Scala Maps and Option

Look up a key without nulls by using Option.

Scala

object Main {
  def main(args: Array[String]): Unit = {
    val ages = Map("Ada" -> 36, "Grace" -> 45)
    println(ages.get("Ada"))
    println(ages.get("Linus"))
    println(ages.getOrElse("Linus", 0))
  }
}

Output

Some(36)
None
0

Explanation

Map("Ada" -> 36) builds an immutable map from key-value pairs. Instead of returning null for a missing key, get returns an Option: Some(value) if the key exists, or None if it does not.

Because the possibility of absence is part of the type, the compiler makes you handle it. getOrElse offers a fallback in one step.

Explore the dry run

Guided walkthroughReady

1object Main {

2 def main(args: Array[String]): Unit = {

3 val ages = Map("Ada" -> 36, "Grace" -> 45)

4 println(ages.get("Ada"))

5 println(ages.get("Linus"))

6 println(ages.getOrElse("Linus", 0))

7 }

8}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
An immutable map with two entries.	`—`	—
The key exists, so the result is wrapped in Some.	`—`	Some(36)
The key is absent, so the result is None.	`—`	Some(36) None
Use the fallback value for the missing key.	`—`	Some(36) None 0

Cost and common mistakes

Lookup is effectively constant time for the default immutable hash map.

Calling ages("Linus") instead of get throws NoSuchElementException for a missing key.
Adding to an immutable Map with + returns a new map; the original is unchanged.

Try it yourself

Predict ages.get("Grace").map(a => a + 1).

Reveal answer and reasoning

Some(46); map applies the function only when a value is present.

Scala Maps and Option

Explanation

Follow the example

Cost and common mistakes

Try it yourself

Further reading