object Main {
  def main(args: Array[String]): Unit = {
    val file = new java.io.PrintWriter("out/output_library_sort")
    var source = scala.io.Source.fromFile("out/testdata")
    source.getLines.map(_.toInt).toSeq.sorted.foreach(x => file.println(f"${x}%07d"))
    source.close()
    file.close()
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val file = new java.io.PrintWriter("out/output_library_sort")
    var source = scala.io.Source.fromFile("out/testdata")
    source.getLines.toSeq.sorted.foreach(file.println)
    source.close()
    file.close()
  }
}

--- 
+++ 
@@ -2,7 +2,7 @@
   def main(args: Array[String]): Unit = {
     val file = new java.io.PrintWriter("out/output_library_sort")
     var source = scala.io.Source.fromFile("out/testdata")
-    source.getLines.toSeq.sorted.foreach(file.println)
+    source.getLines.map(_.toInt).toSeq.sorted.foreach(x => file.println(f"${x}%07d"))
     source.close()
     file.close()
   }
Convert from String to Int before sorting
