How to create a JSONObject from String in Kotlin?

Perhaps I'm misunderstanding the question but it sounds like you are already using org.json which begs the question about why

val answer = JSONObject("""{"name":"test name", "age":25}""")

wouldn't be the best way to do it? What was wrong with the built in functionality of JSONObject?

val rootObject= JSONObject()
rootObject.put("name","test name")
rootObject.put("age","25")

You can use https://github.com/cbeust/klaxon library.

val parser: Parser = Parser()
val stringBuilder: StringBuilder = StringBuilder("{\"name\":\"Cedric Beust\", \"age\":23}")
val json: JsonObject = parser.parse(stringBuilder) as JsonObject
println("Name : ${json.string("name")}, Age : ${json.int("age")}")

Result :

Name : Cedric Beust, Age : 23

The approaches above are a bit dangerous: They don't provide a solution for illegal chars. They don't do the escaping... And we hate to do the escaping ourselves, don't we?

So here's what I did. Not that cute and clean but you have to do it only once.

class JsonBuilder() {
    private var json = JSONObject()

    constructor(vararg pairs: Pair<String, *>) : this() {
        add(*pairs)
    }

    fun add(vararg pairs: Pair<String, *>) {
        for ((key, value) in pairs) {
            when (value) {
                is Boolean -> json.put(key, value)
                is Number -> add(key, value)
                is String -> json.put(key, value)
                is JsonBuilder -> json.put(key, value.json)
                is Array<*> -> add(key, value)
                is JSONObject -> json.put(key, value)
                is JSONArray -> json.put(key, value)
                else -> json.put(key, null) // Or whatever, on illegal input
            }
        }
    }

    fun add(key: String, value: Number): JsonBuilder {
        when (value) {
            is Int -> json.put(key, value)
            is Long -> json.put(key, value)
            is Float -> json.put(key, value)
            is Double -> json.put(key, value)
            else -> {} // Do what you do on error
        }

        return this
    }

    fun <T> add(key: String, items: Array<T>): JsonBuilder {
        val jsonArray = JSONArray()
        items.forEach {
            when (it) {
                is String,is Long,is Int, is Boolean -> jsonArray.put(it)
                is JsonBuilder -> jsonArray.put(it.json)
                else -> try {jsonArray.put(it)} catch (ignored:Exception) {/*handle the error*/}
            }
        }

        json.put(key, jsonArray)

        return this
    }

    override fun toString() = json.toString()
}

Sorry, might have had to cut off types that were unique to my code so I might have broken some stuff - but the idea should be clear

You might be aware that in Kotlin, "to" is an infix method that converts two objects to a Pair. So you use it simply like this:

   JsonBuilder(
      "name" to "Amy Winehouse",
      "age" to 27
   ).toString()

But you can do cuter things:

JsonBuilder(
    "name" to "Elvis Presley",
    "furtherDetails" to JsonBuilder(
            "GreatestHits" to arrayOf("Surrender", "Jailhouse rock"),
            "Genre" to "Rock",
            "Died" to 1977)
).toString()

Kotlin Serialization

import kotlinx.serialization.json.Json
import kotlinx.serialization.json.JsonObject    

return Json.decodeFromString<JsonObject>("""{"name":"test name", "age":25}""")