How to map a JSON string to Kotlin Map

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?

Look at GSON

Ufkoku
– Ufkoku

2017-07-02 12:57:01 +00:00
Commented Jul 2, 2017 at 12:57 — Ufkoku
– Ufkoku, Commented Jul 2, 2017 at 12:57

Arutyun Enfendzhyan · Accepted Answer · 2020-09-22 04:01:36Z

41

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
    }
}

answered Sep 22, 2020 at 4:01

Arutyun Enfendzhyan

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6 Comments

muzzletov Over a year ago

That seems way more Kotlin'esque - upvoted. And the 'value.toMap()' recursive call was funny as hell. Also, that when-is conditional, never seen that before. Learning so much from one post, you deserve at least two upvotes for every single upvote.

Roman Kotenko Over a year ago

Awesome answer. I would only replace the JSONArray part with (0 until value.length()).map { value[it] } - elegant and concise.

Arutyun Enfendzhyan Over a year ago

@RomanKotenko i couldn't because JSONObject takes only maps, not lists. I have to use .toMap recursively, in order to convert JSONObject.NULL to null

Arutyun Enfendzhyan Over a year ago

@muzzletov ahahaha thank you

Jan Over a year ago

sometimes import statements help. No clue where to get JSONObject from

|

tynn · Accepted Answer · 2017-07-02 14:41:50Z

14

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

answered Jul 2, 2017 at 14:41

tynn

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1 Comment

DeenOub Over a year ago

Parser() is deprecated. the lib recommands to use Parser.default() instead

Alexey Soshin · Accepted Answer · 2022-05-03 08:14:38Z

13

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()

edited May 3, 2022 at 8:14

answered Apr 29, 2022 at 14:18

Alexey Soshin

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3 Comments

Shahriar Zaman Over a year ago

it creates Map<String, JsonElement> but I need Map<String, Any>. How can I do so?

Alexey Soshin Over a year ago

@philoopher97 you can cast the results with as Map<String, Any>, although that would be a mistake in most cases.

Hong승원 Over a year ago

Thank you. Thanks to you, I solved it easily. However, you need to add the following in the build.gradle's dependencies. implementation "org.jetbrains.kotlinx:kotlinx-serialization-json:1.0.1" // JVM dependency

Madhu Bhat · Accepted Answer · 2020-04-10 06:54:09Z

9

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

edited Apr 10, 2020 at 6:54

answered Apr 1, 2020 at 7:43

Madhu Bhat

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3 Comments

Raksha Over a year ago

i get an error for "readValue" that says "None of the following functions can be called with the arguments supplied."

Madhu Bhat Over a year ago

@Raksha did that help?

sigma1510 Over a year ago

ObjectMapper().readValue(jsonString, MutableMap::class.java)

Michael Avoyan · Accepted Answer · 2021-09-17 12:06:54Z

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

kassim · Accepted Answer · 2022-11-17 00:45:37Z

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
}

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