I’m updating a Java codebase to Kotlin. I’m stuck ...

# getting-started

[Adam S]

I’m updating a Java codebase to Kotlin. I’m stuck on trying to update Arrays.copyOfRange()

fun copyData(
 codePointsWindow: IntArray,
 pointer: Int,
 dataLength: Int,
 read: Int,
): IntArray {
  return Arrays.copyOfRange(codePointsWindow, pointer, dataLength + read)
}

IntelliJ pops-up a warning, “Should be replaced with Kotlin function”, and provides a quickfix:

return codePointsWindow.copyOfRange(pointer, dataLength + read)

However then a lot of tests fail with

toIndex (11) is greater than size (0).
java.lang.IndexOutOfBoundsException: toIndex (11) is greater than size (0).
	at kotlin.collections.ArraysKt__ArraysJVMKt.copyOfRangeToIndexCheck(ArraysJVM.kt:49)
	at kotlin.collections.ArraysKt___ArraysJvmKt.copyOfRange(_ArraysJvm.kt:1859)

I’ve made an issue IDEA-320926 for the quickfix not producing equivalent code, but can someone help me with a suitable replacement for now?

[Adam S]

I think the difference is that Arrays.copyOfRange() allows for the index to lie outside of the source array’s bounds, while Kotlin’s copyOfRange() performs a check

[Adam S]

alright, this util function seems to work:

/**
 * Like [IntArray.copyOfRange], but allows for [toIndex] to be out-of-bounds.
 */
private fun IntArray.copyOfRangeSafe(fromIndex: Int, toIndex: Int): IntArray =
    IntArray(toIndex - fromIndex) { getOrNull(fromIndex + it) ?: 0 }

thank for rubber ducking rubber duck , any further suggestions or refinements are welcome!

[hho]

Or simpler:

fun IntArray.copyOfRangeSafe(fromIndex: Int, toIndex: Int) =
    copyOfRange(fromIndex, toIndex.coerceAtMost(size))

[Adam S]

that one fails if fromIndex == toIndex

java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0

[hho]

😞 You're right. But I'd still prefer to fix the param ranges and use the copyOfRange function instead of doing the copying yourself.

[Adam S]

me too!

[hho]

is your source array empty? for me, fromIndex == toIndex works…

[Adam S]

yeah, I think it might be

[Adam S]

yeah, it is. println(indices) returns 0..-1

[hho]

But yeah, what IntelliJ suggests definitely is not equivalent

[hho]

I'd probably just use the Java method then.

[Adam S]

the plan is to (eventually) convert to Kotlin Multiplatform, so it would have to be replaced at some point :)

[Klitos Kyriacou]

toIndex.coerceAtMost(size) will not work, because Java's copyOfRange guarantees the returned array will have size toIndex - fromIndex.