How do I use let to execute code only when a value is not null in Kotlin?

Use ?.let { ... }.

The safe-call operator ?. makes sure let is called only when the value is not null. Inside the let block, the value is available as a non-null value, usually named it.

val name: String? = "Kotlin"

name?.let {
    println("Name is $it")
    println("Length is ${it.length}")
}

If name is "Kotlin", the block runs.

If name is null, the block is skipped.

You can also give the value a clearer name:

val name: String? = "Kotlin"

name?.let { nonNullName ->
    println("Name is $nonNullName")
    println("Length is ${nonNullName.length}")
}

To handle the null case too, combine it with the Elvis operator ?::

val name: String? = null

name?.let {
    println("Name is $it")
} ?: println("Name was null")

So the general pattern is:

nullableValue?.let { value ->
    // Runs only when nullableValue is not null
}