EDITED MY QUESTION:
I've been working on my first ever programming and Android project.
In the following code I receive a:
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.google.gson.JsonElement.toString()' on a null object reference
for trying to call a method on my result JSONOBJECT which was retrieved using a networking library: https://github.com/koush/ion and is being parsed using google GSON library.
A toast string message and a log.v reveals that the JSON object is being successfully passed with all the necessary elements included.(Or maybe not passed completely? The Log.v cuts off after some point.)
My confusion lies in the ability to call the toString() GSON method on result which leads to a successful toast message, while attempting to call any other GSON method like result.get(images_count).toString leads to my error. I tried to use if(result.isJsonNull()) before executing my code to make sure that result was not null, but even if result.isJsonNull() returns false my method calls returns the error.
My 2 Part Question
What is causing my returned JsonObject result to successfully execute some GSON library methods sucessfully(like toString()), while returning a NullPointer Exception for others as when I atttempt to reach a json element member through .get(string member)?
In the future, how could I place/recognize checkpoints where an object might return a NullPointerException? So far I only use Logging or Toasts to display current object/variable values. THANKS ANSWERED BY BRONX. I have to use breakpoints.
Code that results in the NullPointer Exception: (I have replaced my CLient-ID with xxxxxxx in order to keep it a secret from the internet).
Future<JsonObject> loading;
private void load() {
// don't attempt to load more if a load is already in progress
if (loading != null && !loading.isDone() && !loading.isCancelled())
return;
// load the tweets from album : gQxwy https://api.imgur.com/3/album/{id}
//https://api.imgur.com/3/album/{id}/images
String url = "https://api.imgur.com/3/album/gQxwy.json";
// This request loads a URL as JsonObject and invokes
// a callback on completion.
loading = Ion.with(this)
.load(url)
.setHeader("Authorization", "Client-ID " + "xxxxxxxxxxxxxxx")
.setLogging("ION_VERBOSE_LOGGING", Log.VERBOSE)
.asJsonObject()
.setCallback(new FutureCallback<JsonObject>() {
@Override
public void onCompleted(Exception e, JsonObject result) {
// this is called back onto the ui thread, no Activity.runOnUiThread or Handler.post necessary.
if (e != null) {
Toast.makeText(Imgur.this, "Error loading tweets", Toast.LENGTH_LONG).show();
return;
}
//Here i test if the object is Null.It goes directly to excute else, showing that the object is not Null.
if(result.isJsonNull()){
Toast.makeText(Imgur.this, result.toString(), Toast.LENGTH_LONG).show();
}else {
Log.v("JSONOBJECT_STRING", result.toString());
Toast.makeText(Imgur.this, result.get("images_count").toString(), Toast.LENGTH_LONG).show();
}
}
});
}
Here is the LogCat which includes the successfully retrieved JSON Object(We can see the JSON Object in the Log.) The Json object DOES contain the element images_count. LogCat:
03-21 21:24:26.395 8035-8061/? D/ION_VERBOSE_LOGGING﹕ (353 ms) https://api.imgur.com/3/album/gQxwy.json: Connection successful
03-21 21:24:26.405 8035-8061/? D/ION_VERBOSE_LOGGING﹕ (363 ms) https://api.imgur.com/3/album/gQxwy.json: Recycling keep-alive socket
03-21 21:24:26.407 8035-8035/? V/JSONOBJECT_STRING﹕ {"data":{"id":"gQxwy","title":"playlist","description":null,"datetime":1425872636,"cover":"c7iJOmN","cover_width":160,"cover_height":120,"account_url":"JohnnyJem","account_id":18800283,"privacy":"public","layout":"blog","views":574,"link":"http://imgur.com/a/gQxwy","favorite":false,"nsfw":null,"section":null,"images_count":10,"images":[{"id":"c7iJOmN","title":"Title","description":"Description","datetime":1425872636,"type":"image/gif","animated":true,"width":160,"height":120,"size":65008,"views":1175,"bandwidth":76384400,"vote":null,"favorite":false,"nsfw":null,"section":null,"account_url":null,"account_id":null,"gifv":"http://i.imgur.com/c7iJOmN.gifv","webm":"http://i.imgur.com/c7iJOmN.webm","mp4":"http://i.imgur.com/c7iJOmN.mp4","link":"http://i.imgur.com/c7iJOmN.gif","looping":true},{"id":"SYRfZPy","title":"Title","description":"Description","datetime":1425872638,"type":"image/gif","animated":true,"width":160,"height":120,"size":139574,"views":1054,"bandwidth":147110996,"vote":null