我在扩展ListFragment的类中有一个AsyncTask。在课程中，任务会查询谷歌附近的地方，并成功返回值。在我的onCreateView中，我创建了一个视图，使用我在本地实例化的变量。在我的onPostExecute方法中，我有一个“循环和日志”段，以及一个空检查来验证我有数据。我发现我的自定义适配器为null，尽管已实例化它。我想我在这里缺少一些基本的东西，任何帮助都会受到赞赏。我在这个问题上搜索了大约35篇其他帖子，但无济于事。 编辑：我还重新观看了Romain Guy关于适配器/列表视图的演示，并重新阅读了开发。关于developer.android的文档。
My Code
package com.example.twigglebeta4;
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONArray;
import org.json.JSONObject;
import com.example.twigglebeta4.PlaceItem;
//import com.google.android.gms.maps.model.LatLng;
import android.content.Context;
import android.location.Location;
import android.location.LocationManager;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v4.app.ListFragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.ViewGroup;
import android.widget.ListView;
//This class is intended to retrieve data,
//and place it into an array for a custom adapter to set the results to listview
public class NearbyPlaceFragment extends ListFragment{
//private int MAX_PLACES=20;
static LocationManager systemLocation;
LayoutInflater inflater;
ViewGroup container;
ArrayList<PlaceItem> placeItems;
//Moving ListView_Adapter out here gives NPE error :
//09-08 15:03:52.190: E/AndroidRuntime(30422): at com.example.twigglebeta4.NearbyPlaceFragment.<init>(NearbyPlaceFragment.java:42)
//ListView_Adapter listViewAdapter = new ListView_Adapter(getActivity().getBaseContext());
ListView_Adapter listViewAdapter;
public NearbyPlaceFragment() {
}
@Override
public ListView onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
//Layout containing listView
ListView rootListView = (ListView) inflater.inflate(R.layout.nearby_list, container, false);
//ID of listView within layout
ListView listView = (ListView) rootListView.findViewById(android.R.id.list);
//Adapter to control cell items
ListView_Adapter listViewAdapter = new ListView_Adapter(getActivity().getBaseContext());
listView.setAdapter(listViewAdapter);
//Start to get places
GetNearbyPlaces();
return rootListView;
}
//Need to retrieve nearby places
private void GetNearbyPlaces() {
//Instantiate wrapper for latitude and longitude
systemLocation = (LocationManager) getActivity().getSystemService(Context.LOCATION_SERVICE);
//Get last location
Location lastKnownLocation = systemLocation.getLastKnownLocation(LocationManager.NETWORK_PROVIDER);
//Get latitude and longitude
double lastLatitude = lastKnownLocation.getLatitude();
double lastLongitude = lastKnownLocation.getLongitude();
//Construct URL for places request
String nearbySearchURL = "https://maps.googleapis.com/maps/api/place/nearbysearch/" +
"json?location="+lastLatitude+","+lastLongitude+
"&radius=1000&sensor=true" + "&types=food%7Crestaurant%7Cbar%7Cstore"+
"&key=AIzaSyC6KLby3iLblyuQY%5Fy%2DoXEvrZyuAZk9OWc";
//Execute request
new ExecuteQuery().execute(nearbySearchURL);
}
//This method is used to execute the query
private class ExecuteQuery extends AsyncTask<String, Void, ArrayList<PlaceItem>> {
@Override
protected ArrayList<PlaceItem> doInBackground(String... nearbyURL) {
// TODO Auto-generated method stub
StringBuilder nearbyPlacesBuilder = new StringBuilder();
//Need to execute query
for(String nearbySearchURL : nearbyURL) {
//Instantiate default Http Client
//This is the container that will execute our query
HttpClient nearbyClient = new DefaultHttpClient();
try {
//Create a new get method to retrieve the data
//This method is the request builder
HttpGet nearbyGet = new HttpGet(nearbySearchURL);
//Get the data from our get
HttpResponse nearbyGetResponse = nearbyClient.execute(nearbyGet);
//Check the status to find whether response was successful or not
if(nearbyGetResponse.getStatusLine().getStatusCode()==200) {
//Create Http Entity of the response to read
HttpEntity nearbyEntity = nearbyGetResponse.getEntity();
//Store the content of our entity, by using an input stream
InputStream nearbyEntityContent = nearbyEntity.getContent();
//Read the content of the input stream to create the input
InputStreamReader nearbyInput = new InputStreamReader(nearbyEntityContent);
//Read the information from the input stream data
BufferedReader nearbyReader = new BufferedReader(nearbyInput);
String lineIn;
while((lineIn = nearbyReader.readLine()) != null) {
//Log.i("com.example.twigglebeta4",lineIn);
