How to Check if Variable is Null in Kotlin: Simple Guide

In Kotlin, you can check if a variable is null by using the equality operator == null or the safe call operator ?.. For example, if (variable == null) checks if the variable is null safely and clearly.

Syntax

To check if a variable is null in Kotlin, use the equality operator == null. You can also use the safe call operator ?. to safely access properties or methods only if the variable is not null.

• variable == null: Returns true if variable is null.
• variable != null: Returns true if variable is not null.
• variable?.property: Accesses property only if variable is not null, otherwise returns null.

kotlin

val variable: String? = null

if (variable == null) {
    println("Variable is null")
} else {
    println("Variable is not null")
}

Output

Variable is null

Example

This example shows how to check if a nullable string variable is null and how to safely access its length using the safe call operator.

kotlin

fun main() {
    val name: String? = null

    if (name == null) {
        println("Name is null")
    } else {
        println("Name length: ${name.length}")
    }

    // Using safe call operator
    println("Name length using safe call: ${name?.length}")
}

Output

Name is null
Name length using safe call: null

Common Pitfalls

One common mistake is trying to access properties or methods on a nullable variable without checking for null, which causes a compile-time error in Kotlin. Another mistake is using === null which checks for reference equality but == null is preferred for null checks.

Always use == null or safe calls ?. to avoid errors.

kotlin

fun main() {
    val text: String? = null

    // Wrong: causes compile error
    // println(text.length)

    // Right: check null before access
    if (text != null) {
        println(text.length)
    } else {
        println("Text is null")
    }

    // Using safe call
    println(text?.length)
}

Output

Text is null
null

Quick Reference

Check Type	Syntax	Description
Check if null	variable == null	Returns true if variable is null
Check if not null	variable != null	Returns true if variable is not null
Safe call	variable?.property	Access property only if variable is not null
Elvis operator	variable ?: defaultValue	Returns variable if not null, else defaultValue

Key Takeaways

Use 'variable == null' to check if a variable is null in Kotlin.

Use safe call operator '?.' to safely access properties or methods on nullable variables.

Avoid accessing nullable variables directly without null checks to prevent errors.

Use 'variable ?: defaultValue' to provide a default when variable is null.

Prefer '==' over '===' for null checks in Kotlin.