fragment 的 onCreate() 方法中的 Android 应用程序运行时错误

Question

我正在为 Android 开发音乐播放器应用。我只使用 ViewPager 创建了一个包含 4 个 fragment 的 Activity 和空布局。到目前为止效果很好。

fragment 是艺术家、专辑、所有歌曲和播放列表。

现在，我试图用设备上所有歌曲的列表填充所有歌曲 fragment 。我为此添加了必要的代码，应用程序编译没有任何错误。但是，当我打开该应用程序时，我收到一条强制关闭消息“不幸的是，播放器已停止”。所以这是 logcat。

E/AndroidRuntime( 2539): FATAL EXCEPTION: main
E/AndroidRuntime( 2539): java.lang.NullPointerException
E/AndroidRuntime( 2539):     at com.example.player.AllSongsFragment.onCreate(AllSongsFragment.java:26)
E/AndroidRuntime( 2539):     at android.support.v4.app.Fragment.performCreate(Fragment.java:1477)
E/AndroidRuntime( 2539):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:904)
E/AndroidRuntime( 2539):     at android.support.v4.app.FragmentManagerImpl.performPendingDeferredStart(FragmentManager.java:834)
E/AndroidRuntime( 2539):     at android.support.v4.app.Fragment.setUserVisibleHint(Fragment.java:841)
E/AndroidRuntime( 2539):     at android.support.v4.app.FragmentPagerAdapter.setPrimaryItem(FragmentPagerAdapter.java:130)
E/AndroidRuntime( 2539):     at android.support.v4.view.ViewPager.populate(ViewPager.java:1066)
E/AndroidRuntime( 2539):     at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
E/AndroidRuntime( 2539):     at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
E/AndroidRuntime( 2539):     at android.view.View.measure(View.java:15775)
E/AndroidRuntime( 2539):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4942)
E/AndroidRuntime( 2539):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
E/AndroidRuntime( 2539):     at android.view.View.measure(View.java:15775)
E/AndroidRuntime( 2539):     at android.widget.LinearLayout.measureVertical(LinearLayout.java:850)
E/AndroidRuntime( 2539):     at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
E/AndroidRuntime( 2539):     at android.view.View.measure(View.java:15775)
E/AndroidRuntime( 2539):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4942)

所以，问题出在 AllSongsFragment 类的 onCreate 方法中的 NullPointerException

这里是AllSongsFragment.java的内容

package com.example.player;

import com.example.player.R;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import android.net.Uri;
import android.content.ContentResolver;
import android.database.Cursor;
import android.widget.ListView;
import com.example.player.adapter.AllSongsAdapter;
import com.example.player.Song;

public class AllSongsFragment extends Fragment {
    private ArrayList<Song> songArray;
    private ListView songListView;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        songListView = (ListView)getView().findViewById(R.id.all_songs_list);
        songArray = new ArrayList<Song>();
        getSongList();
        // Sort the songs
        Collections.sort(songArray, new Comparator<Song>() {
            public int compare(Song a, Song b) {
                return a.getTitle().compareTo(b.getTitle());
            }
        });
        AllSongsAdapter songAdt = new AllSongsAdapter(getActivity(), songArray);
        songListView.setAdapter(songAdt);
    } // onCreate
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.all_songs_fragment_layout, container, false);
        return rootView;
    } //onCreateView()

    public void getSongList() {
    //retrieve song info
         ContentResolver musicResolver = getActivity().getContentResolver();
        Uri musicUri = android.provider.MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
        Cursor musicCursor = musicResolver.query(musicUri, null, null, null, null);
        // Iterate over the List
        if(musicCursor!=null && musicCursor.moveToFirst()) {
        //get columns
        int titleColumn = musicCursor.getColumnIndex(android.provider.MediaStore.Audio.Media.TITLE);
        int idColumn = musicCursor.getColumnIndex(android.provider.MediaStore.Audio.Media._ID);
        int artistColumn = musicCursor.getColumnIndex(android.provider.MediaStore.Audio.Media.ARTIST);
        //add songs to list
        do {
        long thisId = musicCursor.getLong(idColumn);
        String thisTitle = musicCursor.getString(titleColumn);
        String thisArtist = musicCursor.getString(artistColumn);
        songArray.add(new Song(thisId, thisTitle, thisArtist));
        }  while (musicCursor.moveToNext());
        } // if
    } //getSongList()
} //AllSongsFragment

我在 Song.java 中有一个 Song 类来表示每个歌曲对象。

package com.flairsome.player;

public class Song {
    private long id;
    private String title;
    private String artist;
    //Constructor
    public Song(long songID, String songTitle, String songArtist) {
        id=songID;
        title=songTitle;
        artist=songArtist;
    }
    // get methods
    public long getID() {
        return id;
    }
    public String getTitle() {
         return title;
    }
    public String getArtist() {
        return artist;
    }
} //.Song

AllSongsAdapter.java 是

package com.example.player.adapter;

import android.widget.BaseAdapter;
import java.util.ArrayList;
import android.content.Context;
import android.view.LayoutInflater;
import android.widget.LinearLayout;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import com.example.player.Song;
import com.example.player.R;

public class AllSongsAdapter extends  BaseAdapter {
    private ArrayList<Song> songs;
    private LayoutInflater songInf;
    // constructor
    public AllSongsAdapter(Context c, ArrayList<Song> theSongs){
        songs = theSongs;
        songInf = LayoutInflater.from(c);
    }
    @Override
    public int getCount() {
        return songs.size();
    }
    @Override
    public Object getItem(int arg0) {
    // TODO Auto-generated method stub
        return null;
    }
    @Override
    public long getItemId(int arg0) {
        // TODO Auto-generated method stub
        return 0;
    }
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        //map to song layout
        LinearLayout songLay = (LinearLayout)songInf.inflate(R.layout.song_item_layout, parent, false);
        //get title and artist views
        TextView songView = (TextView)songLay.findViewById(R.id.song_title);
        TextView artistView = (TextView)songLay.findViewById(R.id.song_artist);
        //get song using position
        Song currSong = songs.get(position);
        //get title and artist strings
        songView.setText(currSong.getTitle());
        artistView.setText(currSong.getArtist());
        //set position as tag
        songLay.setTag(position);
        return songLay;
    } //getView()
}

虽然我已经发布了大量代码，但从 logcat 来看，问题似乎出在 AllSongsFragment 类的 onCreate() 方法中。

我做错了什么吗？

如果您需要更多信息或我需要改进问题，请发表评论。谢谢。

Answer 1

移动这个

 songListView = (ListView)getView().findViewById(R.id.all_songs_list);

到onActivityCreated

getView() 在您的案例中返回 null。 songListView 为 null，当您将适配器设置为 listview 结果时 NUllPointerException

您也可以将下面的内容移至 onActivityCreated

  songArray = new ArrayList<Song>();
  getSongList();
    // Sort the songs
  Collections.sort(songArray, new Comparator<Song>() {
        public int compare(Song a, Song b) {
            return a.getTitle().compareTo(b.getTitle());
        }
  });
  AllSongsAdapter songAdt = new AllSongsAdapter(getActivity(), songArray);
  songListView.setAdapter(songAdt);