How to Declare Nullable Types in Kotlin: Simple Guide

In Kotlin, you declare a nullable type by adding a ? after the type name, like String?. This tells the compiler the variable can hold either a value or null.

Syntax

To declare a nullable type in Kotlin, add a ? after the type name. This means the variable can hold a value of that type or null.

• TypeName?: The question mark makes the type nullable.
• var or val: Use var for mutable or val for immutable variables.

kotlin

var name: String? = null
val age: Int? = 25

Example

This example shows how to declare nullable variables and safely print their values using Kotlin's safe call operator ?. to avoid errors when the value is null.

kotlin

fun main() {
    var nullableName: String? = null
    println("Name length: " + nullableName?.length) // prints null safely

    nullableName = "Alice"
    println("Name length: " + nullableName?.length) // prints length of "Alice"
}

Output

Name length: null
Name length: 5

Common Pitfalls

Common mistakes include forgetting to add ? for nullable types or trying to use nullable variables without safe calls or null checks, which causes errors.

Always use safe calls ?., the Elvis operator ?:, or explicit null checks before accessing nullable variables.

kotlin

fun main() {
    var name: String? = null
    // Wrong: accessing length directly causes error
    // println(name.length) // ERROR

    // Right: use safe call
    println(name?.length) // prints null safely

    // Or use Elvis operator to provide default
    println(name?.length ?: 0) // prints 0
}

Output

null
0

Quick Reference

Concept	Syntax	Description
Nullable type declaration	var x: Type?	Variable can hold a value or null
Safe call operator	x?.property	Access property only if x is not null
Elvis operator	x ?: default	Provide default if x is null
Non-null assertion	x!!	Assert x is not null (throws error if null)

Key Takeaways

Add a question mark (?) after a type to declare it nullable in Kotlin.

Use safe calls (?.) or null checks to access nullable variables safely.

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

The Elvis operator (?:) helps provide default values when null occurs.

Non-null assertion (!!) should be used carefully as it throws an error if the value is null.