How can I access a JSONObject.keys() set?

MarcoHeleno

Hi!

Is there any way I can access a key set from an JSONObject (using JSONObject.keys()) giving a specific index? Am I able to convert it to some type a an Array?

Thanks.

GoToLoop

// forum.processing.org/two/discussion/5344/
// how-can-i-access-a-jsonobject-keys-set

final JSONObject json = new JSONObject();
String[] properties;

void setup() {
  json.setInt("id", 0);
  json.setString("species", "Panthera leo");
  json.setString("name", "Lion");

  println(json);
  println();

  properties = (String[]) json.keys()
    .toArray(new String[json.size()]);

  println(properties);
  exit();
}

MarcoHeleno

Works perfectly. Thanks!

String[] properties = (String[]) json.keys().toArray(new String[json.size()]);

Can you show me where I can read up more info of what is happening in this line of code? Or explain it? Just want to understand this part: (String[]) json.keys().toArray(new String[json.size()]); I understand that it builds a new String array with the same size as JSONObject, just don't get the (String[])

Thanks!

GoToLoop

(String[]) json.keys().toArray(new String[json.size()]);

Let's do it by parts, shall we?

• Variable json keeps a reference to a JSONObject instance.
• Then the undocumented method keys() is called upon that reference.
• Which in turn invokes method keySet() from its internal HashMap<String, Object> field.
• That returns all the HashMap's keys represented as a Set instance.
• Now as a Set data structure we could choose its iterator() method to get us an Iterator in order to use it in a loop block.
• Instead, I've decided to get a String[] from that via its toArray() method.
• For that, we need a new array instance w/ at least the same size and type of Set<String>.size().
• Next we use that array as the argument for toArray(). All of the key elements in the Set are copied into it.
• Problem is that they are still considered an ordinary Object rather than its true type String.
• Perhaps b/c JSONObject's keys() method returns a generic Set rather than a specific Set<String> type.
• To tell Java we've got a String[] rather than an Object[] type, we use its (cast) operator.
• In this case, we upcast the toArray()'s returned Object[] to String[] via (String[]) typecasting operator.
• So that reference can be validly assigned to a String variable.

MarcoHeleno

Thanks for such detailed description. In response to your first comment I was going to say you were doing a (cast), but I was afraid to say something stupid. And this is the only part that I don’t really get. I've seen this a few times when working with arrays of objects but not yet realized in full what it does. But that's my fault, I have to read about it!

Thanks for the help and explanation!