Kotlin by Example: When Expression

Kotlin 1.9+

Replacing switch statements with this code example showing when expression for value matching, type checking with is operator, range checking with in operator, and when without argument for boolean conditions.

Code

fun describe(obj: Any): String {
    return when (obj) {
        1          -> "One"
        "Hello"    -> "Greeting"
        is Long    -> "Long number"
        !is String -> "Not a string"
        else       -> "Unknown"
    }
}

fun main() {
    val x = 10
    
    // when as a statement (no return value needed)
    when {
        x < 0 -> println("Negative")
        x == 0 -> println("Zero")
        x in 1..10 -> println("Between 1 and 10")
        else -> println("Greater than 10")
    }
    
    println(describe(1))
    println(describe("Hello"))
}

Explanation

The when expression in Kotlin is a flexible and powerful replacement for the traditional switch statement found in languages like Java and C. It can be used both as an expression returning a value and as a statement for side effects. When used as an expression, the else branch is mandatory unless the compiler can prove all possible cases are covered, such as with enums or sealed classes.

When expression capabilities include:

Matching constant values like numbers or strings
Type checking with is operator, automatically smart-casting the variable
Range checking with in operator for inclusive ranges
Arbitrary boolean expressions when used without an argument
Multiple conditions per branch separated by commas

Unlike switch statements, when does not require break statements as execution stops after the first matching branch. When used without an argument, it functions as a cleaner alternative to chained if-else if statements, with each branch condition being an independent boolean expression. The branches are evaluated sequentially until a match is found.

Code Breakdown

5

is Long type check with automatic smart cast to Long.

18

x in 1..10 range check for inclusive range [1, 10].

15

when { ... } without argument acts as if-else chain.

2

when (obj) as expression returns value, no break needed.

Samplebadu

Kotlin by Example: When Expression

Code

Explanation

Code Breakdown