Since you are running your code with Junit, it running on the SDK on your computer. This SDK doesn't provide everything, some are just some skeleton class providing signature of method and documentation but not the code. So you can't execute directly.

Try to add the library in testing

testImplementation 'org.json:json:your_version'

See the version here :

http://mvnrepository.com/artifact/org.json/json

This is based on the answer on this post: JSONObject returns a non null value of "null" after instantiating with string

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?

There is no question that the future of parsing in Kotlin will be with kotlinx.serialization. It is part of Kotlin libraries. Version kotlinx.serialization 1.0 is finally released

https://github.com/Kotlin/kotlinx.serialization

Copyimport kotlinx.serialization.*
import kotlinx.serialization.json.Json

@Serializable
data class MyModel(val a: Int, @Optional val b: String = "42")

fun main(args: Array<String>) {

    // serializing objects
    val jsonData = Json.encodeToString(MyModel.serializer(), MyModel(42))
    println(jsonData) // {"a": 42, "b": "42"}
    
    // serializing lists
    val jsonList = Json.encodeToString(MyModel.serializer().list, listOf(MyModel(42)))
    println(jsonList) // [{"a": 42, "b": "42"}]

    // parsing data back
    val obj = Json.decodeFromString(MyModel.serializer(), """{"a":42}""")
    println(obj) // MyModel(a=42, b="42")
}