在具有片段的活动中为列表视图设置适配器时的Nullpointerexception

时间：2014-04-24 15:32:08

标签： android listview android-fragments adapter

我正在开发一个非常小的音频播放器，但有时我会收到一些让我疯狂的错误。现在，当我为ListView（外部存储上的音频文件列表）设置适配器时，我面临NullPointerException。 我只使用一个带有操作栏和两个标签的活动（两个可以用swype切换的片段）。我还没有为播放器实现控制，我还在开发列表。

MainActivity.java 

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Locale;

import android.app.ActionBar.Tab;
import android.app.ActionBar;
import android.database.Cursor;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.net.Uri;
import android.os.Bundle;
import android.provider.MediaStore.Audio;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ListView;

public class MainActivity extends FragmentActivity implements ActionBar.TabListener {

    private MyAdapter mAdapter;
    private ViewPager mPager;

    private ArrayList<Song> songsList;
    private ListView songsView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        tabs();
    }

    private void tabs() {
        final ActionBar actionBar = getActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        mAdapter = new MyAdapter(getSupportFragmentManager());

        mPager = (ViewPager) findViewById(R.id.pager);
        mPager.setAdapter(mAdapter);

        mPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
                    @Override
                    public void onPageSelected(int position) {
                        actionBar.setSelectedNavigationItem(position);
                    }
                });

        for (int i = 0; i < mAdapter.getCount(); i++) {
            actionBar.addTab(actionBar.newTab()
                    .setText(mAdapter.getPageTitle(i))
                    .setTabListener(this));
        }
    }

    public class MyAdapter extends FragmentPagerAdapter {
        public MyAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public int getCount() {
            return 2;
        }

        @Override
        public Fragment getItem(int position) {
            switch (position) {
            case 0:
                return FilesFragment.newInstance(0);
            case 1:
                return ControlsFragment.newInstance(1);

            default:
                return null;
            }
        }

        @Override
        public CharSequence getPageTitle(int position) {
            Locale l = Locale.getDefault();
            switch (position) {
            case 0:
                return getString(R.string.title_section1).toUpperCase(l);
            case 1:
                return getString(R.string.title_section2).toUpperCase(l);
            }
            return null;
        }
    }

    @Override
    protected void onStart(){
        retrieveAudioFiles();

        songsOrder();

        inflateWithSongs();
    }

    public void retrieveAudioFiles(){
        songsView = (ListView)findViewById(R.id.list);
        songsList = new ArrayList<Song>();

        Uri sd = Audio.Media.EXTERNAL_CONTENT_URI;
        String[] cols = {Audio.Media.TITLE,Audio.Media.ARTIST,Audio.Media.ALBUM};
        String where = Audio.Media.IS_MUSIC;
        Cursor audioCursor = getContentResolver().query(sd,cols,where,null,null);

        while (audioCursor.moveToNext()){
            int posColTitle = audioCursor.getColumnIndex(Audio.Media.TITLE);
            int posColArtist = audioCursor.getColumnIndex(Audio.Media.ARTIST);
            int posColAlbum = audioCursor.getColumnIndex(Audio.Media.ALBUM);

            String songTitle = audioCursor.getString(posColTitle);
            String songArtist = audioCursor.getString(posColArtist);
            String songAlbum = audioCursor.getString(posColAlbum);

            songsList.add(new Song(songTitle,songArtist,songAlbum));
            }
        audioCursor.close();
    }

    public void songsOrder(){
        Collections.sort(songsList, new Comparator<Song>(){
              public int compare(Song a, Song b){
                return a.title.compareTo(b.title);
              }
            });
    }

    public void inflateWithSongs(){
        SongsAdapter songsAdt = new SongsAdapter(this, songsList);
        songsView = (ListView) findViewById(R.id.list);
        songsView.setAdapter(songsAdt); //ERROR HERE!!! SONGSVIEW IS NULL
    }

    public void songPicked(){
        //work in progress
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_files, container,
                    false);
            return rootView;
        }
    }

    @Override
    public void onTabSelected(Tab tab, android.app.FragmentTransaction ft) {
        // TODO Auto-generated method stub
        mPager.setCurrentItem(tab.getPosition());
    }

    @Override
    public void onTabUnselected(Tab tab, android.app.FragmentTransaction ft) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onTabReselected(Tab tab, android.app.FragmentTransaction ft) {
        // TODO Auto-generated method stub

    }
}

FilesFragment.java 

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class FilesFragment extends Fragment {

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

    public static FilesFragment newInstance(int index) {
        FilesFragment f = new FilesFragment();
        Bundle args = new Bundle();
        args.putInt("index", index);
        f.setArguments(args);
        return f;
    }

