Kotlin·Kotlin from syntax to a coroutine app

Null safety

Use nullable types, safe calls, Elvis and smart casts.

Beginner 13 min read Lesson 2 of 300 Challenge included

What you will be able to do

Add ? to allow null.
Use ?. to call safely.
Use ?: to provide a default.
Avoid !! in production code.

In plain English

Kotlin separates nullable and non-null types. A String cannot be null; String? can. Use ?. for safe calls, ?: for defaults and !!. only when you can prove non-null.

Nulls.ktKotlin

fun greet(name: String?): String {
    val safe = name?.trim()?.takeIf { it.isNotEmpty() } ?: "friend"
    return "Hello, $safe"
}

fun main() {
    println(greet(null))
    println(greet(" Ada "))
}

Your turn

Provide a default

Easy

Use the Elvis operator to default a nullable name to "friend".

KOTLIN · starter

val name: String? = null

Check your understanding before moving on.

Knowledge checkQuestion 1

What does ?. do?

Select an answer to continue

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Lesson 2 of 300 · 0% of course