Extracting Keys from a JSONObject using keySet()

Question

I'm trying to extract the keys from a JSON Object. The JSON object, in this case, is obtained by making an API call to a social networking site called SkyRock and looks like this :

{
  "max_page": 2,
  "posts":  {
    "3111623007":  {
      "id_post": 3111623007,
      "media_align": "float_left",
      "tags":  [],
      "nb_comments": 24
    },
    "3114564209":  {
      "id_post": 3114564209,
      "media_align": "float_left",
      "tags":  [],
      "nb_comments": 33
    },
    "3116902311":  {
      "id_post": 3116902311,
      "media_align": "float_left",
      "tags":  [],
      "nb_comments": 29
    }
  }
}

I basically want to store all the post_id values in an ArrayList. In order to do this, am trying to extract the keys from the JSON object and am doing this as follows:

JSONObject posts = (JSONObject) jo.get("posts");
ArrayList<String> keys = (ArrayString<String>) posts.keyset();

The problem is that am not able to find a suitable variable type in which I can store the result obtained from the keyset() method.

I tried searching for the answers, but in most of the cases, keys() is being used to extract the keys (which am not able to use for some reason and I think it's maybe because am using org.json.simple, but am not sure).

Can anyone please help me out here to find a solution to the problem or any alternate method to retrieve the Key values?

Thanks.

Answer 1

The javadoc says:

public interface JsonObject
extends JsonStructure, Map<String,JsonValue>

So, a JSONObject is a Map whose keys are of type String, and whose values are of type JSONValue.

And the javadoc of Map<K, V>.keySet() says:

Set<K> keySet()

Returns a Set view of the keys contained in this map

So, what JSONObject.keySet() returns is a Set<String> (which is quite logical, since keys of JSON objects are strings).

So all that you want is:

Set<String> keys = posts.keySet();

Answer 2

The posts represents Map of JSONObject where key is String

JSONObject mainObject = new JSONObject(jsonString);

JSONObject posts = mainObject.getJSONObject("posts");

Map<String, JSONObject> map = (Map<String,JSONObject>)posts.getMap();

ArrayList<String> list = new ArrayList<String>(map.keySet());

System.out.println(list);

Output:

[3116902311, 3114564209, 3111623007]