Read large (+- 50Mb) Json file using Kotlin - json

Question

I'm starting to work on a weather app using "openweathermap.org" API, and they provide you with a list of available cities in Json format.
Before i continue with the project, i would like to able to work with the data from this Json file.
The problem is that i get Null whenever i try to read and parse that file.
Here is the code:
Main Activity:
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val jsonFileString = getJsonDataFromAsset(applicationContext, "citylist.json")
Log.i("gabs Data", jsonFileString ?: "Empty Data")
val gson = Gson()
val listOfCities = object : TypeToken<List<CityList>>() {}.type
var cities: List<CityList> = gson.fromJson(jsonFileString, listOfCities)
cities.forEachIndexed { idx, city -> Log.i("data", "> Item $idx:\n$city") }
}
}
Utils.kt:
fun getJsonDataFromAsset(context: Context, fileName: String): String? {
val jsonString: String
try {
jsonString = context.assets.open(fileName).bufferedReader().use { it.readText() }
} catch (ioException: IOException) {
ioException.printStackTrace()
return null
}
return jsonString
}
And the data class (Array of cities data):
class CityList : ArrayList<CityList.CityListItem>(){
data class CityListItem(
#SerializedName("coord")
val coord: Coord,
#SerializedName("country")
val country: String,
#SerializedName("id")
val id: Double,
#SerializedName("name")
val name: String,
#SerializedName("state")
val state: String
) {
data class Coord(
#SerializedName("lat")
val lat: Double,
#SerializedName("lon")
val lon: Double
)
}
}
And the error:
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.weatherdisplay/com.example.weatherdisplay.ui.activities.MainActivity}: java.lang.NullPointerException: gson.fromJson(jsonFileString, listOfCities) must not be null.
Caused by: java.lang.NullPointerException: gson.fromJson(jsonFileString, listOfCities) must not be null
at com.example.weatherdisplay.ui.activities.MainActivity.onCreate(MainActivity.kt:21)

Answer 1

There were some problems in your code:
You were not closing the BufferedReader
You should not load the file on the Main thread since it will block the UI
I created some sample data corresponding to your data structure:
[
{
"id": 1,
"country": "Germany",
"state": "Saxony",
"name": "Dresden",
"coord": {
"lat": 0.0,
"lon": 0.0
}
},
{
"id": 2,
"country": "Germany",
"state": "Berlin",
"name": "Berlin",
"coord": {
"lat": 0.0,
"lon": 0.0
}
},
{
"id": 3,
"country": "Germany",
"state": "Baden-Wuerttemberg",
"name": "Stuttgart",
"coord": {
"lat": 0.0,
"lon": 0.0
}
},
{
"id": 4,
"country": "Germany",
"state": "Hessen",
"name": "Frankfurth",
"coord": {
"lat": 0.0,
"lon": 0.0
}
},
{
"id": 5,
"country": "Germany",
"state": "Nordrhine-Westphalia",
"name": "Cologne",
"coord": {
"lat": 0.0,
"lon": 0.0
}
}
]
Your activity:
class MainActivity : AppCompatActivity() {
companion object {
const val TAG = "MyApplication"
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
lifecycleScope.launchWhenStarted {
launch(Dispatchers.IO) {
var reader: BufferedReader? = null
try {
// Create a reader and read the file contents
reader = assets.open("data.json").bufferedReader()
val rawData = reader.use { it.readText() }
// Create a Type token that Gson knows how to parse the raw data
val cityListType = object : TypeToken<List<City>>() {}.type
// Parse the raw data using Gson
val data: List<City> = Gson().fromJson(rawData, cityListType)
// TODO: Do something with the data
} catch (e: IOException) {
// Handle IOException: Gets thrown when the file wasn't found or something similar
Log.e(TAG, "An error occurred while reading in the data:", e)
} catch (e: JsonParseException) {
// Handle JsonParseException: Gets thrown when there is a problem with the contents of the file
Log.e(TAG, "An error occurred while reading in the data:", e)
}
finally {
// Close the reader to release system resources
reader?.close()
}
}
}
}
}
Your data structure:
data class City(
#SerializedName("id")
val id: Int,
#SerializedName("country")
val country: String,
#SerializedName("state")
val state: String,
#SerializedName("name")
val name: String,
#SerializedName("coord")
val coordinate: Coordinate
) {
override fun toString(): String {
return "#$id[$name $state $country]#[${coordinate.lat}|${coordinate.lon}]"
}
}
data class Coordinate(
#SerializedName("lat")
val lat: Double,
#SerializedName("lon")
val lon: Double
)
In the best case you would put the code in which you get the file contents and parse the data in a ViewModel, but this would to go beyond the scope for this answer.
Additional information about ViewModels: https://developer.android.com/topic/libraries/architecture/viewmodel