How to Use when with is Keyword in Kotlin

In Kotlin, you can use the when expression with the is keyword to check the type of a variable and execute code based on that type. This allows you to write clear and concise type checks without explicit casting.

Syntax

The when expression evaluates a variable against multiple conditions. Using is inside when checks if the variable is of a certain type.

• when (variable) { - starts the expression with the variable to check
• is Type -> - checks if variable is of Type
• else -> - default case if no type matches

kotlin

when (variable) {
    is Type1 -> {
        // code for Type1
    }
    is Type2 -> {
        // code for Type2
    }
    else -> {
        // code if no type matches
    }
}

Example

This example shows how to use when with is to check the type of an Any variable and print a message accordingly.

kotlin

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

fun main() {
    println(describe(42))
    println(describe("hello"))
    println(describe(true))
    println(describe(3.14))
}

Output

Integer: 42
String of length 5
Boolean value: true
Unknown type

Common Pitfalls

One common mistake is forgetting that is checks the type but does not cast automatically outside the when branch. Inside each branch, Kotlin smart casts the variable to the checked type.

Another pitfall is not including an else branch, which can cause the when expression to be non-exhaustive if the compiler cannot verify all types are covered.

kotlin

fun checkType(obj: Any) {
    when (obj) {
        is String -> println(obj.length) // smart cast works here
        // Missing else branch can cause warning if not all types covered
    }
}

// Wrong: Trying to use obj as String outside when branch
fun wrongCast(obj: Any) {
    if (obj is String) {
        println(obj.length) // OK
    }
    // println(obj.length) // Error: obj is Any here
}

Quick Reference

Tips for using when with is keyword:

• Use is Type to check variable type inside when.
• Kotlin smart casts variable to the checked type inside the branch.
• Always provide an else branch for completeness.
• Use !is Type to check if variable is NOT of a type.

Key Takeaways

Use when with is to check variable types cleanly in Kotlin.

Inside each is Type branch, Kotlin smart casts the variable to that type automatically.

Always include an else branch to handle unexpected types.

Remember is checks type but does not cast outside the when branch.

You can also use !is to check if a variable is not of a certain type.