如何将 UUID 的 ArrayList 转换为 ByteArray？

Question

object UUIDConversion {
    fun UUID.toByteArray() : ByteArray {
        val byteBuffer = ByteBuffer.wrap(ByteArray(16))
        byteBuffer.putLong(this.mostSignificantBits)
        byteBuffer.putLong(this.leastSignificantBits)
        return byteBuffer.array()
    }
    fun ByteArray.toUUID() : UUID {
        val byteBuffer = ByteBuffer.wrap(this)
        val mostSignificantBits = byteBuffer.long
        val leastSignificantBits = byteBuffer.long
        return UUID(mostSignificantBits, leastSignificantBits)
    }
}

我有上面的代码将 UUID 转换为 ByteArray 并将其转换回来，但我还需要能够将 ArrayList 转换为 ByteArray 并将其转换回来。 我该怎么做呢？ 如果您想知道为什么我需要这样做，那是因为我需要将 HashMap<UUID, ArrayList 存储在键值数据库中，并且我需要将其转换为 ByteArray 并返回使用它。

Java 或 Kotlin 答案都很好。

Answer 1

您可以使用+运算符连接独立的 UUID 字节 arrays，如下所示：

val allUUIDs : ByteArray = listOfUUID.fold(ByteArray(0)) { buffer, uuid -> buffer + uuid.toByteArray() }

但是，如果您要转换的链很长，则性能可能会显着下降。 相反，您可以使用专用方法来读取/写入字节缓冲区：

import java.nio.ByteBuffer
import java.util.*
import kotlin.RuntimeException

fun ByteBuffer.readUUIDs(nbUUIDs : Int = remaining()/16) : Sequence<UUID> {
    if (nbUUIDs <= 0) return emptySequence()
    val nbBytes = nbUUIDs * 16
    // Defensive copy -> resulting sequence becomes independant from receiver buffer
    val defCpy = ByteArray(nbBytes)
    // slice is required to left source buffer untouched
    slice().get(defCpy)
    val view = ByteBuffer.wrap(defCpy)
    return (1..nbUUIDs).asSequence()
        .map { UUID(view.long, view.long) }
}

fun List<UUID>?.write() : ByteBuffer? {
    if (this == null || isEmpty()) return null;
    val buffer = ByteBuffer.allocate(Math.multiplyExact(size, 16))
    forEach { uuid ->
        buffer.putLong(uuid.mostSignificantBits)
        buffer.putLong(uuid.leastSignificantBits)
    }
    buffer.rewind()
    return buffer
}

fun main() {
    val uuids = (0..3).asSequence().map { UUID.randomUUID() }.toList()
    val writtenUUIDs = uuids.write()
    val uuidSequence = writtenUUIDs ?.readUUIDs() ?: throw RuntimeException("Bug")

    // Note that now, we can do whatever we want with the buffer. The sequence is not impacted
    writtenUUIDs.getLong()
    writtenUUIDs.putLong(-1)

    val readBack = uuidSequence?.toList()
    assert(uuids == readBack) { throw RuntimeException("Bug") }
    println(uuids)
    println(readBack)
}