XML Parsing With AsyncTask

Question

I want to take currency from specific Url but AsyncTask is giving me runtime error: I couldn't find what I have to do.

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    WebServisXML xmlRead = new WebServisXML(this);
    xmlRead.execute("http://www.tcmb.gov.tr/kurlar/today.xml");
}

public class WebServisXML extends AsyncTask<String, String, List<String>> {

    private Context context;
    private ListView listView;
    private ProgressDialog dialogBar;
    List<String> dovizListe = new ArrayList<String>();
    HttpURLConnection baglanti = null;

    public WebServisXML(Context context) {
        this.context = context;
        listView = (ListView) ((AppCompatActivity) context).findViewById(R.id.listView);}
        protected void onPreExecute() {
        super.onPreExecute();
        dialogBar = ProgressDialog.show(context, "Lütfen bekleyiniz...", "İşlem yükleniyor...", true);
    }

    @Override
    protected List<String> doInBackground(String... params) {
        try {
            URL adressOfLink = new URL(params[0]);
            baglanti = (HttpURLConnection) adressOfLink.openConnection();
            int responceCode = baglanti.getResponseCode();
            if (responceCode == HttpURLConnection.HTTP_OK) {

                BufferedInputStream stream = new BufferedInputStream(baglanti.getInputStream());
                publishProgress("Döviz kurları okunuyor...");
                DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory.newInstance();
                DocumentBuilder documentBuilder = documentBuilderFactory.newDocumentBuilder();

                Document document = documentBuilder.parse(stream);

                NodeList dovizNodeList = document.getElementsByTagName("Currency");

                for (int i = 0; i < dovizNodeList.getLength(); i++) {

                    Element element = (Element) dovizNodeList.item(i);

                    NodeList nodeBirim = element.getElementsByTagName("Unit");
                    NodeList nodeIsim = element.getElementsByTagName("Isim");
                    NodeList nodeAlis = element.getElementsByTagName("ForexBuying");
                    NodeList nodeSatis = element.getElementsByTagName("ForexSelling");

                    String birim = nodeBirim.item(0).getFirstChild().getNodeValue();
                    String isim = nodeIsim.item(0).getFirstChild().getNodeValue();
                    String alis = nodeAlis.item(0).getFirstChild().getNodeValue();
                    String satis = nodeSatis.item(0).getFirstChild().getNodeValue();

                    dovizListe.add(birim + " " + isim + "  Alış:" + alis + "  Satış:" + satis);
                }
                publishProgress("Liste güncelleniyor...");
            }
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (ParserConfigurationException e) {
            e.printStackTrace();
        } catch (SAXException e) {
            e.printStackTrace();
        }finally {
            if(baglanti!=null)
                baglanti.disconnect();
        }
        return dovizListe;
    }

    @Override
    protected void onProgressUpdate(String... values) {
        super.onProgressUpdate(values);
        dialogBar.setMessage(values[0]);
    }

    @Override
    protected void onPostExecute(List<String> strings) {
        super.onPostExecute(strings);
        ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(context,android.R.layout.simple_expandable_list_item_1,strings);
        listView.setAdapter(arrayAdapter);
        dialogBar.cancel();
    }
}}

This is my error screen:

Answer 1

One of the nodes first child's is null. (Node has no children)

Without proper line numbers I cannot tell you which one. (Line 98 of MainActivity.java)

I suggest adding a check:

String birim = nodeBirim.item(0).getLength() > 0 ? nodeBirim.item(0).getFirstChild().getNodeValue() : "";
String isim = nodeIsim.item(0).getLength() > 0 ? nodeIsim.item(0).getFirstChild().getNodeValue() : "";
String alis = nodeAlis.item(0).getLength() > 0 ? nodeAlis.item(0).getFirstChild().getNodeValue() : "";
String satis = nodeSatis.item(0).getLength() > 0 ? nodeSatis.item(0).getFirstChild().getNodeValue() : "";