How to Use is Keyword in Kotlin: Type Checking Made Easy

In Kotlin, the is keyword checks if a variable is of a specific type and returns a Boolean. It is used like variable is Type to test the type at runtime, enabling safe type checks and smart casts.

Syntax

The is keyword is used to check if a value is of a certain type. The syntax is:

• variable is Type: Returns true if variable is an instance of Type.
• variable !is Type: Returns true if variable is NOT an instance of Type.

This helps you safely check types before using them.

kotlin

if (obj is String) {
    // obj is treated as String here
}

if (obj !is Int) {
    // obj is NOT an Int
}

Example

This example shows how to use is to check a variable's type and then use it safely with smart casting.

kotlin

fun describe(obj: Any): String {
    return if (obj is String) {
        "String of length ${obj.length}"
    } else if (obj is Int) {
        "Integer value $obj"
    } else {
        "Unknown type"
    }
}

fun main() {
    println(describe("Hello"))
    println(describe(42))
    println(describe(3.14))
}

Output

String of length 5
Integer value 42
Unknown type

Common Pitfalls

One common mistake is to check the type but then cast manually instead of relying on Kotlin's smart cast. Also, is only works with non-nullable types unless you handle null explicitly.

Wrong approach:

if (obj is String) {
    val str = obj as String  // unnecessary cast
    println(str.length)
}

Right approach:

if (obj is String) {
    println(obj.length)  // smart cast automatically treats obj as String
}

Quick Reference

Usage	Description
variable is Type	Returns true if variable is an instance of Type
variable !is Type	Returns true if variable is NOT an instance of Type
Smart cast after is check	Variable is automatically cast to Type inside the if block
Works with nullable types	Use with care; null is not an instance of any type

Key Takeaways

Use is to check a variable's type safely at runtime.

Kotlin smart casts the variable inside the if (variable is Type) block automatically.

Avoid unnecessary manual casts after using is.

!is checks if a variable is NOT of a certain type.

Remember that is does not consider null as an instance of any type.