Null check

Learn null checking with if (x != null).

Null check: if (x != null)

When you check a nullable variable with if (x != null), the compiler smart-casts x to its non-null type inside the block. You do not need !!.

val name: String? = "Ava"
if (name != null) {
    println(name.length)  // smart cast: String
}

Outside the block name is still String?; the null-risk remains. This keeps the check valid only inside the block.

Null check and early return

The if (x == null) return pattern handles the null case early at the top of a function, so the rest of the code can be written as non-null. This avoids needless nesting.

fun greet(name: String?) {
    if (name == null) return
    println("Hello $name")
}

After the early return, name is now String; no ?. or !! is needed. This technique is especially useful when validating at the top of a function.

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

Outside the if (name != null) block, what is the type of name?
- String (smart cast)
- Any
- Unit
- String?
Show answer

String? — Outside the block name is still String?; smart cast applies only inside.
Match expression to role

if (x != null)

Inside, x is non-null
if (x == null)

Inside, x is null
smart cast

Type narrows automatically
return

Early exit from block

Fill in the blank: null check

if (name ____ null) { println(name.length) }

Show answer

if (name != null) { println(name.length) }

Next lesson: lateinit

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