Scala Maps and Option

Before you start

Scala collections.

Look up a key without nulls by using Option.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

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.

Code example

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

Example details

Follow the code

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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

Behind the scenes

How Scala processes this code

Scala compiles for the JVM, which invokes the main entry point.

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.