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

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_files, container, false);
        return view;
    }
}

ControlsFragment.java 

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class ControlsFragment extends Fragment {

    public static ControlsFragment newInstance(int index) {
        ControlsFragment f = new ControlsFragment();
        Bundle args = new Bundle();
        args.putInt("index", index);
        f.setArguments(args);
        return f;
    }

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

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

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_controls, container, false);
        return view;
    }
}

Song.java 

import android.graphics.Bitmap;
import android.net.Uri;

public class Song {

    public String title="";
    public String artist="";
    public String album="";
    private Uri path=null;
    private Bitmap cover=null;

    public Song(String t, String ar, String al){
        title=t;
        artist=ar;
        album=al;
    }

    public Song(String t, String ar, String al, Uri p){
        title=t;
        artist=ar;
        album=al;
        path=p;
    }

    public Song(String t, Uri p){
        title=t;
        path=p;
    }

    public Song(){

    }

}

SongsList.java 

import java.util.ArrayList;

public class SongsList extends ArrayList<Song> {

    public SongsList(){
        super();
    }

}

SongsAdapter.java 

import java.util.ArrayList;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.TextView;

public class SongsAdapter extends BaseAdapter {

    private ArrayList<Song> songs;
    private LayoutInflater songInf;

    public SongsAdapter(Context c, ArrayList<Song> theSongs){
        super();  
        songs=theSongs;
        songInf=LayoutInflater.from(c);
    }

    @Override
    public int getCount() {
        // TODO Auto-generated method stub
        return songs.size();
    }

    @Override
    public Object getItem(int position) {
        // TODO Auto-generated method stub
        return songs.get(position);
    }

    @Override
    public long getItemId(int position) {
        // TODO Auto-generated method stub
        return position;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        // TODO Auto-generated method stub
        RowWrapper wrapper;
        if (convertView == null)
        {
            convertView = songInf.inflate(
                R.layout.song_row, null);
            wrapper = new RowWrapper(convertView);
            convertView.setTag(wrapper);
        }
        else
        {
            wrapper = (RowWrapper) convertView.getTag();
        }
        Song song = (Song) getItem(position);
        wrapper.populate(song);

        return convertView;
    }

    private static class RowWrapper
    {
        private TextView titleTextView;
        private TextView artistTextView;
        private TextView albumTextView;

        public RowWrapper(View convertView)
        {
            titleTextView = (TextView) 
                convertView.findViewById(R.id.textTitle);
            artistTextView = (TextView) 
                convertView.findViewById(R.id.textArtist);
            albumTextView = (TextView) 
                convertView.findViewById(R.id.textAlbum);
        }

        public void populate(Song song)
        {
            titleTextView.setText(song.title);
            artistTextView.setText(song.artist);
            albumTextView.setText(song.album);
        }
    }

}

的AndroidManifest.xml 

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.audioplayer"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="14"
        android:targetSdkVersion="19" />
    <uses-permission android:name="android.permission.WAKE_LOCK"/>
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.audioplayer.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

activity_main.xml中 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <android.support.v4.view.ViewPager
        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
         />

</LinearLayout>

fragment_files.xml 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical">

    <ListView
        android:id="@+id/list"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:divider="#242424"
        android:dividerHeight="1dp" />

</LinearLayout>

song_row.xml 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/LinearLayout1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_weight="1"
    android:orientation="vertical"
    android:onClick="songPicked" >

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content" >

        <TextView
            android:id="@+id/labelTitle"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/labelTitle" />

        <TextView
            android:id="@+id/textTitle"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:paddingLeft="5dp"
            android:text="@string/textTitle" />
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content" >

        <TextView
            android:id="@+id/labelArtist"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="0dp"
            android:text="@string/labelArtist" />

        <TextView
            android:id="@+id/textArtist"
            android:layout_width="150dp"
            android:layout_height="wrap_content"
            android:paddingLeft="5dp"
            android:text="@string/textArtist" />

        <TextView
            android:id="@+id/labelAlbum"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="10dp"
            android:text="@string/labelAlbum" />

        <TextView
            android:id="@+id/textAlbum"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:paddingLeft="5dp"
            android:text="@string/textAlbum" />
    </LinearLayout>

</LinearLayout>

fragment_controls.xml ：无所谓

错误日志 

[2014-04-24 17:21:34 - AudioPlayer] Starting activity com.example.audioplayer.MainActivity on device c0808b11451fb7f
[2014-04-24 17:21:35 - AudioPlayer] ActivityManager: Starting: Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] cmp=com.example.audioplayer/.MainActivity }

