Kotlin try/catch/finally Expressions

Like Java, Kotlin has a try/catch/finally construct to let you catch and manage exceptions. If you’ve used Java, there’s only one new feature here: a try expression is truly an expression, meaning you can assign its result to a variable. (See the “Try is an expression” section below for those details.)

Basic syntax

Kotlin’s try/catch/finally syntax looks like this:

try {
    // some exception-throwing code
}
catch (e: SomeException) {
    // code
}
catch (e: SomeOtherException) {
    // code
}
finally {
    // optional
}

As shown in the comment, the finally clause is optional. Everything works just like Java.

Example

Because there are easier ways to read files in Kotlin, this example is a little contrived, but it shows an example of how to use try with multiple catch clauses and a finally clause:

import java.io.*

fun main(args: Array<String>) {

    var linesFromFile = listOf<String>()
    var br: BufferedReader? = null

    try {
        br = File("/etc/passwd2").bufferedReader()
        linesFromFile = br.readLines()
    }
    catch (e: IOException) {
        e.printStackTrace()
    }
    catch (e: FileNotFoundException) {
        e.printStackTrace()
    }
    finally {
        br?.close()
    }

    linesFromFile.forEach { println(">  " + it) }

}

Don’t worry about the ? symbols in the code for now, I just wanted to show a complete try/catch/finally example. Just know that the question marks are needed for working with null values in Kotlin.

Or, if you want to learn about them, jump ahead to the “Nullability” lessons later in this book.

Try is an expression

Try in Kotlin is different than Java because Kotlin’s try is an expression, meaning that it returns a value:

val s = "1"

// using try as an expression
val i = try {
    s.toInt()
} 
catch (e: NumberFormatException) { 
    0 
}

That expression can be read as, “If s converts to an Int, return that integer value; otherwise, if it throws a NumberFormatException, return 0.”

You can write that try expression more concisely:

val i = try { s.toInt() } catch (e: NumberFormatException) { 0 }

Try that expression with different values like these to confirm for yourself that it works as expected:

val s = "1"
val s = "foo"