In my ChecklistAnwer class i added:

   public JSONObject toJsonObject(){
    JSONObject json = new JSONObject();

    try {
        json.put("questionId", questionId);
        json.put("checklistId", checklistId);
        json.put("answer", answer);
        json.put("remark", remark);
    } catch (JSONException e) {
        e.printStackTrace();
    }

    return json;
}

and in my other class:

JSONArray answers = new JSONArray();

ChecklistAnswer answer = new ChecklistAnswer(questions.get(id).id, 2, cb.isChecked(),         text.getText().toString());

answers.put(answer.toJsonObject());

if i filled the array:

String js = answers.toString(1);

and that returns:

[
 {
  "answer": true,
  "questionId": 1,
  "remark": "",
  "checklistId": 2
 },
 {
  "answer": false,
  "questionId": 4,
  "remark": "teesxfgtfghyfj",
  "checklistId": 2
 },
 {
  "answer": true,
  "questionId": 4,
  "remark": "",
  "checklistId": 2
 },
 {
  "answer": true,
  "questionId": 4,
  "remark": "",
  "checklistId": 2
 },
 {
  "answer": true,
  "questionId": 4,
  "remark": "",
  "checklistId": 2
 },
 {
  "answer": true,
  "questionId": 4,
  "remark": "",
  "checklistId": 2
 },
 {
  "answer": true,
  "questionId": 4,
  "remark": "",
  "checklistId": 2
 }
]

thanks to @Selvin

Simplest and correct code is:

public static String[] toStringArray(JSONArray array) {
    if(array==null)
        return new String[0];
    
    String[] arr=new String[array.length()];
    for(int i=0; i<arr.length; i++) {
        arr[i]=array.optString(i);
    }
    return arr;
}

Using List<String> is not a good idea, as you know the length of the array. Observe that it uses arr.length in for condition to avoid calling a method, i.e. array.length(), on each loop.

Input String

[
   {
      "userName": "sandeep",
      "age": 30
   }, 
   {
      "userName": "vivan",
      "age": 5
   }
]

Simple Way to Convert String to JSON

public class Test
{

    public static void main(String[] args) throws JSONException
    {
        String data = "[{\"userName\": \"sandeep\",\"age\":30},{\"userName\": \"vivan\",\"age\":5}]  ";
        JSONArray jsonArr = new JSONArray(data);

        for (int i = 0; i < jsonArr.length(); i++)
        {
            JSONObject jsonObj = jsonArr.getJSONObject(i);

            System.out.println(jsonObj);
        }

    }
}

Output

{"userName":"sandeep","age":30}
{"userName":"vivan","age":5}

You are right! Even @Sean Patrick Floyd's answer is too much boilterplate to covert a JSON array to string[] or any other type of array class. Rather here is what I find to be elegant:

JsonArray jsonArray = input.getAsJsonArray("key");

Gson gson = new Gson();
String[] output = gson.fromJson(jsonArray , String[].class)

NOTE 1: JsonArray must be an array of strings, for the above example, without any property names. Eg:

{key:["Apple","Orange","Mango","Papaya","Guava"]}

Note 2: JsonObject class used above is from com.google.gson library and not the JSONObject class from org.json library.

For your task you could use code as bellow:

String t = "[{\"user_id\": \"someValue\"}]";
JSONParser parser = new JSONParser();
JSONArray obj = (JSONArray) parser.parse(t);

System.out.println(obj.get(0));

And result would be JSONObject.