我正在尝试在我的 fragment 中设置滑动刷新，该 fragment 从解析中获取对象。但每次，我似乎都会在 setadapter 和 ListView 上遇到空指针异常。尽管它们都已正确初始化，但它们都不为空。
我最近的适配器:
package com.astuetz.viewpager.extensions.sample;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.TextView;
import java.util.ArrayList;
import java.util.HashMap;
public class RecentsAdapter extends BaseAdapter
{
LayoutInflater inflater;
Context context;
ArrayList<HashMap<String,String>> items = null;
public RecentsAdapter(Context context,ArrayList<HashMap<String,String>> items)
{
super();
this.context = context;
this.items = items;
inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}
@Override
public int getCount()
{
return items.size();
}
@Override
public Object getItem(int position)
{
return items.get(position);
}
@Override
public long getItemId(int position)
{
return position;
}
@Override
public View getView(int position, View convertView, ViewGroup parent)
{
View v = convertView;
MyHolder holder = null;
if(v == null)
{
v = inflater.inflate(R.layout.recents_list_item,parent,false);
holder = new MyHolder(v);
v.setTag(holder);
}
else
{
holder = (MyHolder) v.getTag();
}
holder.title.setText(items.get(position).get("title"));
holder.author.setText(items.get(position).get("author"));
holder.dept.setText(items.get(position).get("dept"));
holder.price.setText(items.get(position).get("price"));
// holder.place.setText(items.get(position).get("place"));
return v;
}
class MyHolder
{
TextView dept,title,author,price,place;
public MyHolder(View view)
{
title = (TextView) view.findViewById(R.id.book_title);
dept = (TextView)view.findViewById(R.id.dept_name);
author = (TextView) view.findViewById(R.id.author_name);
price = (TextView)view.findViewById(R.id.price_list);
// place = (TextView)view.findViewById(R.id.place_list);
}
}
}
我的recent_card.xml定义了我的 ListView 和滑动刷新:
<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:card_view="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">
<android.support.v4.widget.SwipeRefreshLayout
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent">
<android.support.v7.widget.CardView
android:id="@+id/recents_card"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_margin="8dp"
card_view:cardBackgroundColor="@android:color/white"
card_view:cardCornerRadius="8dp"
android:padding="8dp">
<ListView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/recents_list"
android:divider="@color/dividerColor"
android:dividerHeight="1dp"/>
</android.support.v7.widget.CardView>
</android.support.v4.widget.SwipeRefreshLayout>
</FrameLayout>
和我的RecentsCardFragment.java实现它们:
/*
* Copyright (C) 2013 Andreas Stuetz <<a href="https://stackoverflow.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="02636c66706763712c71767767767842656f636b6e2c616d6f" rel="noreferrer noopener nofollow">[email protected]</a>>
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.astuetz.viewpager.extensions.sample;
import android.app.AlertDialog;
import android.app.ProgressDialog;
import android.content.Context;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.view.ViewCompat;
import android.support.v4.widget.SwipeRefreshLayout;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Adapter;
import android.widget.ListView;
import android.widget.Toast;
import com.parse.FindCallback;
import com.parse.GetCallback;
import com.parse.Parse;
import com.parse.ParseException;
import com.parse.ParseObject;
import com.parse.ParseQuery;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
public class RecentsCardFragment extends Fragment implements SwipeRefreshLayout.OnRefreshListener
{
ListView recentsList = (ListView)getActivity().findViewById(R.id.recents_list);
private SwipeRefreshLayout refreshBooks;
ArrayList<HashMap<String,String>> items = new ArrayList<>();
//private ProgressDialog progress;
public static RecentsCardFragment newInstance() {
RecentsCardFragment f = new RecentsCardFragment();
return f;
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.recents_card,container,false);
ViewCompat.setElevation(rootView,50);
refreshBooks = (SwipeRefreshLayout) rootView.findViewById(R.id.container);
refreshBooks.setOnRefreshListener(this);
getBooks();
RecentsAdapter adapter = new RecentsAdapter(getActivity().getApplicationContext(), items);
recentsList.setAdapter(adapter);
return rootView;
}
private ArrayList<HashMap<String,String>> getBooks(){
final ParseQuery<ParseObject> query = ParseQuery.getQuery("Posted");
query.orderByDescending("createdAt");
query.findInBackground(new FindCallback<ParseObject>() {
@Override
public void done(List<ParseObject> parseObjects, ParseException e) {
if (e == null){
Log.w("Parse","Inside getbooks()");