,"favorite":false,"nsfw":null,"section":null,"account_url":null,"account_id":null,"gifv":"http://i.imgur.com/SYRfZPy.gifv","webm":"http://i.imgur.com/SYRfZPy.webm","mp4":"http://i.imgur.com/SYRfZPy.mp4","link":"http://i.imgur.com/SYRfZPy.gif","looping":true},{"id":"1aD2tdR","title":"Title","description":"Description","datetime":1425872639,"type":"image/gif","animated":true,"width":160,"height":120,"size":117092,"views":882,"bandwidth":103275144,"vote":null,"favorite":false,"nsfw":null,"section":null,"account_url":null,"account_id":null,"gifv":"http://i.imgur.com/1aD2tdR.gifv","webm":"http://i.imgur.com/1aD2tdR.webm","mp4":"http://i.imgur.com/1aD2tdR.mp4","link":"http://i.imgur.com/1aD2tdR.gif","looping":true},{"id":"7UzUF79","title":"Title","description":"Description","datetime":1425872641,"type":"image/gif","animated":true,"width":160,"height":120,"size":146703,"views":785,"bandwidth":115161855,"vote":null,"favorite":false,"nsfw":null,"section":null,"account_url":null,"account_id":null,"gifv":"http://i.imgur.com/7UzUF79.gifv","webm":"http://i.imgur.com/7UzUF79.webm","mp4":"http://i.imgur.com/7UzUF79.mp4","link":"http://i.imgur.com/7UzUF79.gif","looping":true},{"id":"3PKeSiW","title":"Title","description":"Description","datetime":1425872642,"type":"image/gif","animated":true,"width":160,"height":120,"size":136914,"views":753,"bandwidth":103096242,"vote":null,"favorite":false,"nsfw":null,"section":null,"account_url":null,"account_id":null,"gifv":"http://i.imgur.com/3PKeSiW.gifv","webm":"http://i.imgur.com/3PKeSiW.webm","mp4":"http://i.imgur.com/3PKeSiW.mp4","link":"http://i.imgur.com/3PKeSiW.gif","looping":true},{"id":"o3UBVgR","title":"Title","description":"Description","datetime":1425872644,"type":"image/gif","animated":true,"width":160,"height":120,"size":129046,"views":483,"bandwidth":62329218,"vote":null,"favorite":false,"nsfw":null,"section":null,"account_url":null,"account_id":null,"gifv":"http://i.imgur.com/o3UBVgR.gifv","webm":"http://i.imgur.com/o3UBVgR.webm","mp4":"http://i.imgur.com/o3UBVgR.mp4","link":"http://i.imgur.com/o3UBVgR.gif","looping":true},{"id":"qraDqOQ","title":"Title","description":"Description","datetime":1425872645,"type":"image/gif","animated":true,"width":160,"height":120,"size":93116,"views":466,"bandwidth":43392056,"vote":null,"favorite":false,"nsfw":null,"section":null,"account_url":null,"account_id":null,"gifv":"http://i.imgur.com/qraDqOQ.gifv","webm":"http://i.imgur.com/qraDqOQ.webm","mp4":"http://i.imgur.com/qraDqOQ.mp4","link":"http://i.imgur.com/qraDqOQ.gif","looping":true},{"id":"K9PuB3S","title":"Title","description":"Description","datetime":1425872647,"type":"image/gif","animated":true,"width":160,"height":120,"size":84323,"views":481,"bandwidth":40559363,"vote":null,"favorite":false,"nsfw":null,"section":null,"account_url":null,"account_id":null,"gifv":"http://i.imgur.com/K9PuB3S.gifv","webm":"http://i.imgur.com/K9PuB3S.webm","mp4":"http://i.imgur.com/K9PuB3S.mp4","link":"http://i.imgur.com/K9PuB3S.gif","looping":true},{"id":"p3whCAz","title":"Title","descriptio
03-21 21:24:26.408 8035-8035/? D/AndroidRuntime﹕ Shutting down VM
03-21 21:24:26.408 8035-8035/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.johnnymolina.nextphase, PID: 8035
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.google.gson.JsonElement.toString()' on a null object reference
at com.johnnymolina.nextphase.activities.Imgur$2.onCompleted(Imgur.java:116)
at com.johnnymolina.nextphase.activities.Imgur$2.onCompleted(Imgur.java:98)
at com.koushikdutta.async.future.SimpleFuture.handleCallbackUnlocked(SimpleFuture.java:107)
at com.koushikdutta.async.future.SimpleFuture.setComplete(SimpleFuture.java:141)
at com.koushikdutta.async.future.SimpleFuture.setComplete(SimpleFuture.java:128)
at com.koushikdutta.ion.IonRequestBuilder$1.run(IonRequestBuilder.java:246)
at com.koushikdutta.async.AsyncServer$RunnableWrapper.run(AsyncServer.java:57)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
03-21 21:24:26.411 6534-6864/? W/ActivityManager﹕ Force finishing activity com.johnnymolina.nextphase/.activities.Imgur
03-21 21:24:26.412 6534-6864/? W/ActivityManager﹕ Force finishing activity com.johnnymolina.nextphase/.activities.MainActivity
Ok looking at the logCat it seems that the entire JSon Object is not being printed out. Is this a limitation of what Log.v can print? Or am I getting a Null object reference because the network call isn't returning a fully complete object?
