How to correctly handle Byte values greater than 127 in Kotlin?

Question

Imagine I have a Kotlin program with a variable b of type Byte, into which an external system writes values greater than 127. \"External\"

Answer 1

With Kotlin 1.3+ you can use unsigned types. e.g. toUByte (Kotlin Playground):

private fun magicallyExtractRightValue(b: Byte): Int {
    return b.toUByte().toInt()
}

or even require using UByte directly instead of Byte (Kotlin Playground):

private fun magicallyExtractRightValue(b: UByte): Int {
    return b.toInt()
}

For releases prior to Kotlin 1.3, I recommend creating an extension function to do this using and:

fun Byte.toPositiveInt() = toInt() and 0xFF

Example usage:

val a: List<Int> = listOf(0, 1, 63, 127, 128, 244, 255)
println("from ints: $a")
val b: List<Byte> = a.map(Int::toByte)
println("to bytes: $b")
val c: List<Int> = b.map(Byte::toPositiveInt)
println("to positive ints: $c")

Example output:

from ints: [0, 1, 63, 127, 128, 244, 255]
to bytes: [0, 1, 63, 127, -128, -12, -1]
to positive ints: [0, 1, 63, 127, 128, 244, 255]

Answer 2

Good old printf does what we want:

java.lang.String.format("%02x", byte)