Does when fall through like switch in Java?

No. Kotlin when does not fall through. Each branch is independent. No break statement is needed.

Can when be used as an expression?

Yes. val grade = when (score) { in 90..100 -> "A"; in 60..89 -> "B"; else -> "C" } assigns the branch result to grade.

Beginner 7 min readKotlin 2.0

Kotlin when — Pattern Matching Expression

The when expression is Kotlin's powerful replacement for switch. It matches values, ranges, and types — and returns a value when used as an expression.

What You Will Learn

Match values with when
Use ranges in when
Use when without an argument
Use when as an expression
Check types in when

Basic when

when matches an argument against branches. The first matching branch executes.

when — Value Matching

kotlin
fun main() {
    val day = 3
    when (day) {
        1 -> println("Monday")
        2 -> println("Tuesday")
        3 -> println("Wednesday")
        4 -> println("Thursday")
        5 -> println("Friday")
        else -> println("Weekend")
    }
}

Output

Wednesday

day is 3, so the third branch matches. else is the default when no other branch matches.

Beginner Tip: when is more readable than chained if/else if when matching multiple values.

Multiple Values per Branch

Combine multiple values in one branch with commas.

Multiple Values

kotlin
fun main() {
    val num = 0
    when (num) {
        0 -> println("Zero")
        1, 2, 3 -> println("Small positive")
        in 4..100 -> println("Medium")
        else -> println("Large or negative")
    }
}

Output

Zero

1, 2, 3 matches any of those three values. in 4..100 matches any number in that range.

when as an Expression

when can return a value — assign it to a variable.

when Expression

kotlin
fun main() {
    val score = 85
    val grade = when {
        score >= 90 -> "A"
        score >= 80 -> "B"
        score >= 70 -> "C"
        else -> "F"
    }
    println(grade)
}

Output

when without an argument evaluates each condition as a boolean. The first true branch returns its value.

Best Practice: When all branches must be covered (exhaustive), use when with a sealed class — the compiler will warn if you miss a case.

Practice Exercise

Exercisepredict output

What prints? val x = 15 when { x < 10 -> println("Small") x < 20 -> println("Medium") else -> println("Large") }

Quick Quiz

What replaces switch in Kotlin?

Kotlin when — Pattern Matching Expression

What You Will Learn

Basic when

when — Value Matching

Multiple Values per Branch

Multiple Values

when as an Expression

when Expression

Practice Exercise

Quick Quiz

Frequently Asked Questions

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