How to Use Extension Function with Nullable in Kotlin

In Kotlin, you can define an extension function for a nullable type by adding a question mark after the receiver type, like fun String?.yourFunction(). This allows you to safely call the function on null or non-null objects, handling null cases inside the function.

Syntax

An extension function for a nullable type uses a question mark ? after the receiver type. This means the function can be called on objects that might be null.

• fun: keyword to declare a function
• String?: nullable receiver type
• yourFunction(): function name and parameters
• Inside the function, this can be null, so handle it safely

kotlin

fun String?.printLength() {
    if (this == null) {
        println("String is null")
    } else {
        println("Length is ${this.length}")
    }
}

Example

This example shows how to define and use an extension function on a nullable String?. It safely checks if the string is null before printing its length.

kotlin

fun String?.printLength() {
    if (this == null) {
        println("String is null")
    } else {
        println("Length is ${this.length}")
    }
}

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

    name.printLength()      // Calls extension on non-null
    emptyName.printLength() // Calls extension on null
}

Output

Length is 6
String is null

Common Pitfalls

One common mistake is forgetting to mark the receiver type as nullable, which causes the extension function to not accept null values and can lead to crashes.

Also, inside the extension function, you must check for null before accessing properties or methods on this.

kotlin

/* Wrong: receiver is non-nullable, so can't call on null */
fun String.printLengthWrong() {
    println(this.length) // Unsafe if called on null
}

/* Right: receiver is nullable and null-safe check */
fun String?.printLengthRight() {
    if (this == null) {
        println("String is null")
    } else {
        println("Length is ${this.length}")
    }
}

Quick Reference

• Use Type? as receiver to allow null values.
• Always check this for null inside the function.
• Call the extension function safely on nullable variables.

Key Takeaways

Mark the receiver type with ? to make the extension function nullable-safe.

Inside the extension, always check if this is null before using it.

Nullable extension functions let you call methods on null objects safely.

For non-nullable receivers, calling on null causes errors.

Use nullable extensions to add safe utilities to nullable types.