Kotlin

Data Class

data class SportsActivity (
       val totalAveragePaceInMinutesPerKilometre: Double,
       val totalAverageSpeedInKilometresPerHour: Double,
       val totalDurationInSeconds: Int,
       val totalAverageDistanceInMetres: Double, 
       var dateOfActivity: Date
)

Writing to Gson

    var mySportsActivity = SportsActivity(
           0.0,
           0.0,
           0,
           0.0,
           Date())

   val gson = Gson()
   val json = gson.toJson(mySportsActivity)

   FileWriter("D:\\IAIN\\Output.json").use { writer ->
       val gson = GsonBuilder().create()
       gson.toJson(json, writer)
    }