Summary - Null safety

Combine ?, !!, ?: and let to write a null-safe function.

Summary: ?, !!, ?: and let together

In this summary you will combine the null-safe tools. A nullable value can be processed with ?.let and given a fallback with ?:. !! is used only at points where you say "I am sure".

val name: String? = null
name?.let { println("Hello $it") } ?: println("Guest")
// Guest

This chain prints "Guest" when null; otherwise it greets. So one line manages both branches.

Exercise Greet with let and Elvis together

Define a val name: String? = null. Print a null-safe greeting with name?.let { println("Hello $it") } ?: println("Guest").

Show example solution

fun main() {
    val name: String? = null
    name?.let { println("Hello $it") } ?: println("Guest")
}

Exercise Print length with if (name != null)

Define a val name: String? = "Ava". Inside an if (name != null) block, print the length with println(name.length).

Show example solution

fun main() {
    val name: String? = "Ava"
    if (name != null) {
        println(name.length)
    }
}

Test yourself

Which uses let and Elvis together?
- if (name == null) println(name)
- name?.let { println(it) } ?: println("Guest")
- name!! + name?.length
- println(name)
Show answer

name?.let { println(it) } ?: println("Guest") — ?.let runs when not null, ?: gives a default when null.
Match operator to role

?.

Safe call
!!

Non-null assertion
?:

Elvis default
?.let { }

Null-safe block

Next lesson: Task model

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