nearbyPlacesBuilder.append(lineIn);
}
}
}
catch (Exception err) {
err.printStackTrace();
}
}
placeItems = new ArrayList<PlaceItem>();
try {
//Parse JSON object to format results
JSONObject nearbyResultObject = new JSONObject(nearbyPlacesBuilder.toString());
//Create an array from the JSON Object
JSONArray placesArray = nearbyResultObject.getJSONArray("results");
for(int placeCount=0;placeCount < placesArray.length();placeCount++){
//LatLng thisLocation = null;
String thisName = "";
String thisAddress = "";
//Grab the current JSON object
JSONObject thisNearbyObject = placesArray.getJSONObject(placeCount);
//Grab the current name
thisName = thisNearbyObject.getString("name");
//Grab the current address
thisAddress = thisNearbyObject.getString("vicinity");
//Grab this latitude longitude from the object
//JSONObject thisLatLng = thisNearbyObject.getJSONObject("geometry").getJSONObject("location");
//thisLocation = new LatLng(Double.valueOf(thisLatLng.getString("lat")),Double.valueOf(thisLatLng.getString("lng")));
PlaceItem thisPlace = new PlaceItem(R.id.list_cell_icon,thisName,thisAddress);
placeItems.add(thisPlace);
}
}
catch (Exception err) {
err.printStackTrace();
}
return placeItems;
}
protected void onPostExecute(final ArrayList<PlaceItem> placeItems) {
Log.i("onPostExecute", "Starting onPostExecute");
//listViewAdapter.notifyDataSetChanged();
//Layout containing listView
for(int i=0;i<placeItems.size();i++) {
Log.i("onPostExecute Loop", placeItems.get(i).name);
//Problem section start
if(placeItems.get(i).name != null /*&& listViewAdapter != null*/) {//If I remove this condition
Log.i("onPostExecute Loop", "I'm in here now");
listViewAdapter.add(placeItems.get(i).name); //then error here, listViewAdapter is null
}
//Problem section end
}
}
}
}
我的堆栈跟踪：
09-08 15:07:17.290: I/endeffect(31947): AbsListView.onMeasure(), getWidth()=1080, getHeight()=1557, this=android.widget.ListView{4240f490 V.ED.VC. ......I. 0,0-1080,1557 #102000a android:id/list}
09-08 15:07:17.290: D/AbsListView(31947): unregisterIRListener() is called
09-08 15:07:17.290: I/endeffect(31947): AbsListView.onLayout(), getWidth()=1080, getHeight()=1557, this=android.widget.ListView{4240f490 V.ED.VC. ......ID 0,0-1080,1557 #102000a android:id/list}
09-08 15:07:17.650: I/onPostExecute(31947): Starting onPostExecute
09-08 15:07:17.650: I/onPostExecute Loop(31947): Admiral Theatre
09-08 15:07:17.650: I/onPostExecute Loop(31947): I'm in here now
09-08 15:07:17.650: D/AndroidRuntime(31947): Shutting down VM
09-08 15:07:17.650: W/dalvikvm(31947): threadid=1: thread exiting with uncaught exception (group=0x4127fac8)
09-08 15:07:17.660: E/AndroidRuntime(31947): FATAL EXCEPTION: main
09-08 15:07:17.660: E/AndroidRuntime(31947): java.lang.NullPointerException
09-08 15:07:17.660: E/AndroidRuntime(31947): at com.example.twigglebeta4.NearbyPlaceFragment$ExecuteQuery.onPostExecute(NearbyPlaceFragment.java:180)
09-08 15:07:17.660: E/AndroidRuntime(31947): at com.example.twigglebeta4.NearbyPlaceFragment$ExecuteQuery.onPostExecute(NearbyPlaceFragment.java:1)
09-08 15:07:17.660: E/AndroidRuntime(31947): at android.os.AsyncTask.finish(AsyncTask.java:631)
09-08 15:07:17.660: E/AndroidRuntime(31947): at android.os.AsyncTask.access$600(AsyncTask.java:177)
09-08 15:07:17.660: E/AndroidRuntime(31947): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
09-08 15:07:17.660: E/AndroidRuntime(31947): at android.os.Handler.dispatchMessage(Handler.java:99)
09-08 15:07:17.660: E/AndroidRuntime(31947): at android.os.Looper.loop(Looper.java:137)
09-08 15:07:17.660: E/AndroidRuntime(31947): at android.app.ActivityThread.main(ActivityThread.java:5328)
09-08 15:07:17.660: E/AndroidRuntime(31947): at java.lang.reflect.Method.invokeNative(Native Method)
09-08 15:07:17.660: E/AndroidRuntime(31947): at java.lang.reflect.Method.invoke(Method.java:511)
09-08 15:07:17.660: E/AndroidRuntime(31947): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
09-08 15:07:17.660: E/AndroidRuntime(31947): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
09-08 15:07:17.660: E/AndroidRuntime(31947): at dalvik.system.NativeStart.main(Native Method)
答案 0 :(得分：2)
尝试：
//Adapter to control cell items
listViewAdapter = new ListView_Adapter(getActivity().getBaseContext());