Safe Cast Operator (as?) in Kotlin

Introduction

Safe Cast Operator (as?) 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 safe cast operator as? tries to cast a value to another type. 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 safe cast operator as? tries to cast a value to another type.
• If the cast fails, it returns null instead of throwing ClassCastException.
• It is useful when you are not sure about the runtime type of a value.

Syntax

val result = value as? TargetType

Safe Cast Operator (as?) in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    val value: Any = "Little Drops"
    val text = value as? String
    val number = value as? Int
    println("Text: $text")
    println("Number: $number")
}

Sample Output

Text: Little Drops
Number: null

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. val value: Any = "Little Drops" assigns or updates a value used later in the program.

3. val text = value as? String assigns or updates a value used later in the program.

4. val number = value as? Int assigns or updates a value used later in the program.

5. The println("Text: $text") statement writes a line to the console — this produces part of the sample output below.

6. The println("Number: $number") statement writes a line to the console — this produces part of the sample output below.

7. The safe cast operator as? tries to cast a value to another type.

8. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown 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 safe cast operator as? tries to cast a value to another type.
• If the cast fails, it returns null instead of throwing ClassCastException.
• It is useful when you are not sure about the runtime type of a value.
• 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.