How to Use let with Null Check in Kotlin

In Kotlin, you can use let with a null check by calling it on a nullable variable using the safe call operator ?.. This runs the let block only if the variable is not null, allowing safe access to the non-null value inside the block.

Syntax

The syntax uses the safe call operator ?. followed by let. Inside the let block, the variable is non-null and accessible as it or a named parameter.

• nullableVariable?.let { nonNullValue -> /* use nonNullValue safely */ }
• The block runs only if nullableVariable is not null.

kotlin

val nullableString: String? = "Hello"
nullableString?.let { nonNullString ->
    println("String length is ${nonNullString.length}")
}

Output

String length is 5

Example

This example shows how to use let to print the length of a nullable string only if it is not null.

kotlin

fun main() {
    val name: String? = "Kotlin"
    name?.let {
        println("Name length: ${it.length}")
    }

    val emptyName: String? = null
    emptyName?.let {
        println("This won't print because the value is null")
    }
}

Output

Name length: 6

Common Pitfalls

A common mistake is to call let without the safe call operator ?., which causes the block to run even if the variable is null, leading to a NullPointerException.

Also, using let without handling the null case separately may cause missed logic for null values.

kotlin

val nullableValue: String? = null

// Wrong: This will throw an error
// nullableValue.let { println(it.length) }

// Right: Use safe call operator
nullableValue?.let { println(it.length) }

Quick Reference

Use ?.let { } to run code only when a variable is not null. Inside the block, the variable is non-null and safe to use.

• variable?.let { /* safe code */ } - runs if variable is not null
• variable.let { } - runs always, can cause errors if variable is null

Key Takeaways

Use the safe call operator ?. before let to run code only when the variable is not null.

Inside the let block, the variable is non-null and accessible as it or a named parameter.

Avoid calling let without ?. on nullable variables to prevent NullPointerException.

let helps write concise and safe null checks in Kotlin.

Use let for executing code blocks only when a value is present.