Android : Android - Getting SharedPreferences and putting it in a class

I have this class wherein it gets the sharedPreferences put it in a variable and use that variable to $POST to my PHP file.

public class NetworkOperations extends AsyncTask<String,Void,String> {

String awesomeString = PreferenceManager.getDefaultSharedPreferences(this).getString("key", "");

@Override
protected String doInBackground(String... arg0) {

    HttpClient httpClient = new DefaultHttpClient();
    HttpPost httpPost = new HttpPost("http://usamobileapp.pe.hu/webservice/viewGradesJSON.php");
    try {
        List<NameValuePair> nameValuePair = new ArrayList<NameValuePair>(1);
        nameValuePair.add(new BasicNameValuePair("id", "somevalue"));
        httpPost.setEntity(new UrlEncodedFormEntity(nameValuePair));
        HttpResponse response = httpClient.execute(httpPost);
        Log.d("Http Response:", (response.getEntity()).toString());
    } catch (ClientProtocolException e) {

    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        httpClient.getConnectionManager().shutdown();
    }
    return null;
}

}

Here's the problem:

I'm a beginner so I don't seem to grasp what the problem is.

Thank you for the help.