finally

Learn the cleanup block with finally.

finally: the always-run block

The finally block always runs, whether or not an error occurs. It is ideal for closing resources or cleanup.

try {
    val x = 10 / 0
} catch (e: ArithmeticException) {
    println("Error")
} finally {
    println("Done")
}
// Error, Done

Even with an error, finally runs; that makes it safe for closing files or databases. It can be written without catch; then it only does cleanup. finally cannot be skipped.

finally and resource cleanup

After opening a resource you must close it. finally guarantees this even if an error occurs.

var open = true
try {
    // use resource
} finally {
    open = false  // always close
}

With or without an error, open = false runs. This prevents resource leaks. Even a return inside try still triggers the close in finally.

Exercise Set up try-catch-finally

Inside a try block write val x = 10 / 0. Catch ArithmeticException and print "Error". Then in a finally block print "Done".

Show example solution

fun main() {
    try {
        val x = 10 / 0
    } catch (e: ArithmeticException) {
        println("Error")
    } finally {
        println("Done")
    }
}

Test yourself

Match block to job

try

Trial block
catch

Error catching
finally

Always runs
throw

Error raising

Fill in the blank: write finally

try {
    val x = 1
} ____ {
    println("Done")
}

Show answer

try {
    val x = 1
} finally {
    println("Done")
}

When does finally run?
- Only when an error occurs
- Always
- Only when no error
- Never
Show answer

Always — finally runs whether or not there is an error.

Next lesson: try expression

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