How to get an unsigned integer from a byte array in Kotlin JVM?

As mentioned in the comments there is no out of the box solution in the JVM version of Kotlin. An extension function doing the same as the Kotlin/Native function might look like this:

fun ByteArray.getUIntAt(idx: Int) =
    ((this[idx].toUInt() and 0xFFu) shl 24) or
            ((this[idx + 1].toUInt() and 0xFFu) shl 16) or
            ((this[idx + 2].toUInt() and 0xFFu) shl 8) or
            (this[idx + 3].toUInt() and 0xFFu)

fun main(args: Array<String>) {

    // 16843009
    println(byteArrayOf(1, 1, 1, 1).getUIntAt(0))

    // 4294967295, which is UInt.MAX_VALUE
    println(byteArrayOf(-1, -1, -1, -1).getUIntAt(0))
}

To expand upon @Alexander's excellent answer, I wrote a version that accepts ByteArrays that have fewer than four values. I've found this particularly useful when parsing ByteArrays with a mix of signed and unsigned integers (in my case bluetooth characteristic update notifications) that I need to slice into sub-arrays.

fun ByteArray.fromUnsignedBytesToInt(): Int {
//Note: UInt is always 32 bits (4 bytes) regardless of platform architecture
//See https://kotlinlang.org/docs/basic-types.html#unsigned-integers
    val bytes = 4
    val paddedArray = ByteArray(bytes)
    for (i in 0 until bytes-this.size) paddedArray[i] = 0
    for (i in bytes-this.size until paddedArray.size) paddedArray[i] = this[i-(bytes-this.size)]

    return (((paddedArray[0].toULong() and 0xFFu) shl 24) or
            ((paddedArray[1].toULong() and 0xFFu) shl 16) or
            ((paddedArray[2].toULong() and 0xFFu) shl 8) or
            (paddedArray[3].toULong() and 0xFFu)).toInt()
}

The test class below confirms the expected values:

import org.junit.Test
import org.junit.Assert.*

internal class ByteArrayExtKtTest {
    @Test
    fun testAsUnsignedToIntTwoBytes() {
        val bytes = byteArrayOf (0x0A.toByte(), 0xBA.toByte())
        assertEquals(2746, bytes.fromUnsignedBytesToInt())
    }

    @Test
    fun testAsUnsignedToIntFourBytes() {
        val bytes = byteArrayOf(1,1,1,1)
        assertEquals(16843009, bytes.fromUnsignedBytesToInt())
    }
}