Kotlin: The Spead Operator

Recently ran into a brick wall trying to pass a varargs parameter to another function that also has a varargs parameter.

A colleague mentioned the "spread" operator to me and it took me a little while to find information about it.

An example

package org.mrbear.kotlin

enum class ErrorCode(val description: String) {
    OBJECT_NOT_FOUND("Object %s not found."), NO_DEFAULT_PROVIDED("No default provided for parameter %s."), MALFORMED_URL(
        "Malformed url (%s)"
    )
}

abstract class MyException : Exception {
    constructor(errorCode: ErrorCode, cause: Throwable, vararg params: Any) : super(
        String.format(
            errorCode.description,
            *params
        ), cause
    )

    constructor(errorCode: ErrorCode, vararg params: Any) : super(String.format(errorCode.description, *params))
}

class ObjectNotFoundException(vararg params: Any) : MyException(ErrorCode.OBJECT_NOT_FOUND, *params)

Now to throw it in a test.

class ExceptionTest {

    @Test(expectedExceptions = [ObjectNotFoundException::class], expectedExceptionsMessageRegExp = "Object User mrbear not found.")
    fun testException() {
        throw ObjectNotFoundException("User mrbear")
    }
}

References

Kotlin - Variable number of arguments (varargs)

https://kotlinlang.org/docs/functions.html#variable-number-of-arguments-varargs

Kotlin - Java varargs

https://kotlinlang.org/docs/java-interop.html#java-varargs

Baeldung - Convert Kotlin Array to Varargs

https://www.baeldung.com/kotlin/array-to-varargs

Baeldung - Varargs in Java

https://www.baeldung.com/java-varargs