Checking if exists subObject in JSON

Question

I'm checking if an object in a JSON string exists using this:

JSONObject json = null; try { json = new JSONObject(myJsonString); } catch (JSONException e) { e.printStackTrace(); } if(json.has("myObject")) System.out.println("EXISTS"); else System.out.println("DOESN'T EXIST");

The problem appears when I attempt to check if a sub object exists. e.g:

...,"queue":{"building":{"q0":{"id":177779,...

Queue always exists and building also, but q0 is not always there. So, how can I check the existence of q0? And, is there a way to check it using the Gson library?

Thank you in advance!

Peter Ajtai · Accepted Answer · 2012-04-08 17:11:11Z

You can simply give it a try and return null if the try failed. Or you can break your attempt up into little pieces to monitor where it fails.

/** * This method will return the JSONObject q0, if it exists * If it doesn't exist it will return NULL * */ private JSONObject getQZero(JSONObject json) { try { return json.getJSONObject("queue").getJSONObject("building").getJSONObject("q0"); } catch (JSONException e) { // This could be triggered either because there is no q0 // or because the JSON structure is different from what was expected. return null; } }

You could also go step by step, if you want to print logs for each level;

/** * This method will show where your jsonparsing fails. * It will throw a JSONOException if the json is way different from what * was expected, and otherwise it will print a log of where the parsing * failed. */ private JSONObject getQZero(JSONObject json) throws JSONException { // Stop if no queue if (! myObject.has("queue") { Log.d(TAG, "no queue!"); return null; } JSONObject queue = myObject.getJSONObject("queue"); // Stop if no building if (! queue.has("building") { Log.d(TAG, "no building!"); return null; } JSONObject building = queue.getJSONObject("building") // Stop if no q0 if (! building.has("q0")) { Log.d(TAG, "no q0!"); return null; } JSONObject q0 = building.getJSONObject("q0"); // Q0 is returned here. If the method returned earlier, it returned NULL // You could also do nested ifs, but the indentation gets crazy return q0; }

Blundell · Accepted Answer · 2012-04-08 16:04:46Z

1

Use the exceptions to your advantage

 try { JSONObject i = json. getJSONObject("q0"); // Is there do something } catch (JSONException e) { // Isn't there }

http://www.json.org/javadoc/org/json/JSONObject.html#getJSONObject(java.lang.String)

JSONException - if the key is not found or if the value is not a JSONObject.

edited Apr 8, 2012 at 16:04

answered Apr 8, 2012 at 15:38

Blundell

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3 Comments

Peter Ajtai Over a year ago

Looks like q0 is a JSONObject and not an int.

anonymous Over a year ago

@PeterAjtai Yes, it is a JSONObject that I want to check its existence

Peter Ajtai Over a year ago

@KrLx_roller - if (building.has("q0"))... - see the second part of my answer

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