Scala Cookbook

11.13. Choosing a Map Implementation

Problem

You need to choose a map class for a particular problem.

Solution

Scala has a wealth of map types to choose from, and you can even use Java map classes.

If you’re looking for a basic map class, where sorting or insertion order doesn’t matter, you can either choose the default, immutable Map, or import the mutable Map, as shown in the previous recipe.

If you want a map that returns its elements in sorted order by keys, use a SortedMap:

scala> import scala.collection.SortedMap
import scala.collection.SortedMap

scala> val grades = SortedMap("Kim" -> 90,
     | "Al" -> 85,
     | "Melissa" -> 95,
     | "Emily" -> 91,
     | "Hannah" -> 92
     | )
grades: scala.collection.SortedMap[String,Int] =
    Map(Al -> 85, Emily -> 91, Hannah -> 92, Kim -> 90, Melissa -> 95)

If you want a map that remembers the insertion order of its elements, use a LinkedHashMap or ListMap. Scala only has a mutable LinkedHashMap, and it returns its elements in the order you inserted them:

scala> import scala.collection.mutable.LinkedHashMap
import scala.collection.mutable.LinkedHashMap

scala> var states = LinkedHashMap("IL" -> "Illinois")
states: scala.collection.mutable.LinkedHashMap[String,String] =
  Map(IL -> Illinois)

scala> states += ("KY" -> "Kentucky")
res0: scala.collection.mutable.LinkedHashMap[String,String] =
  Map(IL -> Illinois, KY -> Kentucky)

scala> states += ("TX" -> "Texas")
res1: scala.collection.mutable.LinkedHashMap[String,String] =
  Map(IL -> Illinois, KY -> Kentucky, TX -> Texas)