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" means that I cannot change the type of the value it returns.

val a:Int = 128 val b:Byte = a.toByte()

Both a.toByte() and b.toInt() return -128.

Imagine I want to get the correct value (128) from the variable b. How can I do it?

In other words: What implementation of magicallyExtractRightValue would make the following test run?

@Test fun testByteConversion() {     val a:Int = 128     val b:Byte = a.toByte()      System.out.println(a.toByte())     System.out.println(b.toInt())      val c:Int = magicallyExtractRightValue(b)      Assertions.assertThat(c).isEqualTo(128) }  private fun magicallyExtractRightValue(b: Byte): Int {     throw UnsupportedOperationException("not implemented") } 

Update 1: This solution suggested by Thilo seems to work.

private fun magicallyExtractRightValue(o: Byte): Int = when {     (o.toInt() < 0) -> 255 + o.toInt() + 1     else -> o.toInt() } 

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]