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

问题

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()
}

回答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]

回答2:

Good old printf does what we want:

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

来源：https://stackoverflow.com/questions/38651192/how-to-correctly-handle-byte-values-greater-than-127-in-kotlin

标签

types

type-conversion

kotlin