
IndexedSeq implies immutable, indexed

Creating collections instances is very consistent:

val a = Vector(1, 2, 3)
val b = List(1, 2, 3)
val c = ArrayBuffer(1, 2, 3)
val d = Set(1, 1, 1, 2, 2, 2, 3)
val e = Map(1 -> "a", 2 -> "b")

Using them in for expressions is also consistent, as are append, prepend, update, and delete methods. (See the Vector and ArrayBuffer videos for examples.)

Map

A Map is a list of key/value pairs, where the keys are unique.
The immutable Map is available by default.
You have to import the mutable Map if you want to use it.

val a = Map("a" -> 1, "b" -> 2)
val m = Map(
    "first_name" -> "Alvin",
    "last_name"  -> "Alexander"
)

Set

A Set is a collection of unique elements.
The immutable Set is available by default.
You have to import the mutable Set if you want to use it.

val set = Set(1, 1, 2, 2, 3, 3, 3)
// result: Set(1, 2, 3)

Range

A Range is a sequence of equally spaced values.
Typically created with to (and until).

1 to 10
1 to 10 by 2

'a' to 'e'
'a' to 'e' by 2

for i <- 1 to 10 yield i * 10

Tuple

Not really a collection class, but similar.
A heterogenous collection of values.

(1, 2.2, 3L, 'a', "yo")    // (Int, Double, Long, Char, String)

val t = (1, 2.2, 3L, 'a', "yo")
t(0)
t(1)

def getUserInfo(): (String, Int, Double) =
    // do some stuff here, then return a 3-element tuple
    ("Al", 42, 200.0)

val (name, age, weight) = getUserInfo()

See the “Tuples” video for more details.

More resources

Detailed blog posts on my website, with hundreds of examples: