我有一个ArrayAdapter，它通过从API获取数据来更新ListView。 API工作正常，我使用AsyncTask获取数据。
OnPostExecute方法应清除ArrayAdapter并向其添加新数据。
但我面临一些错误java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ArrayAdapter.clear()' on a null object reference
package com.example.sunshine;
import android.content.Context;
import android.net.Uri;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.text.format.Time;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.ProtocolException;
import java.net.URL;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
/**
* A simple {@link Fragment} subclass.
* Activities that contain this fragment must implement the
* {@link WeatherFragment.OnFragmentInteractionListener} interface
* to handle interaction events.
* Use the {@link WeatherFragment#newInstance} factory method to
* create an instance of this fragment.
*/
public class WeatherFragment extends Fragment {
// 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";
private ArrayAdapter<String> mAdapter;
// TODO: Rename and change types of parameters
private String mParam1;
private String mParam2;
public EditText locationCity;
private OnFragmentInteractionListener mListener;
public WeatherFragment() {
// Required empty public constructor
}
/**
* 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 WeatherFragment.
*/
// TODO: Rename and change types and number of parameters
public static WeatherFragment newInstance(String param1, String param2) {
WeatherFragment fragment = new WeatherFragment();
Bundle args = new Bundle();
args.putString(ARG_PARAM1, param1);
args.putString(ARG_PARAM2, param2);
fragment.setArguments(args);
return fragment;
}
@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 view = inflater.inflate(R.layout.fragment_weather, container, false);
String[] forecastArray = {"Mon 6/23 - Sunny - 31/17",
"Tue 6/24 - Foggy - 21/8",
"Wed 6/25 - Cloudy - 22/17",
"Thurs 6/26 - Rainy - 18/11",
"Fri 6/27 - Foggy - 21/10",
"Sat 6/28 - TRAPPED IN WEATHERSTATION - 23/18",
"Sun 6/29 - Sunny - 20/7"};
List<String> weekForecast = new ArrayList<>(Arrays.asList(forecastArray));
mAdapter = new ArrayAdapter<>(getActivity(), android.R.layout.simple_list_item_1, weekForecast );
mAdapter.notifyDataSetChanged();
ListView listView = (ListView) view.findViewById(R.id.listView1);
Log.d("Log", "If listview is null "+listView);
listView.setAdapter(mAdapter);
return view;
}
@Override
public void onAttach(Context context) {
super.onAttach(context);
if (context instanceof OnFragmentInteractionListener) {
mListener = (OnFragmentInteractionListener) context;
} else {
throw new RuntimeException(context.toString()
+ " must implement OnFragmentInteractionListener");
}
}
@Override
public void onDetach() {
super.onDetach();
mListener = null;
}
/**
* This interface must be implemented by activities that contain this
* fragment to allow an interaction in this fragment to be communicated
* to the activity and potentially other fragments contained in that
* activity.
* <p/>
* See the Android Training lesson <a href=
* "http://developer.android.com/training/basics/fragments/communicating.html"
* >Communicating with Other Fragments</a> for more information.
*/
public interface OnFragmentInteractionListener {
// TODO: Update argument type and name
void onFragmentInteraction(Uri uri);
}
public class FetchWeatherTask extends AsyncTask<String, Void, String[]> {
private final String LOG_TAG = FetchWeatherTask.class.getSimpleName();
/* The date/time conversion code is going to be moved outside the asynctask later,
* so for convenience we're breaking it out into its own method now.
*/
private String getReadableDateString(long time){
// Because the API returns a unix timestamp (measured in seconds),
// it must be converted to milliseconds in order to be converted to valid date.
SimpleDateFormat shortenedDateFormat = new SimpleDateFormat("EEE MMM dd");
return shortenedDateFormat.format(time);
}
/**
* Prepare the weather high/lows for presentation.
*/
private String formatHighLows(double high, double low) {
// For presentation, assume the user doesn't care about tenths of a degree.
long roundedHigh = Math.round(high);
long roundedLow = Math.round(low);
return roundedHigh + "/" + roundedLow;
}
/**
* Take the String representing the complete forecast in JSON Format and
* pull out the data we need to construct the Strings needed for the wireframes.
*
* Fortunately parsing is easy: constructor takes the JSON string and converts it
* into an Object hierarchy for us.
*/
private String[] getWeatherDataFromJson(String forecastJsonStr, int numDays)
throws JSONException {
// These are the names of the JSON objects that need to be extracted.
final String OWM_LIST = "list";
final String OWM_WEATHER = "weather";
final String OWM_TEMPERATURE = "temp";
final String OWM_MAX = "max";
final String OWM_MIN = "min";
final String OWM_DESCRIPTION = "main";
JSONObject forecastJson = new JSONObject(forecastJsonStr);
JSONArray weatherArray = forecastJson.getJSONArray(OWM_LIST);
// OWM returns daily forecasts based upon the local time of the city that is being
// asked for, which means that we need to know the GMT offset to translate this data
// properly.
// Since this data is also sent in-order and the first day is always the
// current day, we're going to take advantage of that to get a nice
// normalized UTC date for all of our weather.
Time dayTime = new Time();
dayTime.setToNow();
// we start at the day returned by local time. Otherwise this is a mess.
int julianStartDay = Time.getJulianDay(System.currentTimeMillis(), dayTime.gmtoff);
// now we work exclusively in UTC
dayTime = new Time();
String[] resultStrs = new String[numDays];
for(int i = 0; i < weatherArray.length(); i++) {
// For now, using the format "Day, description, hi/low"
String day;
String description;
String highAndLow;
// Get the JSON object representing the day
JSONObject dayForecast = weatherArray.getJSONObject(i);
// The date/time is returned as a long. We need to convert that
// into something human-readable, since most people won't read "1400356800" as
// "this saturday".
long dateTime;
// Cheating to convert this to UTC time, which is what we want anyhow
dateTime = dayTime.setJulianDay(julianStartDay+i);
day = getReadableDateString(dateTime);
// description is in a child array called "weather", which is 1 element long.
