Question

I am doing an HttpPost to get data from a php server using async task. Basically the php script will either return a JSON array or null. It works fine when the json array is returned, however if the script returns null my if statement is not being picked up on and I am being returned this error:

Error parsing data org.json.JSONException: Value null of type org.json.JSONObject$1 cannot be converted to JSONArray

This is a snippet of my script:

    @Override
        protected Void doInBackground(String... params) {
            String url_select = "http://localhost/test.php";
            HttpClient httpClient = new DefaultHttpClient();
            HttpPost httpPost = new HttpPost(url_select);
            List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(3);
        nameValuePairs.add(new BasicNameValuePair("id", id));
            try {
                httpPost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
                HttpResponse httpResponse = httpClient.execute(httpPost);
                HttpEntity httpEntity = httpResponse.getEntity();
                //read content
                is =  httpEntity.getContent();  

            } catch (Exception e) {
                Log.e("log_tag", "Error in http connection "+e.toString());
                }

            try {
                BufferedReader br = new BufferedReader(new InputStreamReader(is));
                StringBuilder sb = new StringBuilder();
                String line = "";
                    while((line=br.readLine())!=null){
                        sb.append(line+"\n");
                    }
                is.close();
                result=sb.toString();
            } catch (Exception e) {
                Log.e("log_tag", "Error converting result "+e.toString());
            }
            return null;

        }

        protected void onPostExecute(Void v) {

        if(result == "null"){
         this.progressDialog.dismiss();
             startActivity(new Intent(viewRandom.this, allDone.class));
        }else{

        try {
            JSONArray Jarray = new JSONArray(result);
            for(int i=0;i<Jarray.length();i++){
                JSONObject Jasonobject = null;
                Jasonobject = Jarray.getJSONObject(i);
                String id = Jasonobject.getString("id");
        }
            this.progressDialog.dismiss();

        } catch (Exception e) {
            Log.e("log_tag", "Error parsing data "+e.toString());
        }
        }
}

Answer 1

Change if(result == "null") to if(result == null).

If you want to check for the string "null" do it with .equals(): if ("null".equals(result))

I am not sure if you really send a “null” string back from your server but anyway. As you might end returning null (not the string!), you should check for that, too.

Edit: Why is "null".equals(result) better than result.equals("null")? The answer is: the first one is null-safe which means it will not throw a NullPointerException when result is null. The second one will result in an exception in that case.