you can get this data as a JsonArray

Your root JSON is an Array, so first create a JSONArray from your String.

Do this:

JSONArray arr = new JSONArray(jstring);
for (int i = 0; i < arr.length(); i++) { // Walk through the Array.
    JSONObject obj = arr.getJSONObject(i);
    JSONArray arr2 = obj.getJSONArray("fileName");
    // Do whatever.
}

For more info, please refer to the docs on JSONArray and JSONObject.

Instead of getting a List<String> from the JSON Object, you can access the array of fields in the same way you are accessing the array of errors:

public static String getField(JSONObject json, String errorsIndex, String fieldIndex) {
    JSONObject errorJson = json.getJSONArray("errors").getJSONObject(errorIndex);
    String value = errorJson.getJSONArray("fields").getString(fieldIndex);
    return value;
}

Note that get(fieldIndex) has changed to getString(fieldIndex). That way you don't have to cast an Object to a String.

If anyone else is stuck here, It turned out I was on the right track, I can access the List using getJSONArray, but when iterating for each member, I use getString

 public static Person parsePersonJson(String json) {

            JSONObject currentPerson;
            String name;

            try {
                currentPerson = new JSONObject(json);

                // so I can access the name like

                name = currentPerson.getString("name");

                List<String> alsoKnownAs= new ArrayList<>();

                //use getJSONArray to get the list

                JSONArray arrayKnownAs = currentPerson.getJSONArray("alsoKnownAs");

                for (int i = 0, l = arrayKnownAs.length(); i < l; i++) {

                 //This is where I was getting it wrong, i needed to use getString to access list items

                  alsoKnownAs.add(arrayKnownAs.getString(i));
                   }



               Person thisPerson =  new Person(

                //I instantiate person object here

                );
                return thisPerson;

            } catch (org.json.JSONException e) {
             // error
            }
            return null;
        }

see this code what i am used in my application

String data="{'foo':'bar','coolness':2.0, 'altitude':39000, 'pilot':{'firstName':'Buzz','lastName':'Aldrin'}, 'mission':'apollo 11'}";

I retrieved like this

JSONObject json = (JSONObject) JSONSerializer.toJSON(data);        
    double coolness = json.getDouble( "coolness" );
    int altitude = json.getInt( "altitude" );
    JSONObject pilot = json.getJSONObject("pilot");
    String firstName = pilot.getString("firstName");
    String lastName = pilot.getString("lastName");

    System.out.println( "Coolness: " + coolness );
    System.out.println( "Altitude: " + altitude );
    System.out.println( "Pilot: " + lastName );

String loudScreaming = json.getJSONObject("LabelData").getString("slogan");

You are asking Jackson to parse a StudentList. Tell it to parse a List (of students) instead. Since List is generic you will typically use a TypeReference

List<Student> participantJsonList = mapper.readValue(jsonString, new TypeReference<List<Student>>(){});

You can iterate through the JSONArray and store each value to the list, and return that.

JSONObject jo = new JSONObject(jsonString); //
JSONArray ja = jo.getJSONArray("id_list"); // get the JSONArray
List<String> keys = new ArrayList<>();

for(int i=0;i<ja.length();i++){
    keys.add(ja.getString(i)); // iterate the JSONArray and extract the keys
}

return keys; // return the list

Have you tried using JSONArray.getJSONObject(int), and JSONArray.length() to create your for-loop:

for (int i = 0; i < recs.length(); ++i) {
    JSONObject rec = recs.getJSONObject(i);
    int id = rec.getInt("id");
    String loc = rec.getString("loc");
    // ...
}

There are several ways to retrieve the json array value:

Assume we have a jsonString

jsonString = "{\n" + "     \"firstNames\": [ \n" + "          \"Aaron\",\n" + "          \"Abigail\",\n" + "          \"Albert\",\n" + "          \"Bob\"\n" + "     ]\n" + "}";

(since many classes share similar names, I am using the groupId and artifactId for distinction.)

Simple cases: use generic JSONObjects and JSONArrays.

json-simple (which OP is using) json-simple website, maven :

org.json.simple.parser.JSONParser jsonParser = new org.json.simple.parser.JSONParser();
org.json.simple.JSONObject firstObject = (org.json.simple.JSONObject) jsonParser.parse(jsonString);
org.json.simple.JSONArray jsonArray = (org.json.simple.JSONArray) firstObject.get("firstNames");
System.out.println(jsonArray);

JSON in Java (mentioned in adendrata's answer): JSON-Java doc, maven

org.json.JSONObject secondObject = new org.json.JSONObject(jsonString);
org.json.JSONArray jsonArray2 = secondObject.getJSONArray("firstNames");
System.out.println(jsonArray2);

gson: Gson, maven

com.google.gson.JsonObject thirdObject = com.google.gson.JsonParser.parseString(jsonString).getAsJsonObject();
System.out.println(thirdObject.get("firstNames").getAsJsonArray());

For more complicated use cases, if you'd like to define your own class, and want to deserialize JSON string to your class, then you can use Gson or Jackson:

// Create your own class:
/*
public class YourOwnClass {
    private List<String> firstNames;

    public List<String> getFirstNames() {
        return firstNames;
    }
}
*/

Gson gson = new Gson();
YourOwnClass customObject1 = gson.fromJson(jsonString, YourOwnClass.class);
System.out.println(customObject1.getFirstNames());

ObjectMapper mapper = new ObjectMapper();
YourOwnClass customObject2 = mapper.readValue(jsonString, YourOwnClass.class);
System.out.println(customObject2.getFirstNames());