JSONObject weatherObject = dayForecast.getJSONArray(OWM_WEATHER).getJSONObject(0);
description = weatherObject.getString(OWM_DESCRIPTION);
// Temperatures are in a child object called "temp". Try not to name variables
// "temp" when working with temperature. It confuses everybody.
JSONObject temperatureObject = dayForecast.getJSONObject(OWM_TEMPERATURE);
double high = temperatureObject.getDouble(OWM_MAX);
double low = temperatureObject.getDouble(OWM_MIN);
highAndLow = formatHighLows(high, low);
resultStrs[i] = day + " - " + description + " - " + highAndLow;
}
for (String s : resultStrs) {
Log.v(LOG_TAG, "Forecast entry: " + s);
}
return resultStrs;
}
@Override
protected String[] doInBackground(String... params) {
if(params.length == 0){
return null;
}
HttpURLConnection urlConnection = null;
BufferedReader reader = null;
String format = "json";
String units = "metric";
int numDays = 7;
String forecastJSONStr = null;
try {
final String BASE_URL = "http://api.openweathermap.org/data/2.5/forecast/daily?";
final String QUERY_PARAM = "q";
final String FORMAT_PARAM = "mode";
final String UNITS_PARAM = "units";
final String COUNT_PARAM = "cnt";
final String APPID_PARAM = "APPID";
Uri builtUri = Uri.parse(BASE_URL).buildUpon()
.appendQueryParameter(QUERY_PARAM, params[0])
.appendQueryParameter(FORMAT_PARAM, format)
.appendQueryParameter(UNITS_PARAM, units)
.appendQueryParameter(COUNT_PARAM, Integer.toString(numDays))
.appendQueryParameter(APPID_PARAM, BuildConfig.OWMAPIKey)
.build();
URL url = new URL(builtUri.toString());
Log.v(LOG_TAG, "Built Uri"+ builtUri.toString());
urlConnection = (HttpURLConnection) url.openConnection();
urlConnection.setRequestMethod("GET");
urlConnection.connect();
InputStream inputStream = urlConnection.getInputStream();
StringBuilder buffer = new StringBuilder();
if(inputStream == null){
forecastJSONStr = null;
}
if(inputStream != null){
reader = new BufferedReader(new InputStreamReader(inputStream));
}
String line;
if(reader != null){
while((line = reader.readLine()) != null){
buffer.append(line).append('\n');
}
}
if(buffer.length() == 0){
forecastJSONStr = null;
}
forecastJSONStr = buffer.toString();
Log.e(LOG_TAG, "Forecast String " + forecastJSONStr);
} catch (ProtocolException | MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
Log.e(LOG_TAG, "onCreateView: Error", e);
forecastJSONStr = null;
}finally {
if(urlConnection != null){
urlConnection.disconnect();
}
if(reader != null){
try {
reader.close();
} catch (final IOException e) {
Log.e(LOG_TAG, "Error closing stream ", e);
}
}
}
try {
if(forecastJSONStr != null){
Log.v(LOG_TAG, "doInBackground: "+forecastJSONStr);
String[] results= getWeatherDataFromJson(forecastJSONStr, numDays);
Log.v(LOG_TAG, "doInBackground: Check result" +Arrays.toString(results));
return results;
// return getWeatherDataFromJson(forecastJSONStr, numDays);
}
else{
Log.e(LOG_TAG, "Check your internet!" );
}
}catch (JSONException e){
Log.e(LOG_TAG, e.getMessage(), e);
e.printStackTrace();
}
return null;
}
@Override
protected void onPostExecute(String[] result) {
Log.v(LOG_TAG, "onPostExecute: Check result object" + Arrays.toString(result));
Log.i(LOG_TAG, "onPostExecute: "+mAdapter);
mAdapter.clear();
mAdapter.addAll(result);
// if(result != null){
//// for(String dayForecastStr : result) {
//// mAdapter.add(dayForecastStr);
//// }
//
// }
// super.onPostExecute(result);
}
}
}
检查Line 351以查找导致问题的确切行。
道歉，如果它是一个菜鸟问题，但我是Android开发新手。
答案 0 :(得分：3)
这是一个非常普遍的问题。考虑这个问题的一种方法可能是在AsyncTask开始之后，你真的无法控制它何时结束，因为这取决于许多因素，任务规模等。所以，调用AsyncTask本身的视图可能无法使用。
因为在您的情况下，mAdapter设置为null因此导致NullPointerException，我建议您执行以下操作之一。
在onPostExecute（..）中，在UI线程上启动一个活动，首先重新初始化mAdapter，如果它是null（如果view本身有变得不活跃，这不会工作）
将回调处理函数传递给AsyncTask;该函数必须在WeatherFragment中，以确保在AsyncTask返回时初始化并运行该片段;如果它为null，则再次重新初始化mAdapter，并根据从AsyncTask收到的数据相应地更新UI。
不幸的是，鉴于您没有发布更具体的问题，这些只是我想到的两条建议。您将能够在谷歌上找到这些设计选择的众多资源。如果有任何一种方法，请告诉我。
答案 1 :(得分：1)
在第25行的WeatherFragment.java
mAdapter.clear（）;
首先尝试使用此代码检查isEmpty
if(mAdapter.getCount()!=0 && mAdapter!=null){
mAdapter.clear();
}
否则您可以指定新的
mAdapter = new ArrayAdapter<String>(result);
这个并删除mAdapter.clear（）;