Scala’s Seq is an ordered sequence of values. It further breaks into two categories:

  • 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.

  • coll :+ elem append
  • elem +: coll prepend


A Vector is the immutable equivalent of ArrayBuffer. It is implemented as trees, and each node has up to 32 children.


It represents an immutable integer sequence. It only stores start, end, and imcrement. It is constructed using to and until methods.

Stack, Queue, PriorityQueue

Just as normal data structures.

Immutable Lists

A List in Scala is either Nil (empty) or an object with a head element and a tail (that is a List). So a List is like a node in a linked list.

The :: operator makes a new list from given head and tail:

9 :: List (4, 2)

It is right-associative. It can also destructure the list into head and tail.

It is natural to use recursion to traverse a list.

Mutable Lists

LinkedList: you can modify the head (lst.elem) and tail ( DoubleLinkedList: has a mutable prev reference.

  • elem :: lst, elem +: lst : a new list with elem prepended to lst.
  • lst2 ::: lst, lst2 ++: lst: a new list with lst2 prepended to lst.