android - 使用 asyncTask、接口(interface)和 fragment 时出现 NullPointerException

Question

我有一个加载项目 ListView 的异步任务。我目前正在尝试将 onClick 设置为加载一个新 fragment ，该 fragment 具有从单击的列表项中检索的“id”。 Android Studio 向我显示的代码中没有错误。

当我运行应用程序并单击 ListView 中的项目时，我得到了这个 FC:

02-13 19:49:56.813  20334-20334/com.beerportfolio.beerportfoliopro E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.NullPointerException
            at com.beerportfolio.beerportfoliopro.ReadJSONResult$1.onItemClick(ReadJSONResult.java:140)
            at android.widget.AdapterView.performItemClick(AdapterView.java:298)
            at android.widget.AbsListView.performItemClick(AbsListView.java:1237)
            at android.widget.ListView.performItemClick(ListView.java:4555)
            at android.widget.AbsListView$PerformClick.run(AbsListView.java:3037)
            at android.widget.AbsListView$1.run(AbsListView.java:3724)
            at android.os.Handler.handleCallback(Handler.java:730)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:158)
            at android.app.ActivityThread.main(ActivityThread.java:5789)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1027)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:843)
            at dalvik.system.NativeStart.main(Native Method)
02-13 19:50:42.112  20864-20870/? E/jdwp﹕ Failed sending reply to debugger: Broken pipe

ReadJSONResult 中的第 140 行是:

listenerBeer.onArticleSelected(idToSend);

该行是整个 onClick 的一部分:

//set up clicks
                lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
                    @Override
                    public void onItemClick(AdapterView<?> arg0, View arg1,
                                            int arg2, long arg3) {
                        BeerData beerInfo = beerList.get(arg2);

                        String idToSend = beerInfo.beerId;

                        //todo: launch beer fragment
                        listenerBeer.onArticleSelected(idToSend);




                    }
                });

ReadJSONResult 的所有代码是:

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

        Context c;
        private ProgressDialog Dialog;

        public ReadJSONResult(Context context)
        {
            c = context;
            Dialog = new ProgressDialog(c);
        }

        //code for on click
        OnArticleSelectedListener listenerBeer;
        public interface OnArticleSelectedListener{
            public void onArticleSelected(String myString);


        }
    public void setOnArticleSelectedListener(OnArticleSelectedListener listener){
        this.listenerBeer = listener;


    }
    //end code for onClick

        @Override
        protected String doInBackground(String... arg0) {
            // TODO Auto-generated method stub
            return readJSONFeed(arg0[0]);
        }

        protected void onPreExecute() {
            Dialog.setMessage("Searching Beer Cellar");

            Dialog.setTitle("Loading");
            Dialog.setCancelable(false);
            Dialog.show();
        }

        protected void onPostExecute(String result){
            //decode json here
            try{

                JSONObject json = new JSONObject(result);

                //acces listview
                ListView lv = (ListView) ((Activity) c).findViewById(android.R.id.list);


                //make array list for beer
                final List<BeerData> beerList = new ArrayList<BeerData>();



                //get json items
                for(int i = 0; i < json.getJSONArray("data").length(); i++) {

                    String beerId = GetBeerDataFromJSON(i,"id", json);
                    String beerName = GetBeerDataFromJSON(i,"name", json);
                    String beerDescription = GetBeerDataFromJSON(i,"description" , json);
                    String beerAbv = GetBeerDataFromJSON(i,"abv" , json);
                    String beerIbu = GetBeerDataFromJSON(i,"ibu" , json);
                    String beerIcon = GetBeerIconsFromJSON(i, "icon",json );
                    String beerMediumIcon = GetBeerIconsFromJSON(i, "medium",json );
                    String beerLargeIcon = GetBeerIconsFromJSON(i, "large",json );
                    String beerGlass = GetBeerGlassFromJSON(i, json );
                    String beerStyle = GetBeerStyleFromJSON(i,"name", json );
                    String beerStyleDescription = GetBeerStyleFromJSON(i,"description", json );
                    String beerBreweryId = GetBeerBreweryInfoFromJSON(i, "id", json );
                    String beerBreweryName = GetBeerBreweryInfoFromJSON(i, "name", json );
                    String beerBreweryDescription = GetBeerBreweryInfoFromJSON(i, "description", json );
                    String beerBreweryWebsite = GetBeerBreweryInfoFromJSON(i, "website", json );

                    //get long and latt
                    String beerBreweryLat = GetBeerBreweryLocationJSON(i, "longitude", json );
                    String beerBreweryLong = GetBeerBreweryLocationJSON(i, "latitude", json );

                    String beerBreweryYear = GetBeerBreweryInfoFromJSON(i, "established", json );
                    String beerBreweryIcon = GetBeerBreweryIconsFromJSON(i,"large",json);



