json - ListView 中的空对象引用

Question

我在一个片段中有一个 ListView ，我正在用来自 JSON 调用的信息填充它。

我在弄湿片段中的 ListView 时遇到问题。我的片段只扩展了片段，我假设这可能是我的问题所在。我用来帮助我导入 json 数据的示例代码没有使用片段，所以我有点困惑。

我的错误是:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference

at com.peekatu.Fcc4me.watchFragment$GetContacts.onPostExecute(watchFragment.java:192)
            at com.peekatu.Fcc4me.watchFragment$GetContacts.onPostExecute(watchFragment.java:119)
            at android.os.AsyncTask.finish(AsyncTask.java:632)
            at android.os.AsyncTask.access$600(AsyncTask.java:177)
            at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            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)

片段

public class watchFragment extends Fragment {


private static final String TAG_DATA = "data";
private static final String TAG_TITLE = "title";
private static final String TAG_URL = "video_url";
private static final String TAG_IMAGE = "image";
public ListView lv;

private ProgressDialog pDialog;

// URL to get contacts JSON
private static String url = "/load.php";

// contacts JSONArray
JSONArray data = null;

// Hashmap for ListView
ArrayList<HashMap<String, String>> dataList;

private WebView web_v;
// TODO: Rename parameter arguments, choose names that match
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
private static final String ARG_PARAM1 = "param1";
private static final String ARG_PARAM2 = "param2";

// TODO: Rename and change types of parameters
private String mParam1;
private String mParam2;


/**
 * Use this factory method to create a new instance of
 * this fragment using the provided parameters.
 *
 * @param param1 Parameter 1.
 * @param param2 Parameter 2.
 * @return A new instance of fragment watchFragment.
 */
// TODO: Rename and change types and number of parameters
public static watchFragment newInstance(String param1, String param2) {
    watchFragment fragment = new watchFragment();
    Bundle args = new Bundle();
    args.putString(ARG_PARAM1, param1);
    args.putString(ARG_PARAM2, param2);
    fragment.setArguments(args);
    return fragment;
}
public watchFragment() {
    // Required empty public constructor
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (getArguments() != null) {
        mParam1 = getArguments().getString(ARG_PARAM1);
        mParam2 = getArguments().getString(ARG_PARAM2);
    }

}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment

    View v= inflater.inflate(R.layout.fragment_watch, container, false);

    //btn = (Button) v.findViewById(R.id.button1);
    dataList = new ArrayList<HashMap<String, String>>();

    ListView lv = (ListView) v.findViewById(R.id.listView1);


    // Calling async task to get json
    new GetVideos().execute();



    return v;
}

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

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        // Showing progress dialog
        pDialog = new ProgressDialog(getActivity());
        pDialog.setMessage("Please wait...");
        pDialog.setCancelable(false);
        pDialog.show();

    }

    @Override
    protected Void doInBackground(Void... arg0) {
        // Creating service handler class instance
        ServiceHandler sh = new ServiceHandler();

        // Making a request to url and getting response
        String jsonStr = sh.makeServiceCall(url, ServiceHandler.GET);

        Log.d("Response: ", "> " + jsonStr);

        if (jsonStr != null) {
            try {
                JSONObject jsonObj = new JSONObject(jsonStr);

                // Getting JSON Array node
                data = jsonObj.getJSONArray(TAG_DATA);

                // looping through All Contacts
                for (int i = 0; i < data.length(); i++) {
                    JSONObject d = data.getJSONObject(i);

                    String title = d.getString(TAG_TITLE);
                    String image = d.getString(TAG_IMAGE);
                    String url = d.getString(TAG_URL);

                    // tmp hashmap for single contact
                    HashMap<String, String> data = new HashMap<String, String>();

                    // adding each child node to HashMap key => value
                    data.put(TAG_TITLE, title);
                    data.put(TAG_IMAGE, image);
                    data.put(TAG_URL, url);

                    // adding contact to contact list
                    dataList.add(data);
                }
            } catch (JSONException e) {
                e.printStackTrace();
            }
        } else {
            Log.e("ServiceHandler", "Couldn't get any data from the url");
        }

        return null;
    }
    @Override
    protected void onPostExecute(Void result) {
        super.onPostExecute(result);
        // Dismiss the progress dialog
        if (pDialog.isShowing())
            pDialog.dismiss();
        /**
         * Updating parsed JSON data into ListView
         * */
        ListAdapter adapter = new SimpleAdapter(
                getActivity(), dataList,
                R.layout.list_item, new String[] { TAG_TITLE, TAG_IMAGE
        }, new int[] { R.id.title,
                R.id.image});

        lv.setAdapter(adapter);
    }
}
}

XML 布局

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.peekatu.Fcc4me.watchFragment">

    <!-- TODO: Update blank fragment layout -->

    <VideoView
        android:layout_width="wrap_content"
        android:layout_height="225dp"
        android:id="@+id/videoView"
        android:layout_gravity="center_horizontal|top" />

    <ListView
        android:id="@+id/listView1"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/videoView" />

    </RelativeLayout>

第 192 行是 lv.setAdapter(adapter);

Answer 1

ListView lv 未初始化(您在 onCreateView 中注释掉了该行):

//ListView lv = (ListView) v.findViewById(R.id.listView1);

你应该取消注释:

ListView lv = (ListView) v.findViewById(R.id.listView1);

编辑:好的，好的，对不起，您需要分配给全局 lv 变量而不是本地变量，试试这个:

lv = (ListView) v.findViewById(R.id.listView1);

这应该让您获得正确的 View 并使空异常消失...