How to map a JSON string to Kotlin Map - json

Question

I have a simple Kotlin program that access a Mongo database and produce a JSON string as below;
"{
"_id" : { "$oid" : "593440eb7fa580d99d1abe85"} ,
"name" : "Firstname Secondname" ,
"reg_number" : "ATC/DCM/1016/230" ,
"oral" : 11 ,
"oral_percent" : 73 ,
"cat_1" : 57 ,
"cat_2" : 60 ,
"cat_average" : 59 ,
"assignment" : 90
}"
How do I map this in Kotlin Map/MutableMap? Is there an API in Kotlin to read JSON and map it to Map/MutableMap?

Answer 1

No additional library is needed:
val jsonObj = JSONObject(jsonString)
val map = jsonObj.toMap()
where toMap is:
fun JSONObject.toMap(): Map<String, *> = keys().asSequence().associateWith {
when (val value = this[it])
{
is JSONArray ->
{
val map = (0 until value.length()).associate { Pair(it.toString(), value[it]) }
JSONObject(map).toMap().values.toList()
}
is JSONObject -> value.toMap()
JSONObject.NULL -> null
else -> value
}
}

Answer 2

This can be done with Klaxon. With this you can easily read the Json data as JsonObject which is actually a MutableMap.
val json: JsonObject = Parser().parse(jsonData) as JsonObject

Answer 3

Using Jackson's kotlin module, you can create a Map/MutableMap as below:
val jsonString = "{\n" +
" \"_id\": {\n" +
" \"\$oid\": \"593440eb7fa580d99d1abe85\"\n" +
" },\n" +
" \"name\": \"Firstname Secondname\",\n" +
" \"reg_number\": \"ATC/DCM/1016/230\",\n" +
" \"oral\": 11,\n" +
" \"oral_percent\": 73,\n" +
" \"cat_1\": 57,\n" +
" \"cat_2\": 60,\n" +
" \"cat_average\": 59,\n" +
" \"assignment\": 90\n" +
"}"
val map = ObjectMapper().readValue<MutableMap<Any, Any>>(jsonString)
Note: In case you're getting the below compilation error
None of the following functions can be called with the arguments supplied
Please ensure that you have added the the dependency of jackson-module-kotlin (for gradle: compile "com.fasterxml.jackson.module:jackson-module-kotlin:${jackson_version}") and have added the import for the readValue implementation as import com.fasterxml.jackson.module.kotlin.readValue in the place where you're using readValue

Answer 4

This can be done without any third party library:
#Throws(JSONException::class)
fun JSONObject.toMap(): Map<String, Any> {
val map = mutableMapOf<String, Any>()
val keysItr: Iterator<String> = this.keys()
while (keysItr.hasNext()) {
val key = keysItr.next()
var value: Any = this.get(key)
when (value) {
is JSONArray -> value = value.toList()
is JSONObject -> value = value.toMap()
}
map[key] = value
}
return map
}
#Throws(JSONException::class)
fun JSONArray.toList(): List<Any> {
val list = mutableListOf<Any>()
for (i in 0 until this.length()) {
var value: Any = this[i]
when (value) {
is JSONArray -> value = value.toList()
is JSONObject -> value = value.toMap()
}
list.add(value)
}
return list
}
Usage to convert JSONObject to Map:
val jsonObject = JSONObject(jsonObjStr)
val map = jsonObject.toMap()
Usage to convert JSONArray to List:
val jsonArray = JSONArray(jsonArrStr)
val list = jsonArray.toList()
More info is here

Answer 5

This is now also possible with kotlinx.serialization:
import kotlinx.serialization.json.Json
import kotlinx.serialization.json.jsonObject
val input = """{
"_id" : { "some_id" : "593440eb7fa580d99d1abe85"} ,
"name" : "Firstname Secondname" ,
"reg_number" : "ATC/DCM/1016/230" ,
"oral" : 11 ,
"oral_percent" : 73 ,
"cat_1" : 57 ,
"cat_2" : 60 ,
"cat_average" : 59 ,
"assignment" : 90
}"""
val json = Json.parseToJsonElement(input)
val map = json.jsonObject.toMap()

Answer 6

The latest version of the org.json library has JSONObject.toMap() so you can just do
val map = JSONObject(string).toMap()
So just add the org.json:json:20220924 dependency to your project.
If you're writing for Android (and thus have an old version of the org.json library) then the following can be used.
import org.json.JSONArray
import org.json.JSONObject
fun JSONObject.toMap(): Map<String, Any?> =
keys().asSequence().associateWith { key -> toValue(get(key)) }
fun JSONArray.toList(): List<Any?> =
(0 until length()).map { index -> toValue(get(index)) }
private fun toValue(element: Any) = when (element) {
JSONObject.NULL -> null
is JSONObject -> element.toMap()
is JSONArray -> element.toList()
else -> element
}