04-24 17:27:03.647: D/ActivityThread(23489): handleBindApplication:com.example.audioplayer
04-24 17:27:03.647: D/ActivityThread(23489): setTargetHeapUtilization:0.75
04-24 17:27:03.647: D/ActivityThread(23489): setTargetHeapMinFree:524288
04-24 17:27:03.663: W/ActivityThread(23489): Application com.example.audioplayer is waiting for the debugger on port 8100...
04-24 17:27:03.671: I/System.out(23489): Sending WAIT chunk
04-24 17:27:03.983: I/dalvikvm(23489): Debugger is active
04-24 17:27:04.077: I/System.out(23489): Debugger has connected
04-24 17:27:04.077: I/System.out(23489): waiting for debugger to settle...
04-24 17:27:04.272: I/System.out(23489): waiting for debugger to settle...
04-24 17:27:04.475: I/System.out(23489): waiting for debugger to settle...
04-24 17:27:04.686: I/System.out(23489): waiting for debugger to settle...
04-24 17:27:04.889: I/System.out(23489): waiting for debugger to settle...
04-24 17:27:05.085: I/System.out(23489): waiting for debugger to settle...
04-24 17:27:05.280: I/System.out(23489): waiting for debugger to settle...
04-24 17:27:05.483: I/System.out(23489): debugger has settled (1485)
04-24 17:27:05.835: D/AndroidRuntime(23489): Shutting down VM
04-24 17:27:05.835: W/dalvikvm(23489): threadid=1: thread exiting with uncaught exception (group=0x418bbce0)
04-24 17:27:05.850: E/AndroidRuntime(23489): FATAL EXCEPTION: main
04-24 17:27:05.850: E/AndroidRuntime(23489): Process: com.example.audioplayer, PID: 23489
04-24 17:27:05.850: E/AndroidRuntime(23489): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.audioplayer/com.example.audioplayer.MainActivity}: java.lang.NullPointerException
04-24 17:27:05.850: E/AndroidRuntime(23489):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2215)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2265)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at android.app.ActivityThread.access$800(ActivityThread.java:145)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1206)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at android.os.Handler.dispatchMessage(Handler.java:102)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at android.os.Looper.loop(Looper.java:136)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at android.app.ActivityThread.main(ActivityThread.java:5149)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at java.lang.reflect.Method.invokeNative(Native Method)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at java.lang.reflect.Method.invoke(Method.java:515)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:605)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at dalvik.system.NativeStart.main(Native Method)
04-24 17:27:05.850: E/AndroidRuntime(23489): Caused by: java.lang.NullPointerException
04-24 17:27:05.850: E/AndroidRuntime(23489):    at com.example.audioplayer.MainActivity.inflateWithSongs(MainActivity.java:151)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at com.example.audioplayer.MainActivity.onStart(MainActivity.java:112)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1171)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at android.app.Activity.performStart(Activity.java:5241)
04-24 17:27:05.850: E/AndroidRuntime(23489):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2178)
04-24 17:27:05.850: E/AndroidRuntime(23489):    ... 11 more
04-24 17:27:09.147: I/Process(23489): Sending signal. PID: 23489 SIG: 9

1 个答案:

答案 0 :(得分：1)

您混合了初始化序列：

首先，创建并启动活动（onCreate - ＆gt; onStart），之后活动中的ViewPager将请求合适的片段（取决于{{的当前位置/页面） 1}}）通过您的ViewPager实施。

只有在FragmentPagerAdapter#getItem中实例化的相应片段才会被附加和膨胀（回调到getItem）。因此，到Fragment#onCreateView时，还没有包含您要查找的Activity#onStart的片段，因此listView会为您提供空值。

因此，我们的想法是在虚增之前不要访问findViewById。这意味着您应该将逻辑从listView移到Activity#inflateWithSongs：

FilesFragment#onCreateView

要从您的片段中访问// 1. get hold of `songList` : two options, see below // 2. create new SongsAdapter SongsAdapter songsAdt = new SongsAdapter(getActivity(), songsList); // 3. inflate root view , locate listView in it and set the adapter View view = inflater.inflate(R.layout.fragment_files, container, false); ListView songsView = (ListView) view.findViewById(R.id.list); songsView.setAdapter(songsAdt); // 4. return inflated root view return view; ，您可以：

  1. 将songList列表完全移动到您的片段以及从ArrayList<Song> songsList填充它的代码（顺便说一下，您应该避免在UI线程上从contentProvider加载，就像现在这样）。 / p>

  2. 将contentProvider保留在活动中，并在活动中添加额外的方法来访问它：


    3. songsList

    ，所以从Fragment你可以：


    public ArrayList<Song> getSongsLists() {
    return songsList;
}
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