Scala’s collection classes begin with the Traversable and Iterable traits, and extend into the three main categories of sequences (Seq), sets (Set), and maps (Map). Sequences further branch off into indexed and linear sequences.

The Traversable trait lets you traverse an entire collection, and its Scaladoc states that it “implements the behavior common to all collections in terms of a foreach method,” which lets you traverse the collection repeatedly.

The Iterable trait yields an Iterator that can be used as:

val coll = ... // some collection
val iter = coll.iter

When using an iterator, the collection can be traversed only once, because each element is consumed during the iteration process.

  • Seq is an ordered sequence of values:
  • IndexSeq indicates that reandom access of elements is efficient through index. By default, IndexSeq creates a Vector.
  • LinearSeq implis that the collection can be efficiently split into head and tail. By default, LinearSeq creates a List, which is a singly linked list.
  • Set is an unordered collection of values.
  • Map is a set of (key, value) pairs.

Scala prefers immutable collections: companion objects in scala.collection package produce immutable collections. And Predef object (which is always imported) has alias to immutable List, Set and Map.

Best practice: to use both immutable and mutable collection, import scala.collection.mutable and you can get Map and immutable map and mutable.Map as mutable.

Work with immutable collections: you can create new collections out of old ones.

coll ++ coll2 and coll ++: coll2 returns a collection of the same type with all elements combined.

For mutable collections:

  • coll += elem, coll -= elem: add or remove elem to coll.
  • coll += (e1, e2, ...), coll -= (e1, e2, ...): add or remove elements to coll.
  • coll ++= coll2, coll --= coll2: add or remove coll2 to coll.

Strict and lazy collections

A transformer method is a method that constructs a new collection from an existing collection. This includes methods like map, filter, reverse, etc. — any method that transforms the input collection to a new output collection. Given that definition, collections can also be thought of in terms of being strict or lazy.

In a strict collection, memory for the elements is allocated immediately, and all of its elements are immediately evaluated when a transformer method is invoked.

In a lazy collection, memory for the elements is not allocated immediately, and transformer methods do not construct new elements until they are demanded. All of the collection classes except Stream are strict, but the other collection classes can be converted to a lazy collection by creating a view on the collection.

Common Methods

  • map: applies the one-to-one function to all elements of a collection, and return a new collection.
  • take and takeRight: select first or last n elements.
  • drop and dropRight: select all elements except first or last n.
  • flatMap: if an element of a collection is a collection, treat its elements as in the outer collection and apply the function on to them directly. For example, Option is a collection with 0 or 1 element, so flapMap will only apply to those match to Some().
  • foreach: similar to map but it doesn’t return a value.
  • filter: yields all elements of a collection taht matches a condition.
  • reduceLeft: takes a binary function, and applies to all elements of a sequence from left to right. For example,
    (1 to 100).reduceLeft(_ + _)

    sums all numbers from 1 to 100. In _ + _ each underscore represents a separate parameter.

  • sortWith: takes a binary comparison function for sorting.
  • /:: Apply a binary operator to a start value and all elements of this traversable or iterator, from left to right. :\ does the same from right to left. Note the order of parameters for this binary operator.
def removeAll(collection: TraversableOnce[A], items: Iterable[A]) = (collection /: items) { (coll, item) => coll.remove(item) }

def addAll(collection: TraversableOnce[A], items: Iterable[A]) = (items :\ collection) { (item, coll) => coll.add(item) }


Tuple is a set of values, up to 22, enclosed in parentheses. Underlying they are instances of Tuple1 to Tuple22 classes. To access its components, using _1, _2, etc. Note it starts from 1.

(a, b) and (a -> b) both produce a Tuple2. However, latter form is easier to read.

Tuples are usually for multiple assignments: val (first, second, third) = t. It can be useful when a function or method returns multiple values. Unnecessary components are marked out with _ placeholder.

zip method

  • zip bundles elements in two collections into a sequence of tuples. If one collection is longer than the other, the remaining is ignored.
  • zipAll fills the shorter collection with default value.
  • zipped, available on 2- and 3-tuple of lists, does 2- or 3-way zip of multiple lists.
  • zipWithIndex converts a list into a list of (elem, index) pairs. Note the index is the second element in the tuple.


A generic type to hold some optional information. It can be matched to Some(a) or None. It’s better than null check. You can think of it as an iterable that holds either zero or one values, so you can use foreach to perform an operation if it is not empty.


Either[A, B] is more general than Option[A]’s Some()/None pair. To get the concrete type, match against Left(A) and Right(B).