Collections – Seq, Set and Map

As part of this topic we will see one of the important topics for building Spark applications using Scala i.e Collections.


Scala collections are categorized into 3 types

  1. Seq
  2. Set
  3. Map


  • Sequence have length
  • Elements in Sequence can be accessed using prefix
  • eg: scala.Array, scala.collection.immutable.List etc
  • Classes are divided into Seq classes and Buffer classes
  • Vector and List are considered as Seq classes and they are more frequently used
  • Array is special type of collection analogous to Java Array
  • We will get into the details of Array and List
  • Array is mutable while List is immutable
  • Seq have 2 sub traits – IndexedSeq and LinearSeq


  • Set is iterable which contain unique elements (no duplicates)
  • As duplicates are not allowed, there is no length (Seq have both length and index)
  • Even though we can access data using index, it might not return same value always
  • By default Set is immutable, but there is mutable one as well
  • Let us see how we can use mutable and immutable Set in a demo
  • If the Set contain elements for which implicit Ordering defined, we can use SortedSet to sort the data


  • Map is Iterable of Key and Value pairs
  • Each element can be defined as key -> value or (key, value)

We will look into few collection classes such as Array,List,Set and Map.

Declaration of Array

val a = Array(1,2,3,4)

Declaration of List

val l = List(1,2,3,4)

Declaration of set

val s = Set(1,1,2,2,3,4)

Declaration of Map

val m = Map("Hello" -> 1 , "World" -> 2)

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