Kotlin error Function

Table of Contents

This article was translated from Japanese by Claude Code.

The Kotlin code in this article was verified with kotlin-stdlib-common 1.3.72.

I found this and thought it was convenient so here’s a note 📝.

In Java, when a value expected to be non-null is actually null, you write code like this to throw an exception:

if (nonNullIsExpceted == null) {
  throw new IllegalStateException("nonNullIsExpceted should not be null.");
}

In Kotlin, you can write similar code like this:

Pattern 1: Using throw
#

if (nonNullIsExpceted == null) {
  throw IllegalStateException("nonNullIsExpceted should not be null.")
}

val valid = nonNullIsExpected ?: throw IllegalStateException("nonNullIsExpceted should not be null.")

Pattern 2: Using error function
#

if (nonNullIsExpceted == null) {
  error("nonNullIsExpceted should not be null.")
}

val valid = nonNullIsExpected ?: error("nonNullIsExpceted should not be null.")

Both patterns in Kotlin have the same meaning. The error function is implemented like this:

/**
 * Throws an [IllegalStateException] with the given [message].
 *
 * @sample samples.misc.Preconditions.failWithError
 */
@kotlin.internal.InlineOnly
public inline fun error(message: Any): Nothing = throw IllegalStateException(message.toString())

I thought it was concise and intuitive to write. Also, the fact that the message parameter is non-null and requires some error message in the API specification is nice too.

kotlinlang.org

Pattern 1: Using throw#

Pattern 2: Using error function#

Pattern 1: Using throw
#

Pattern 2: Using error function
#