for (ParseObject book : parseObjects) {
HashMap<String, String> test = new HashMap<>();
String dept = book.getString("Department");
String title = book.getString("Title");
String author = book.getString("Author");
Number price_num = book.getNumber("Price");
String price = String.valueOf(price_num);
String place = book.getString("Place");
String desp = book.getString("Description");
test.put("dept", dept);
test.put("title", title);
test.put("author", author);
test.put("price", price);
test.put("place", place);
test.put("description", desp);
items.add(test);
}
} else {
Log.d("Books", "Error: " + e.getMessage());
}
}
});
return items;
}
@Override
public void onRefresh(){
refreshBooks.setRefreshing(true);
updateBooks();
}
private void updateBooks(){
Toast.makeText(getActivity(),"Refresh", Toast.LENGTH_SHORT).show();
items.addAll(getBooks());
refreshBooks.setRefreshing(false);
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
}
}
这是我的日志:
06-14 02:44:52.485 2373-2373/com.astuetz.viewpager.extensions.sample I/Process﹕ Sending signal. PID: 2373 SIG: 9
06-14 02:58:26.624 2502-2524/com.astuetz.viewpager.extensions.sample D/OpenGLRenderer﹕ Render dirty regions requested: true
06-14 02:58:26.644 2502-2502/com.astuetz.viewpager.extensions.sample D/﹕ HostConnection::get() New Host Connection established 0xabb86710, tid 2502
06-14 02:58:27.720 2502-2502/com.astuetz.viewpager.extensions.sample D/Atlas﹕ Validating map...
06-14 02:58:28.692 2502-2502/com.astuetz.viewpager.extensions.sample I/Choreographer﹕ Skipped 57 frames! The application may be doing too much work on its main thread.
06-14 02:58:28.716 2502-2502/com.astuetz.viewpager.extensions.sample D/AndroidRuntime﹕ Shutting down VM
06-14 02:58:28.720 2502-2502/com.astuetz.viewpager.extensions.sample E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.astuetz.viewpager.extensions.sample, PID: 2502
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.support.v4.app.FragmentActivity.findViewById(int)' on a null object reference
at com.astuetz.viewpager.extensions.sample.RecentsCardFragment.<init>(RecentsCardFragment.java:54)
at com.astuetz.viewpager.extensions.sample.RecentsCardFragment.newInstance(RecentsCardFragment.java:68)
at com.astuetz.viewpager.extensions.sample.MainActivity$MyPagerAdapter.getItem(MainActivity.java:133)
at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:97)
at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:837)
at android.support.v4.view.ViewPager.populate(ViewPager.java:987)
at android.support.v4.view.ViewPager.populate(ViewPager.java:919)
at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1441)
at android.view.View.measure(View.java:17430)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:875)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2560)
at android.view.View.measure(View.java:17430)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2001)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1166)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1372)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1054)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5779)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
at android.view.Choreographer.doCallbacks(Choreographer.java:580)
at android.view.Choreographer.doFrame(Choreographer.java:550)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
06-14 02:58:32.992 2502-2502/com.astuetz.viewpager.extensions.sample I/Process﹕ Sending signal. PID: 2502 SIG: 9
我的 ListView 最近列表或最近列表.ssetadapter(adapter) 行中不断出现空指针异常。
尽管有几个关于同一主题的已回答问题，但无法使用它们解决我的问题。
请帮帮我! 提前致谢!
最佳答案
您的recent_list为 null 并且您在 onCreateView 之外引用它在你的RecentsCardFragment里面，这就是为什么NullPointerException正在发生。移动您的recent_list声明到您的onCreateView方法，像这样:
public class RecentsCardFragment extends Fragment implements SwipeRefreshLayout.OnRefreshListener
{
ListView recentsList;
private SwipeRefreshLayout refreshBooks;
ArrayList<HashMap<String,String>> items = new ArrayList<>();
//private ProgressDialog progress;
public static RecentsCardFragment newInstance() {
RecentsCardFragment f = new RecentsCardFragment();
return f;
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.recents_card,container,false);
recentsList = (ListView)rootView.findViewById(R.id.recents_list);
ViewCompat.setElevation(rootView,50);
refreshBooks = (SwipeRefreshLayout) rootView.findViewById(R.id.container);
refreshBooks.setOnRefreshListener(this);
getBooks();
RecentsAdapter adapter = new RecentsAdapter(getActivity().getApplicationContext(), items);
recentsList.setAdapter(adapter);
return rootView;
}
关于java - fragment 中的 ListView 空指针异常，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/30826920/