50. Exceptions

Kotlin handles errors with try/catch/finally. There are no checked exceptions, and try is an expression — it can produce a value. You raise an error with throw, and you can define your own exception by extending Exception.

class MyError(message: String) : Exception(message)

fun parsePositive(text: String): Int {
    val n = text.toIntOrNull() ?: throw MyError("not a number: $text")
    if (n < 0) throw MyError("negative: $n")
    return n
}

fun main() {
    // try/catch/finally
    try {
        val n = parsePositive("oops")
        println("parsed $n")
    } catch (e: MyError) {
        println("caught: ${e.message}")
    } finally {
        println("done parsing")
    }

    // try as an expression: it evaluates to a value.
    val value = try {
        parsePositive("42")
    } catch (e: MyError) {
        -1
    }
    println("value = $value")
}

The finally block always runs — after a successful try, after a caught exception, or while one propagates. When used as an expression, the value of try is the last expression of the block that completes (the try body, or the matching catch).

Running it:

$ kotlin run
caught: not a number: oops
done parsing
value = 42

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