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Kotlin で Int を Big-Endian なバイト配列に変換するプログラムと、Big-Endian なバイト配列を Int に変換するプログラムを掲載しています。

fun Int.toByteArray(): ByteArray {
    val array = ByteArray(4)
    array[0] = (this shr 24 and 0xFF).toByte()
    array[1] = (this shr 16 and 0xFF).toByte()
    array[2] = (this shr 8 and 0xFF).toByte()
    array[3] = (this and 0xFF).toByte()

    return array
}

fun ByteArray.toInt(): Int {
    if (this.size != 4) throw IllegalStateException("ByteArray size must be 4, but this is ${this.size}.")
    return (this[0].toInt() and 0xFF shl 24) or
        (this[1].toInt() and 0xFF shl 16) or
        (this[2].toInt() and 0xFF shl 8) or
        (this[3].toInt() and 0xFF)
}