Nullable types and safe calls

A student may not have a recorded mark yet. Zero is a valid numerical value and should not also silently mean missing. A nullable type such as Int? permits either a number or null, making absence part of the declared contract.

What you’ll learn

Represent absence with a nullable type.
Use safe calls and meaningful fallbacks.

Nullable types and safe calls in the example

1
Absent score
A student may not have a recorded mark yet.
```
Absent score
```
→2
Safe access
The compiler restricts unsafe access to nullable values.
```
Safe access
```
→3
Explicit fallback
The absent score uses the display fallback -1.
```
Explicit fallback
```

The absent score uses the display fallback -1. The name is present and has four code units. The optional score itself remains absent; printing a fallback does not create a recorded mark.

Understand nullable types and safe calls

The compiler restricts unsafe access to nullable values. A safe call such as name?.length produces null when the receiver is absent. The Elvis operator ?: provides a chosen fallback. The non-null assertion !! can throw if its assumption is false.

Follow nullable types and safe calls

Save the complete source as Main.kt and use the Kotlin/JVM environment. No separate input file is needed.

Kotlin/JVM · complete program

fun main() {
  val score: Int? = null
  val name: String? = "Diya"
  println(score ?: -1)
  println(name?.length ?: 0)
}

Expected output

-1
4

Follow the execution

The absent score uses the display fallback -1. The name is present and has four code units. The optional score itself remains absent; printing a fallback does not create a recorded mark.

Try this in the Kotlin compiler by pasting the complete program.

Practice: Nullable types and safe calls

Your task: Would replacing null with zero preserve missingness?

Kotlin/JVM · complete program

fun main() {
  val score: Int? = null
  val name: String? = "Diya"
  println(score ?: -1)
  println(name?.length ?: 0)
}

Worked answer

No. A real zero and an absent result would become indistinguishable without another convention. Keep the nullable representation and choose an honest display message.