                    //create beer object
                    BeerData thisBeer = new BeerData(beerName, beerId, beerDescription, beerAbv, beerIbu, beerIcon,
                            beerMediumIcon,beerLargeIcon, beerGlass, beerStyle, beerStyleDescription, beerBreweryId, beerBreweryName,
                            beerBreweryDescription, beerBreweryYear, beerBreweryWebsite,beerBreweryIcon, beerBreweryLat, beerBreweryLong);

                    //add beer to list
                    beerList.add(thisBeer);


                }

                //update listview
                BeerSearchAdapter adapter1 = new BeerSearchAdapter(c ,R.layout.listview_item_row, beerList);
                lv.setAdapter(adapter1);

                //set up clicks
                lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
                    @Override
                    public void onItemClick(AdapterView<?> arg0, View arg1,
                                            int arg2, long arg3) {
                        BeerData beerInfo = beerList.get(arg2);

                        String idToSend = beerInfo.beerId;

                        //todo: launch beer fragment
                        listenerBeer.onArticleSelected(idToSend);




                    }
                });




            }
            catch(Exception e){

            }

            Dialog.dismiss();

        }

        //todo: all the get functions go here
        private String GetBeerDataFromJSON(int position, String whatToGet, JSONObject json ) {
            String whatIsTheKeyYouAreLookFor = whatToGet;
            int whereInTheJSONArrayForLoopIsTheData = position;
            String holder = "";
            try{
                holder = json.getJSONArray("data").getJSONObject(whereInTheJSONArrayForLoopIsTheData).getString(whatIsTheKeyYouAreLookFor);

            } catch (JSONException e) {
                holder = "N/A";
            }



            return holder;
        }


        //get icons
        private String GetBeerBreweryIconsFromJSON(int position, String whatToGet, JSONObject json ) {
            String whatIsTheKeyYouAreLookFor = whatToGet;
            int whereInTheJSONArrayForLoopIsTheData = position;
            String holder = "";
            try{
                holder = json.getJSONArray("data").getJSONObject(whereInTheJSONArrayForLoopIsTheData).getJSONArray("breweries").getJSONObject(0).getJSONObject("images").getString(whatIsTheKeyYouAreLookFor);;
            } catch (JSONException e) {
                holder = "N/A";
            }



            return holder;
        }

        //get icons
        private String GetBeerIconsFromJSON(int position, String whatToGet, JSONObject json ) {
            String whatIsTheKeyYouAreLookFor = whatToGet;
            int whereInTheJSONArrayForLoopIsTheData = position;
            String holder = "";
            try{
                holder = json.getJSONArray("data").getJSONObject(whereInTheJSONArrayForLoopIsTheData).getJSONObject("labels").getString(whatIsTheKeyYouAreLookFor);

            } catch (JSONException e) {
                holder = "N/A";
            }



            return holder;
        }

        //get style information
        private String GetBeerStyleFromJSON(int position, String whatToGet, JSONObject json ) {
            String whatIsTheKeyYouAreLookFor = whatToGet;
            int whereInTheJSONArrayForLoopIsTheData = position;
            String holder = "";
            try{
                holder = json.getJSONArray("data").getJSONObject(whereInTheJSONArrayForLoopIsTheData).getJSONObject("style").getString(whatIsTheKeyYouAreLookFor);

            } catch (JSONException e) {
                holder = "N/A";
            }



            return holder;
        }

        //get location data
        private String GetBeerBreweryLocationJSON(int position, String whatToGet, JSONObject json ) {
            String whatIsTheKeyYouAreLookFor = whatToGet;
            int whereInTheJSONArrayForLoopIsTheData = position;
            String holder = "";
            try{
                holder = json.getJSONArray("data").getJSONObject(whereInTheJSONArrayForLoopIsTheData).getJSONArray("breweries").getJSONObject(0).getJSONArray("locations").getJSONObject(0).getString(whatIsTheKeyYouAreLookFor);

            } catch (JSONException e) {
                holder = "N/A";
            }



            return holder;
        }

        //get brewery information
        //get style information
        private String GetBeerBreweryInfoFromJSON(int position, String whatToGet, JSONObject json ) {
            String whatIsTheKeyYouAreLookFor = whatToGet;
            int whereInTheJSONArrayForLoopIsTheData = position;
            String holder = "";
            try{
                holder = json.getJSONArray("data").getJSONObject(whereInTheJSONArrayForLoopIsTheData).getJSONArray("breweries").getJSONObject(0).getString(whatIsTheKeyYouAreLookFor);

            } catch (JSONException e) {
                holder = "N/A";
            }



            return holder;
        }

        //get glass
        private String GetBeerGlassFromJSON(int position, JSONObject json ) {

            int whereInTheJSONArrayForLoopIsTheData = position;
            String holder = "";
            try{
                holder = json.getJSONArray("data").getJSONObject(whereInTheJSONArrayForLoopIsTheData).getJSONObject("glass").getString("name");

