具有空指針異常的RecyclerView

Question

我從CMS中獲取了一些新聞數據，並希望在RecyclerView中顯示它們。問題是我在使用LinearLayoutManager的行中收到了空指針異常。 這是logcat的輸出。

Caused by: java.lang.NullPointerException
  at testing.theo.manchesterunitedapp.newsandfeatures.FeaturesFragment.onCreateView(FeaturesFragment.java:65)
  at android.support.v4.app.Fragment.performCreateView(Fragment.java:1962)
  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1067)
  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1248)
  at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
  at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1613)
  at android.support.v4.app.FragmentController.execPendingActions(FragmentController.java:330)
  at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:547)
  at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1171)
  at android.app.Activity.performStart(Activity.java:5241)
  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2157)
  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2233) 
  at android.app.ActivityThread.access$800(ActivityThread.java:135) 
  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) 
  at android.os.Handler.dispatchMessage(Handler.java:102) 
  at android.os.Looper.loop(Looper.java:136) 
  at android.app.ActivityThread.main(ActivityThread.java:5001) 
  at java.lang.reflect.Method.invokeNative(Native Method) 
  at java.lang.reflect.Method.invoke(Method.java:515)

我在其中進行Web服務的片段非常簡單。 我使用回調方法。 我要在其中初始化ArrayList和RecyclerView的onCreateView，然后是我正在運行Web服務的onActivityCreate。

public class FeaturesFragment extends Fragment {

public static final String TAG = "ManuApp";
private static final String IMAGE_URL = "xxxxxxxxxxxx/xxxx/features_images/" ;
private List<FeaturesObject> listItemsList;

private RecyclerView mRecyclerView;
private FeaturesAdapter adapter;

public FeaturesFragment() {
    // Required empty public constructor
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.features_row, container, false);
    // Inflate the layout for this fragment
    listItemsList = new ArrayList<>();
    mRecyclerView = (RecyclerView)v.findViewById(R.id.features_recycler_view);
    //mRecyclerView.addItemDecoration(new HorizontalDividerItemDecoration.Builder(getActivity()).color(Color.BLACK).build());

    final LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity());
    mRecyclerView.setLayoutManager(linearLayoutManager);


    return v;
}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    updateFeaturesList();
}

public void updateFeaturesList() {




    //declare the adapter and attach it to the recyclerview
    adapter = new FeaturesAdapter(getActivity(), listItemsList);
    mRecyclerView.setAdapter(adapter);

    // Instantiate the RequestQueue.
    RequestQueue queue = Volley.newRequestQueue(getActivity());



    // Request a string response from the provided URL.
    JsonArrayRequest jsObjRequest = new JsonArrayRequest(Request.Method.GET, Config.URL_FEATURES, new Response.Listener<JSONArray>() {

        @Override
        public void onResponse(JSONArray response) {

            Log.d(TAG, response.toString());
            //hidePD();

            // Parse json data.
            // Declare the json objects that we need and then for loop through the children array.
            // Do the json parse in a try catch block to catch the exceptions
            try {



                for (int i = 0; i < response.length(); i++) {

                    JSONObject post = response.getJSONObject(i);
                    FeaturesObject item = new FeaturesObject();
                    item.setTitle(post.getString("title"));
                    item.setImage(IMAGE_URL + post.getString("features_image"));
                    item.setArticle(post.getString("article"));



                    listItemsList.add(item);

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

            // Update list by notifying the adapter of changes
            adapter.notifyDataSetChanged();
        }
    }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            VolleyLog.d(TAG, "Error: " + error.getMessage());
            //hidePD();
        }
    });
    jsObjRequest.setRetryPolicy(new RetryPolicy() {
        @Override
        public int getCurrentTimeout() {
            return 50000;
        }

        @Override
        public int getCurrentRetryCount() {
            return 50000;
        }

        @Override
        public void retry(VolleyError error) throws VolleyError {

        }
    });
    queue.add(jsObjRequest);

}
}

這部分代碼會導致問題。

final LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity());
    mRecyclerView.setLayoutManager(linearLayoutManager);

這很奇怪，因為我使用相同的技術來獲取應用程序另一個片段中的數據。

有任何想法嗎？

謝謝。

Answer 1

您的RecyclerView為空。 您確定要在正確的布局文件中尋找正確的ID嗎？ 我的猜測是您正在填充錯誤的布局文件，或者正在尋找RecyclerView的錯誤視圖ID。

Answer 2

可能不是這種情況，但是...通常，所有需要活動實例的初始化都必須在onActivityCreated() （或Fragment生命周期中的下一個方法）。 在onCreateView()內部，不能保證getActivity（）返回非空值。 在某些情況下，它返回null。

Answer 3

好像您要查找的id不在features_row ，這就是為什么在該行上獲得NullPointerException的原因：

mRecyclerView = (RecyclerView)v.findViewById(R.id.features_recycler_view);

如果您仍然遇到錯誤，可以嘗試以下方法：

final FragmentActivity c = getActivity();
final LinearLayoutManager linearLayoutManager = new LinearLayoutManager(c);
mRecyclerView.setLayoutManager(linearLayoutManager );