An example of what a Json object retreived from Imgur's Api looks like: https://api.imgur.com/models/album
{
"data": {
"id": "lDRB2",
"title": "Imgur Office",
"description": null,
"datetime": 1357856292,
"cover": "24nLu",
"account_url": "Alan",
"account_id": 4,
"privacy": "public",
"layout": "blog",
"views": 13780,
"link": "http://alanbox.imgur.com/a/lDRB2",
"images_count": 11,
"images": [
{
"id": "24nLu",
"title": null,
"description": null,
"datetime": 1357856352,
"type": "image/jpeg",
"animated": false,
"width": 2592,
"height": 1944,
"size": 855658,
"views": 135772,
"bandwidth": 116174397976,
"link": "http://i.imgur.com/24nLu.jpg"
},
{
"id": "Ziz25",
"title": null,
"description": null,
"datetime": 1357856394,
"type": "image/jpeg",
"animated": false,
"width": 2592,
"height": 1944,
"size": 919391,
"views": 135493,
"bandwidth": 124571044763,
"link": "http://i.imgur.com/Ziz25.jpg"
},
{
"id": "9tzW6",
"title": null,
"description": null,
"datetime": 1357856385,
"type": "image/jpeg",
"animated": false,
"width": 2592,
"height": 1944,
"size": 655028,
"views": 135063,
"bandwidth": 88470046764,
"link": "http://i.imgur.com/9tzW6.jpg"
},
{
"id": "dFg5u",
"title": null,
"description": null,
"datetime": 1357856378,
"type": "image/jpeg",
"animated": false,
"width": 2592,
"height": 1944,
"size": 812738,
"views": 134704,
"bandwidth": 109479059552,
"link": "http://i.imgur.com/dFg5u.jpg"
},
{
"id": "oknLx",
"title": null,
"description": null,
"datetime": 1357856338,
"type": "image/jpeg",
"animated": false,
"width": 1749,
"height": 2332,
"size": 717324,
"views": 32938,
"bandwidth": 23627217912,
"link": "http://i.imgur.com/oknLx.jpg"
},
{
"id": "OL6tC",
"title": null,
"description": null,
"datetime": 1357856321,
"type": "image/jpeg",
"animated": false,
"width": 2592,
"height": 1944,
"size": 1443262,
"views": 32346,
"bandwidth": 46683752652,
"link": "http://i.imgur.com/OL6tC.jpg"
},
{
"id": "cJ9cm",
"title": null,
"description": null,
"datetime": 1357856330,
"type": "image/jpeg",
"animated": false,
"width": 2592,
"height": 1944,
"size": 544702,
"views": 31829,
"bandwidth": 17337319958,
"link": "http://i.imgur.com/cJ9cm.jpg"
},
{
"id": "7BtPN",
"title": null,
"description": null,
"datetime": 1357856369,
"type": "image/jpeg",
"animated": false,
"width": 2592,
"height": 1944,
"size": 844863,
"views": 31257,
"bandwidth": 26407882791,
"link": "http://i.imgur.com/7BtPN.jpg"
},
{
"id": "42ib8",
"title": null,
"description": null,
"datetime": 1357856424,
"type": "image/jpeg",
"animated": false,
"width": 2592,
"height": 1944,
"size": 905073,
"views": 30945,
"bandwidth": 28007483985,
"link": "http://i.imgur.com/42ib8.jpg"
},
{
"id": "BbwIx",
"title": null,
"description": null,
"datetime": 1357856360,
"type": "image/jpeg",
"animated": false,
"width": 1749,
"height": 2332,
"size": 662413,
"views": 30107,
"bandwidth": 19943268191,
"link": "http://i.imgur.com/BbwIx.jpg"
},
{
"id": "x7b91",
"title": null,
"description": null,
"datetime": 1357856406,
"type": "image/jpeg",
"animated": false,
"width": 1944,
"height": 2592,
"size": 618567,
"views": 29259,
"bandwidth": 18098651853,
"link": "http://i.imgur.com/x7b91.jpg"
}
]
},
"success": true,
"status": 200
}
if you can post an example of the json response we can help you better. However from what i can see the JsonElement "images_count" doesn't exists, and so you get the NullPointerException.
From what i can see, if i try that calls from the browser i get this json response:
{"data":{"error":"Authentication required","request":"\/3\/album\/gQxwy.json","method":"GET"},"success":false,"status":401}
It is possibile that you don't add some query parameter request from the WebService.
P.S.: To display current object/variable values you need to use breakpoints and run your application in debug mode