Listview适配器正在处理mainactivity类，但不在Fragment中工作

Question

因为我是android的新手 我的片段类如下 -

/**
 * A simple {@link Fragment} subclass.
 */
public class DeviceListFragment extends android.support.v4.app.Fragment {

    //Initialize of device list and device adapter
    private ListView listview;

    private ArrayList<DeviceList> deviceLists;



    String deviceUrl;

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

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
    }

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


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


        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_geo_fence, container, false);
        listview = (ListView) view.findViewById(R.id.device_list);

        MyLiveTracking activity = (MyLiveTracking) getActivity();
        String username = activity.getUserNameData();
        String token = activity.getTokenData();
        String privateKeyData = activity.getPrivateKeyData();

        deviceLists = new ArrayList<DeviceList>();

        deviceUrl = "https://agdtrack.herokuapp.com/api/getdevices/"+token+"/"+privateKeyData;

        new JSONAsyncTask().execute(deviceUrl);

        DeviceListAdapter deviceListAdapter = new DeviceListAdapter(getActivity().getApplicationContext(), R.layout.device_row, deviceLists);

        listview.setAdapter(deviceListAdapter);


       // listview.setOnItemClickListener(new AdapterView.OnItemClickListener() {

            //@Override
           // public void onItemClick(AdapterView<?> arg0, View arg1, int position,long id) {
                // TODO Auto-generated method stub
           //     Toast.makeText(getActivity(), deviceLists.get(position).getCarImei(), Toast.LENGTH_LONG).show();
          //      Toast.makeText(getActivity(), deviceLists.get(position).getCarModel(), Toast.LENGTH_LONG).show();


           // }
       // });

        return view;
    }

    private void populateList() {

    }

    //Get user's Devices list form live server
    class JSONAsyncTask extends AsyncTask<String, Void, Boolean> {

        ProgressDialog dialog;

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            dialog = new ProgressDialog(getActivity());
            dialog.setMessage("Loading, please wait");
            dialog.setTitle("Connecting server");
            dialog.show();
            dialog.setCancelable(false);
        }

        @Override
        protected Boolean doInBackground(String... urls) {

            //Call Json Parser to get Json data
            JSONParser jsonParser = new JSONParser();
            JSONArray json = jsonParser.getJSONFromUrl(urls[0]);
            //System.out.println(json);
            try {
                for (int i=0; i<json.length(); i++) {
                    JSONObject object = json.getJSONObject(i);

                    DeviceList deviceList = new DeviceList();

                    deviceList.setCarName(object.getString("_id"));
                    deviceList.setCarModel(object.getString("hardware_model"));
                    deviceList.setCarImei(object.getString("imei"));
                    deviceList.setUserId(object.getString("users_id"));

                    deviceLists.add(deviceList);
                }
                return true;
            }catch (Exception e){ e.printStackTrace();}
            return false;
        }

        protected void onPostExecute(Boolean result) {
            dialog.cancel();
            //deviceListAdapter.notifyDataSetChanged();
            if(result == false) {
                Toast.makeText(getActivity(), "Unable to fetch data from server", Toast.LENGTH_LONG).show();
            }

        }
    }

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

    @Override
    public void onDestroy() {
        super.onDestroy();
    }

    @Override
    public void onDetach() {
        super.onDetach();
    }

}

我的适配器类位于 -

之下

/**
 * Created by tanvinn on 11/15/2015.
 */
public class DeviceListAdapter extends ArrayAdapter<DeviceList> {

    ArrayList<DeviceList> deviceLists;
    LayoutInflater vi;
    int Resource;
    ViewHolder holder;

    public DeviceListAdapter(Context context, int resource, ArrayList<DeviceList> objects) {
        super(context, resource, objects);
        vi = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);


        Resource = resource;
        deviceLists = objects;
    }

    static class ViewHolder {

        public TextView tvCarName;
        public TextView tvCarModel;
        public TextView tvImei;
        public TextView tvUserId;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        // convert view = design
        View v = convertView;


        if (v == null) {
            holder = new ViewHolder();
            v = vi.inflate(Resource, null);



            holder.tvCarName = (TextView) v.findViewById(R.id.tvCarName);
            holder.tvCarModel = (TextView) v.findViewById(R.id.tvCarModel);
            holder.tvImei = (TextView) v.findViewById(R.id.tvImei);
            holder.tvUserId = (TextView) v.findViewById(R.id.tvUserId);

            v.setTag(holder);
        } else {
            holder = (ViewHolder) v.getTag();
        }

        holder.tvCarName.setText(deviceLists.get(position).getCarName());
        holder.tvCarModel.setText(deviceLists.get(position).getCarModel());
        holder.tvImei.setText(deviceLists.get(position).getCarImei());
        holder.tvUserId.setText(deviceLists.get(position).getUserId());

        return v;

    }
}

这是我的DeviceList

/**
 * Created by tanvinn on 11/15/2015.
 */
public class DeviceList {

    private String carname;
    private String carmodel;
    private String carimei;
    private String userid;

    public DeviceList() {
        // TODO Auto-generated constructor stub
    }

    public DeviceList(String carname, String carmodel, String carimei, String userid) {
        super();
        this.carname = carname;
        this.carmodel = carmodel;
        this.carimei = carimei;
        this.userid = userid;
    }


    public String getCarName() {
        return carname;
    }

    public void setCarName(String carname) {
        this.carname = carname;
    }

    public String getCarModel() {
        return carmodel;
    }

    public void setCarModel(String carmodel) {
        this.carmodel = carmodel;
    }

    public String getCarImei() {
        return carimei;
    }

    public void setCarImei(String carimei) {
        this.carimei = carimei;
    }

    public String getUserId() {
        return userid;
    }

    public void setUserId(String userid) {
        this.userid = userid;
    }
}

我得到的错误如下 -

 9910-9910/bd.com.agdits.agdittracking E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: bd.com.agdits.agdittracking, PID: 9910
    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
            at bd.com.agdits.agdittracking.DeviceListFragment.onCreateView(DeviceListFragment.java:70)
            at android.support.v4.app.Fragment.performCreateView(Fragment.java:1962)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1026)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1207)
            at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1572)
            at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:493)
            at android.os.Handler.handleCallback(Handler.java:739)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5253)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:948)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:743)