如何正确处理Kotlin中大于127的字节值? [英] How to correctly handle Byte values greater than 127 in Kotlin?

问题描述

想象一下，我有一个Kotlin程序，其类型为Byte的变量b，外部系统在其中写入了大于127的值. 外部"表示我无法更改其返回值的类型.

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()

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

a.toByte()和b.toInt()都返回-128.

想象一下我想从变量b获得正确的值(128).我该怎么办?

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

换句话说:magicallyExtractRightValue的哪种实现将使以下测试运行?

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")
}

更新1:此解决方案由 Thilo 建议.

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

推荐答案

使用Kotlin 1.3+，您可以使用 toUByte (

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

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

甚至需要直接使用UByte代替Byte(科特琳游乐场):

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

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

对于Kotlin 1.3之前的版本，我建议创建一个扩展功能使用 and :

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

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

示例用法:

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")

示例输出:

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]

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