Scala - Mutable and Immutable Collections

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Scala has a collection library that is located under the scala.collection package. Collections in Scala are basics to store, retrieve, and manipulate groups of objects efficiently. There are types of collections: mutable and immutable. We will explain both of these in this chapter.

Mutable Collections

You can update and modify immutable type collections even after declaring these. You can update mutable collections in-place, so it will not require you to create other collections for these updates. You update mutable collections in-place. Mutable collections are located at scala.collection.mutable. So you will need to import this library when you work on immutable collections. Mutable collections are used when you need to frequently update the collection.

Examples are: ListBuffer, HashSet, HashMap, ArrayBuffer, Queue, Stack.

Declaring Mutable Collections

The following is the syntax for declaring various mutable collections in Scala -

Syntax

import scala.collection.mutable.ListBuffer
import scala.collection.mutable.HashSet
import scala.collection.mutable.HashMap

val mutableList: ListBuffer[String] = ListBuffer("Apple", "Banana", "Cherry")
val mutableSet: HashSet[String] = HashSet("Apple", "Banana", "Cherry")
val mutableMap: HashMap[String, String] = HashMap
   ("red" -> "#FF0000", "green" -> "#00FF00", "blue" -> "#0000FF")

Note that it is necessary to import their library otherwise it will not compile.

Example

Below is an example program showing how to create, initialize, and process mutable collections

import scala.collection.mutable.ListBuffer
import scala.collection.mutable.HashSet
import scala.collection.mutable.HashMap

object Demo {
   def main(args: Array[String]) = {
      val mutableList: ListBuffer[String] = ListBuffer("Apple", "Banana", "Cherry")
      val mutableSet: HashSet[String] = HashSet("Apple", "Banana", "Cherry")
      val mutableMap: HashMap[String, String] = HashMap
         ("red" -> "#FF0000", "green" -> "#00FF00", "blue" -> "#0000FF")

      // Print the collections
      println(mutableList)
      println(mutableSet)
      println(mutableMap)

      // Modify the collections (in-place)
      mutableList += "Grapes"
      mutableSet += "Grapes"
      mutableMap += ("yellow" -> "#FFFF00")

      // Print the modified collections
      println(mutableList)
      println(mutableSet)
      println(mutableMap)
   }
}

Save the above program in Demo.scala. Use the following commands to compile and execute this program.

Command

> scalac Demo.scala
> scala Demo

Output

ListBuffer(Apple, Banana, Cherry)
Set(Apple, Banana, Cherry)
Map(red -> #FF0000, green -> #00FF00, blue -> #0000FF)
ListBuffer(Apple, Banana, Cherry, Grapes)
Set(Apple, Banana, Cherry, Grapes)
Map(red -> #FF0000, green -> #00FF00, blue -> #0000FF, yellow -> #FFFF00)

Immutable Collections

You cannot update and modify immutable type collections once you create these. If you try to add, remove, and update immutable collections then the compiler will return a new collection. The original collection will remain the same. Immutable collections are located at scala.collection.immutable. So you will need to import this library when you work on immutable collections.

Examples are: List, Set, Map, Vector, Queue, Stack, Range, Option, Tuple, Iterator.

Declaring Immutable Collections

The following is the syntax for declaring various immutable collections in Scala -

Syntax

import scala.collection.immutable.List
import scala.collection.immutable.Set
import scala.collection.immutable.Map

val immutableList: List[String] = List("Apple", "Banana", "Cherry")
val immutableSet: Set[String] = Set("Apple", "Banana", "Cherry")
val immutableMap: Map[String, String] = Map
   ("red" -> "#FF0000", "green" -> "#00FF00", "blue" -> "#0000FF")

Since these are immutable by default, so even if you do not import their immutable collection library then it will also work, like this -

val immutableList: List[String] = List("Apple", "Banana", "Cherry")
val immutableSet: Set[String] = Set("Apple", "Banana", "Cherry")
val immutableMap: Map[String, String] = Map
   ("red" -> "#FF0000", "green" -> "#00FF00", "blue" -> "#0000FF")

Example

object Demo {
   def main(args: Array[String]) = {
      val immutableList: List[String] = List("Apple", "Banana", "Cherry")
      val immutableSet: Set[String] = Set("Apple", "Banana", "Cherry")
      val immutableMap: Map[String, String] = Map
         ("red" -> "#FF0000", "green" -> "#00FF00", "blue" -> "#0000FF")

      // Print the collections
      println(immutableList)
      println(immutableSet)
      println(immutableMap)

      // Attempt to modify collections (will return new collections)
      val newList = immutableList :+ "Grapes"
      val newSet = immutableSet + "Grapes"
      val newMap = immutableMap + ("yellow" -> "#FFFF00")

      // Print the new collections
      println(newList)
      println(newSet)
      println(newMap)
   }
}

Save the above program in Demo.scala. Use the following commands to compile and execute this program.

Command

> scalac Demo.scala
> scala Demo

Output

List(Apple, Banana, Cherry)
Set(Apple, Banana, Cherry)
Map(red -> #FF0000, green -> #00FF00, blue -> #0000FF)
List(Apple, Banana, Cherry, Grapes)
Set(Apple, Banana, Cherry, Grapes)
Map(red -> #FF0000, green -> #00FF00, blue -> #0000FF, yellow -> #FFFF00)

Difference between Mutable and Immutable Collections