Question

我正在创建一个显示来自tumblr帐户的图片的应用。它一次显示20个图像，并且在底部有一个botten，按下时应该将20个新图像加载到列表中（仅显示新的20个图像）。我正在使用AsyncTask来执行该过程。我已经走得很远但是当点击“加载更多”按钮时，应用程序会在NullPointerException类中的tumblrs.add(tumblr);行与DoInBackground崩溃。我似乎无法找到问题而且在这一点上陷入困境。

代码如下所示：

public class Example extends Activity {

ArrayList<Tumblr> tumblrs;
Context context = null;
ListView listView = null;
TextView footer;
int offset = 0;
ProgressDialog pDialog;

private String searchUrl = "http://api.tumblr.com/v2/blog/factsandchicks.com/posts?api_key=API_KEY&offset=0";

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    ArrayList<Tumblr> tumblrs;
    try {
        tumblrs = getTumblrs();
        listView = (ListView) findViewById(R.id.list);
        View v = getLayoutInflater().inflate(R.layout.footer_layout, null);
        footer = (TextView) v.findViewById(R.id.tvFoot);
        listView.addFooterView(v);
        listView.setAdapter(new UserItemAdapter(this, R.layout.listitem,
                tumblrs));

    } catch (ClientProtocolException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } catch (JSONException e) {
        e.printStackTrace();
    }

    footer.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            new loadMoreListView().execute();
        }
    });

}

public class UserItemAdapter extends ArrayAdapter<Tumblr> {
    private ArrayList<Tumblr> tumblrs;

    public UserItemAdapter(Context context, int imageViewResourceId,
            ArrayList<Tumblr> tumblrs) {
        super(context, imageViewResourceId, tumblrs);
        this.tumblrs = tumblrs;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View v = convertView;
        if (v == null) {

            LayoutInflater vi = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            v = vi.inflate(R.layout.listitem, null);

        }

        Tumblr tumblr = tumblrs.get(position);
        if (tumblr != null) {

            ImageView image = (ImageView) v.findViewById(R.id.avatar);

            if (image != null) {
                image.setImageBitmap(getBitmap(tumblr.image_url));
            }
        }
        return v;
    }
}

public Bitmap getBitmap(String bitmapUrl) {
    try {
        URL url = new URL(bitmapUrl);
        return BitmapFactory.decodeStream(url.openConnection()
                .getInputStream());
    } catch (Exception ex) {
        return null;
    }
}

public ArrayList<Tumblr> getTumblrs() throws ClientProtocolException,
        IOException, JSONException {
    searchUrl = "http://api.tumblr.com/v2/blog/factsandchicks.com/posts?api_key=API_KEY&offset=0";

    ArrayList<Tumblr> tumblrs = new ArrayList<Tumblr>();

    HttpClient client = new DefaultHttpClient();
    HttpGet get = new HttpGet(searchUrl);

    ResponseHandler<String> responseHandler = new BasicResponseHandler();

    String responseBody = null;
    try {
        responseBody = client.execute(get, responseHandler);
    } catch (Exception ex) {
        ex.printStackTrace();
    }

    JSONObject jsonObject = new JSONObject(responseBody);

    JSONArray posts = jsonObject.getJSONObject("response").getJSONArray(
            "posts");
    for (int i = 0; i < posts.length(); i++) {
        JSONArray photos = posts.getJSONObject(i).getJSONArray("photos");
        for (int j = 0; j < photos.length(); j++) {
            JSONObject photo = photos.getJSONObject(j);
            String url = photo.getJSONArray("alt_sizes").getJSONObject(0)
                    .getString("url");

            Tumblr tumblr = new Tumblr(url);
            tumblrs.add(tumblr);
        }
    }
    return tumblrs;
}

public class Tumblr {

    public String image_url;

    public Tumblr(String url) {

        this.image_url = url;
    }
}

private class loadMoreListView extends AsyncTask<Void, Void, Void> {

    @Override
    protected void onPreExecute() {
        // Showing progress dialog before sending http request
        pDialog = new ProgressDialog(Example.this);
        pDialog.setMessage("Please wait..");
        pDialog.setIndeterminate(true);
        pDialog.setCancelable(false);
        pDialog.show();
    }

    @Override
    protected Void doInBackground(Void... unused) {
        // TODO Auto-generated method stub
        runOnUiThread(new Runnable() {
            public void run() {
                // increment current page
                offset += 20;

                // Next page request

                searchUrl = "http://api.tumblr.com/v2/blog/factsandchicks.com/posts?api_key=API_KEY&offset="
                        + offset;

                HttpClient client = new DefaultHttpClient();
                HttpGet get = new HttpGet(searchUrl);

                ResponseHandler<String> responseHandler = new BasicResponseHandler();

                String responseBody = null;
                try {
                    responseBody = client.execute(get, responseHandler);
                } catch (Exception ex) {
                    ex.printStackTrace();
                }

                JSONObject jsonObject;
                try {
                    jsonObject = new JSONObject(responseBody);

                    JSONArray posts = jsonObject.getJSONObject("response")
                            .getJSONArray("posts");
                    for (int i = 0; i < posts.length(); i++) {
                        JSONArray photos = posts.getJSONObject(i)
                                .getJSONArray("photos");
                        for (int j = 0; j < photos.length(); j++) {
                            JSONObject photo = photos.getJSONObject(j);
                            String url = photo.getJSONArray("alt_sizes")
                                    .getJSONObject(0).getString("url");

                            Tumblr tumblr = new Tumblr(url);
                            tumblrs.add(tumblr);

                        }
                    }

                } catch (JSONException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

                // get listview current position - used to maintain scroll
                // position
                int currentPosition = listView.getFirstVisiblePosition();

                // Setting new scroll position
                listView.setSelectionFromTop(currentPosition + 1, 0);

            }
        });

        return null;
    }

    protected void onPostExecute(Void unused) {
        pDialog.dismiss();
    }

}

}

帮助很高。

Answer 1

tumblrs字段未初始化。在onCreate方法中，您初始化一个名为tumblrs的本地变量。

Answer 2

您可能无法在onCreate方法中初始化字段“tumblrs”（在第3行中声明），因为您将其重新声明为第17行中的局部变量。

将新数据附加到Arraylist

2 个答案: