在Android的ListViews中显示MP3文件

Question

我是Android编程的初学者。我想制作一个简单的listView，列出了从SD卡中列出所有MP3文件。我尝试了很多示例代码，但并非所有这些代码都完全工作。

的帮助将被应用。

public class PlayListActivity extends Activity {
private ListView lv;
//Songs List
public ArrayList<HashMap<String, String>> songsList = new ArrayList<HashMap<String, String>>();
SimpleAdapter adapter;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.playlist);
    lv = (ListView) findViewById(android.R.id.list);
    ArrayList<HashMap<String, String>> songsListData = new ArrayList<HashMap<String, String>>();
    //ArrayList<String> songsListData = new ArrayList<>();
    SongsManager plm = new SongsManager();
    this.songsList = plm.getPlayList();
    for(int i=0; i < songsList.size();i++) {
        HashMap<String, String> song = songsList.get(i);
        songsListData.add(song);
    }
    ListAdapter adapter = new SimpleAdapter(this, songsListData,
            R.layout.playlist_item, new String [] {"songTitle"},new int[] {
            R.id.songTitle});
    //setListAdapter(adapter);
    lv.setAdapter(adapter);
    //ListView lv = getListView();
}
}

这是Songnmanager类

public class SongsManager {
//SDCARD
final String MEDIA_PATH = Environment.getExternalStorageDirectory().getPath() + "/";
private ArrayList<HashMap<String, String>> songsList = new ArrayList<HashMap<String, String>>();
private String mp3Pattern=".mp3";
//Constructor
public SongsManager(){
}
public ArrayList<HashMap<String, String >> getPlayList(){
    System.out.println(MEDIA_PATH);
    if (MEDIA_PATH !=null){
        File home = new File(MEDIA_PATH);
        File[] listFiles = home.listFiles();
        if(listFiles != null && listFiles.length>0){
            for(File file:listFiles){
                System.out.println(file.getAbsolutePath());
                if (file.isDirectory()){
                    scanDirectory(file);
                }else{
                    addSongToList(file);
                }
            }
        }
    }
    return songsList;
}
private void scanDirectory(File directory) {
    if(directory != null){
        File[]listFiles = directory.listFiles();
        if(listFiles != null && listFiles.length>0){
            for (File file : listFiles){
                if(file.isDirectory()){
                    scanDirectory(file);
                }else{
                    addSongToList(file);
                }
            }
        }
    }
}
private void addSongToList(File song) {
    if(song.getName().endsWith(mp3Pattern)){
        HashMap<String, String> songMap = new HashMap<String ,String>();
        songMap.put("songTitle",song.getName().substring(0,(song.getName().length()-4)));
        songMap.put("songPath",song.getPath());
        songsList.add(songMap);
    }
}
class FileExtensionFilter implements FilenameFilter {
    public boolean accept(File dir, String name) {
        return (name.endsWith(".mp3") || name.endsWith(".MP3"));
    }
}
}

它给出错误消息：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.a45v.praransemen/com.example.a45v.praransemen.PlayListActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
                                                                              at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2434)
                                                                              at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2494)
                                                                              at android.app.ActivityThread.access$900(ActivityThread.java:153)
                                                                              at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1347)
                                                                              at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                              at android.os.Looper.loop(Looper.java:148)
                                                                              at android.app.ActivityThread.main(ActivityThread.java:5451)
                                                                              at java.lang.reflect.Method.invoke(Native Method)
                                                                              at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                                                                           Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
                                                                              at com.example.a45v.praransemen.PlayListActivity.onCreate(PlayListActivity.java:41)
                                                                              at android.app.Activity.performCreate(Activity.java:6323)
                                                                              at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
                                                                              at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2387)
                                                                              at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2494) 
                                                                              at android.app.ActivityThread.access$900(ActivityThread.java:153) 
                                                                              at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1347) 
                                                                              at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                              at android.os.Looper.loop(Looper.java:148) 
                                                                              at android.app.ActivityThread.main(ActivityThread.java:5451) 
                                                                              at java.lang.reflect.Method.invoke(Native Method) 
                                                                              at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                                                                              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

这是playlist.xml

<ListView
    android:id="@+id/list"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    tools:layout_editor_absoluteX="8dp"
    tools:layout_editor_absoluteY="8dp" />

这是playlist_item.xml

<TextView
    android:id="@+id/songTitle"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_weight="1"
    android:text="TextView" />

Answer 1

显然， res/layout/playlist.xml不包含 android:id值@android:id/list的ListView。