sbt "runMain KatakanaRomajiConvert.Main キョウト"

build.sbt

lazy val root = (project in file(".")).
  settings(
    inThisBuild(List(
      scalaVersion := "2.12.7",
      version      := "0.1.0-SNAPSHOT"
    )),
    name := "KatakanaRomajiConvert",
    libraryDependencies += "org.scalatest" %% "scalatest" % "3.0.5" % Test,
    libraryDependencies += "com.ibm.icu" % "icu4j" % "63.1",
  )

Main.scala

package KatakanaRomajiConvert

import com.ibm.icu.text.Transliterator

class KatakanaRomajiConvert(val word: String = "") {
  private val transliterator = Transliterator.getInstance("Katakana-Latin")
  def toRomaji: String = {
    val kana: Option[String] = Some(word)
    transliterator.transliterate(kana.getOrElse(""))
  }
}

object Main extends App {
  if (args.length != 0) {
    val katakana: String = args(0)
    val katakanaObj = new KatakanaRomajiConvert(katakana)
    println(katakanaObj.toRomaji)
  } else {
    println("")
  }
}

MainSpec.scala

package KatakanaRomajiConvert

import org.scalatest._

class KatakanaRomajiConvertSpec extends FlatSpec with Matchers {
  "Input strings \"トウキョウ\"" should "output strings toukyou" in {
    new KatakanaRomajiConvert("トウキョウ").toRomaji shouldEqual "toukyou"
  }
  "Input no string" should "" in {
    new KatakanaRomajiConvert("").toRomaji shouldEqual ""
  }
  "Argument is null" should "" in {
    new KatakanaRomajiConvert(null).toRomaji shouldEqual ""
  }
  "No argument" should "" in {
    new KatakanaRomajiConvert().toRomaji shouldEqual ""
  }
  "Input strings \"あいうアイウabcＡＢＣ\"" should "" in {
    new KatakanaRomajiConvert("あいうアイウabcＡＢＣ").toRomaji shouldEqual "あいうaiuabcＡＢＣ"
  }
  "Input strings \"あいう アイウ abc ＡＢＣ\"" should "" in {
    new KatakanaRomajiConvert("あいう アイウ abc ＡＢＣ").toRomaji shouldEqual "あいう aiu abc ＡＢＣ"
  }
}