build.sbt

name := """minimal-scala"""

version := "1.0"

scalaVersion := "2.11.7"

libraryDependencies += "com.typesafe.play" %% "play-json" % "2.5.13"

inventoryitem.scala

object InventoryItem {
  implicit val inventoryItemWrites: Writes[InventoryItem] = (
      (JsPath \ "name").write[String] and
      (JsPath \ "serialNumber").write[String] and
      (JsPath \ "value").write[Float]
    )(unlift(InventoryItem.unapply))

  implicit val inventoryItemReads: Reads[InventoryItem] = (
      (JsPath \ "name").read[String] and
      (JsPath \ "serialNumber").read[String] and
      (JsPath \ "value").read[Float]
  )(InventoryItem.apply _)
}

case class InventoryItem(name: String, serialNumber: String, value: Float) {}

readingjson.scala

object ReadingJson extends App {
  var inventory: List[InventoryItem] = List()
  val itemJsonString = Source.fromFile("D:\\Path\\To\\My.json").mkString
  val json = Json.parse(itemJsonString)

  json.validate[List[InventoryItem]] match {
    case s: JsSuccess[List[InventoryItem]] => inventory = s.get
    case e: JsError => println("Oh dog, what done")
  } 
}

result.json

[
  {
    "name": "Item",
    "serialNumber": "asdsad",
    "value": 22
  },
  {
    "name": "Another Item",
    "serialNumber": "SRS-001",
    "value": 33
  }
]

writingjson.scala

object WritingJson extends App {
   var inventory = List[InventoryItem]()
   inventory = inventory :+ InventoryItem("Name", "SR-001", 33.0)
  
   val json = Json.toJson(inventory)
   println(Json.stringify(json))
}