let Function for Null Safety in Kotlin

Introduction

let Function for Null Safety is a fundamental concept every Kotlin developer should understand. Null safety is one of Kotlin’s signature features. It catches null-related bugs at compile time instead of crashing at runtime with a NullPointerException.

The let function executes a block only when the object is not null. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• The let function executes a block only when the object is not null.
• Inside the block, the object is available as it (or a named parameter).
• It is commonly used with the safe call operator ?.

Syntax

nullableValue?.let { value ->
    // use value safely
}

let Function for Null Safety in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    var name: String? = "Kotlin"
    name?.let { value ->
        println("Length of $value is ${value.length}")
    }
    name = null
    name?.let { value ->
        println("This will not print")
    }
    println("Program completed")
}

Sample Output

Length of Kotlin is 6
Program completed

When to use

Use nullable types when a value may legitimately be absent — optional fields, parsed input, or database lookups that can miss.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. var name: String? = "Kotlin" assigns or updates a value used later in the program.

3. name?.let { value -> demonstrates null-safe or type-safe Kotlin syntax in action.

4. The println("Length of $value is ${value.length}") statement writes a line to the console — this produces part of the sample output below.

5. name = null assigns or updates a value used later in the program.

6. name?.let { value -> demonstrates null-safe or type-safe Kotlin syntax in action.

7. The println("This will not print") statement writes a line to the console — this produces part of the sample output below.

8. The println("Program completed") statement writes a line to the console — this produces part of the sample output below.

Best Practices

• Prefer non-nullable types by default; add ? only when null is a valid state.
• Use safe call (?.) and Elvis (?:) instead of force unwrap (!!) unless you are certain.
• Handle nullable collections and nested properties with ?. chains or let blocks.

Common Mistakes

• Using !! everywhere instead of handling null safely — this reintroduces NPE risk.
• Forgetting that platform types from Java are implicitly nullable.
• Comparing nullable strings with == without considering both sides may be null.

Key Points

• The let function executes a block only when the object is not null.
• Inside the block, the object is available as it (or a named parameter).
• It is commonly used with the safe call operator ?.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.