Convert Seq[Byte] in Scala to byte[]/ InputStream

wouldn't

val a: Seq[Byte] = List()
a.toArray

do the job?

You can copy the contents of a Seq With copyToArray.

val myseq: Seq[Byte] = ???
val myarray = new Array[Byte](myseq.size)
myseq.copyToArray(myarray)

Note that this will iterate through the Seq twice, which may be undesirable, impossible, or just fine, depending on your use.

A sensible option:

val byteSeq: Seq[Byte] = ???
val byteArray: Array[Byte] = bSeq.toArray
val inputStream = java.io.ByteArrayInputStream(byteArray)

A less sensible option:

object HelloWorld {

  implicit class ByteSequenceInputStream(val byteSeq: Seq[Byte]) extends java.io.InputStream {
    private var pos = 0
    val size = byteSeq.size
    override def read(): Int = pos match {
        case `size` => -1 // backticks match against the value in the variable
        case _ => {
          val result = byteSeq(pos).toInt
          pos = pos + 1
          result
        }
      }
  }

  val testByteSeq: Seq[Byte] = List(1, 2, 3, 4, 5).map(_.toByte)
  def testConversion(in: java.io.InputStream): Unit = {
    var done = false
    while (! done) {
      val result = in.read()
      println(result)
      done = result == -1
    }
  }

  def main(args: Array[String]): Unit = {
    testConversion(testByteSeq)
  }
}