Scala Cookbook

11.12. Creating Maps

Problem

You want to use a mutable or immutable Map in a Scala application.

Solution

To use an immutable map, you don’t need an import statement, just create a Map:

scala> val states = Map("AL" -> "Alabama", "AK" -> "Alaska")
states: scala.collection.immutable.Map[String,String] =
  Map(AL -> Alabama, AK -> Alaska)

This expression creates an immutable Map with type [String, String]. For the first element, the string AL is the key, and Alabama is the value.

As noted, you don’t need an import statement to use a basic, immutable Map. The Scala Predef object brings the immutable Map trait into scope by defining a type alias:

type Map[A, +B] = immutable.Map[A, B]
val Map         = immutable.Map

To create a mutable map, either use an import statement to bring it into scope, or specify the full path to the scala.collection.mutable.Map class when you create an instance. You can define a mutable Map that has initial elements:

scala> var states = collection.mutable.Map("AL" -> "Alabama")
states: scala.collection.mutable.Map[String,String] = Map(AL -> Alabama)

You can also create an empty, mutable Map initially, and add elements to it later:

scala> var states = collection.mutable.Map[String, String]()
states: scala.collection.mutable.Map[String,String] = Map()

scala> states += ("AL" -> "Alabama")
res0: scala.collection.mutable.Map[String,String] = Map(AL -> Alabama)

Discussion

Like maps in other programming languages, maps in Scala are a collection of key/value pairs. If you’ve used maps in Java, ...