Maps, Options, and Tuples

This chapter is from the book

Scala for the Impatient, 3rd Edition

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This chapter is from the book

This chapter is from the book 

Scala for the Impatient, 3rd Edition

Learn More Buy

4.7 The Option Type

The Option class in the standard library expresses values that might or might not be present. The subclass Some wraps a value. The object None indicates that there is no value.

var friend: Option[String] = Some("Fred")
friend = None // No friend

This is less ambiguous than using an empty string and safer than using null for a missing value.

Option is a generic type. For example, Some("Fred") is an Option[String].

The get method of the Map class returns an Option. If there is no value for a given key, get returns None. Otherwise, it wraps the value inside Some.

val scores = Map("Alice" -> 10, "Bob" -> 7, "Cindy" -> 8)
val alicesScore = scores.get("Alice") // Some(10)
val dansScore = scores.get("Dan") // None

To find out what is inside an Option instance, you can use the isEmpty and get methods:

if alicesScore.isEmpty
  then println("No score")
  else println(alicesScore.get)

That’s tedious, though. It is better to use the getOrElse method:

println(alicesScore.getOrElse("No score"))

If alicesScore is None, then getOrElse returns "No score".

alicesScore.getOrElse(System.getProperty("DEFAULT_SCORE"))

A more powerful way of working with options is to consider them as collections that have zero or one element. You can visit the element with a for loop:

for score <- alicesScore do println(score)

If alicesScore is None, nothing happens. If it is a Some, then the loop executes once, with score bound to the contents of the option.

You can also use methods such as map, filter, or foreach. For example,

val biggerScore = alicesScore.map(_ + 1) // Some(score + 1) or None
val acceptableScore = alicesScore.filter(_ > 5) // Some(score) if score > 5 or None
alicesScore.foreach(println) // Prints the score if it exists