            } catch (JSONException e) {
                holder = "N/A";
            }



            return holder;
        }



        public String readJSONFeed(String URL) {
            StringBuilder stringBuilder = new StringBuilder();
            HttpClient httpClient = new DefaultHttpClient();
            HttpGet httpGet = new HttpGet(URL);
            try {
                HttpResponse response = httpClient.execute(httpGet);
                StatusLine statusLine = response.getStatusLine();
                int statusCode = statusLine.getStatusCode();
                if (statusCode == 200) {
                    HttpEntity entity = response.getEntity();
                    InputStream inputStream = entity.getContent();
                    BufferedReader reader = new BufferedReader(
                            new InputStreamReader(inputStream));
                    String line;
                    while ((line = reader.readLine()) != null) {
                        stringBuilder.append(line);
                    }
                    inputStream.close();
                } else {
                    Log.d("JSON", "Failed to download file");
                }
            } catch (Exception e) {
                Log.d("readJSONFeed", e.getLocalizedMessage());
            }
            return stringBuilder.toString();
        }

    }

BeerSearchAdapter 是:

public class BeerSearchAdapter extends ArrayAdapter<BeerData> {

    Context context;
    int layoutResourceId;
    List<BeerData> data = null;

    public BeerSearchAdapter(Context context, int layoutResourceId, List<BeerData> data) {
        super(context, layoutResourceId, data);
        this.layoutResourceId = layoutResourceId;
        this.context = context;
        this.data = data;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View row = convertView;
        beerHolder holder = null;

        if(row == null)
        {
            LayoutInflater inflater = ((Activity)context).getLayoutInflater();
            row = inflater.inflate(layoutResourceId, parent, false);

            holder = new beerHolder();
            holder.txtBrewery = (TextView)row.findViewById(R.id.beerBreweryNameList);
            holder.txtTitle = (TextView)row.findViewById(R.id.beerNameList);

            row.setTag(holder);
        }
        else
        {
            holder = (beerHolder)row.getTag();
        }

        BeerData beer = data.get(position);
        holder.txtTitle.setText(beer.beerName);
        holder.txtBrewery.setText(beer.beerBreweryName);

        return row;
    }

    static class beerHolder
    {
        TextView txtBrewery;
        TextView txtTitle;
    }
}

我的 Search.java 接口(interface)形式在这里:

public class Search extends Fragment implements SearchView.OnQueryTextListener, ReadJSONResult.OnArticleSelectedListener {

    private ListView lv;
    View v;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        //set layout here
        v = inflater.inflate(R.layout.activity_search, container, false);
        setHasOptionsMenu(true);
        getActivity().setTitle("Search");


        //get user information
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(getActivity());
        String userName = prefs.getString("userName", null);
        String userID = prefs.getString("userID", null);


        //todo: code body goes here





        // Inflate the layout for this fragment
        return v;


}


    @Override
    public void onCreateOptionsMenu (Menu menu, MenuInflater inflater) {
        // Inflate the menu; this adds items to the action bar if it is present.

        super.onCreateOptionsMenu(menu, inflater);

        Log.d("click", "inside the on create");

        //inflater.inflate(R.menu.main, menu);
        SearchView searchView = (SearchView) menu.findItem(R.id.menu_search2).getActionView();
        searchView.setIconified(false);
        searchView.setOnQueryTextListener(this);
    }




    public boolean onQueryTextSubmit (String query) {

        //toast query

        //make json variables to fill

        // url to make request
        String url = "myURL";



        try {
            query = URLEncoder.encode(query, "UTF-8");
        } catch (UnsupportedEncodingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }


        String jsonUrl = url + query;



        //todo: get json
        new ReadJSONResult(getActivity()).execute(jsonUrl);

        return false;
    }





    @Override
    public boolean onQueryTextChange(String newText) {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public void onArticleSelected(String b){

        //code to execute on click
        Fragment Fragment_one;
        FragmentManager man= getFragmentManager();
        FragmentTransaction tran = man.beginTransaction();

        //todo: set to beer fragment
        Fragment_one = new StylePage2();
        final Bundle bundle = new Bundle();
        bundle.putString("beerIDSent", b);
        Fragment_one.setArguments(bundle);
        tran.replace(R.id.main, Fragment_one);//tran.
        tran.addToBackStack(null);
        tran.commit();

    }



}

如果您需要任何其他代码，请告诉我，我很忙，可以使用第二双眼睛。谢谢。

Answer 1

从错误消息中可以看出，listenerBeer 引用是null。

我看到这段代码:

public void setOnArticleSelectedListener(OnArticleSelectedListener listener){
    this.listenerBeer = listener;
}

但我没有看到任何调用它的代码。除非您没有包含设置 listenerBeer 引用的代码，否则这将解释为什么它是 null。

在您的代码中的某处，您必须执行如下操作:

. . .
setOnArticleSelectedListener(new OnArticleSelectedListener() {
  public void onArticleSelected(String myArticle) {
    // Do something here....
  }
});
. . .

HTH