Android：我的应用程序出现 Java.NullPointerException 错误

Question

我是新来的，但我每天都会在这个论坛上阅读问题。 我正在构建一个基于左侧侧抽屉的音乐播放器（使用库“简单侧抽屉”），当我触摸向上按钮（在操作栏上）时，左侧抽屉打开。 在左边的抽屉里有一个列表视图（简单），里面有你所有的歌曲。当我点击一个项目时，它应该开始播放......但是应用程序崩溃了。 这是错误

12-08 14:33:58.760: E/AndroidRuntime(12420): FATAL EXCEPTION: main
12-08 14:33:58.760: E/AndroidRuntime(12420): java.lang.NullPointerException
12-08 14:33:58.760: E/AndroidRuntime(12420):    at com.xand.holomusicplayer.Maincu$1.onItemClick(Maincu.java:58)
12-08 14:33:58.760: E/AndroidRuntime(12420):    at android.widget.AdapterView.performItemClick(AdapterView.java:297)
12-08 14:33:58.760: E/AndroidRuntime(12420):    at android.widget.AbsListView.performItemClick(AbsListView.java:1123)
12-08 14:33:58.760: E/AndroidRuntime(12420):    at android.widget.AbsListView$PerformClick.run(AbsListView.java:2931)
12-08 14:33:58.760: E/AndroidRuntime(12420):    at android.widget.AbsListView$2.run(AbsListView.java:3616)
12-08 14:33:58.760: E/AndroidRuntime(12420):    at android.os.Handler.handleCallback(Handler.java:730)
12-08 14:33:58.760: E/AndroidRuntime(12420):    at android.os.Handler.dispatchMessage(Handler.java:92)
12-08 14:33:58.760: E/AndroidRuntime(12420):    at android.os.Looper.loop(Looper.java:137)
12-08 14:33:58.760: E/AndroidRuntime(12420):    at android.app.ActivityThread.main(ActivityThread.java:5287)
12-08 14:33:58.760: E/AndroidRuntime(12420):    at java.lang.reflect.Method.invokeNative(Native Method)
12-08 14:33:58.760: E/AndroidRuntime(12420):    at java.lang.reflect.Method.invoke(Method.java:525)
12-08 14:33:58.760: E/AndroidRuntime(12420):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:738)
12-08 14:33:58.760: E/AndroidRuntime(12420):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:554)
12-08 14:33:58.760: E/AndroidRuntime(12420):    at dalvik.system.NativeStart.main(Native Method)

不知道为什么，代码如下：

private String[] mAudioPath;
 private MediaPlayer mMediaPlayer;
 private String[] mMusicList;
 private SimpleSideDrawer mSlidingMenu;
 ListView list;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main_activity);
    getActionBar().setDisplayHomeAsUpEnabled(true);
    mSlidingMenu = new SimpleSideDrawer(this);
    mSlidingMenu.setLeftBehindContentView(R.layout.playlist);
    list = (ListView) findViewById (R.id.listView1);
    mMediaPlayer = new MediaPlayer();
    mMusicList = getAudioList();
    ArrayAdapter<String> mAdapter = new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1, mMusicList);
    list.setAdapter(mAdapter);
    list.setOnItemClickListener(new OnItemClickListener() {

        @Override
        public void onItemClick(AdapterView<?> arg0, View arg1, int arg2,
        long arg3) {
        try {
            playSong(mAudioPath[arg2]);
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        } catch (IllegalStateException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        }
       });
    }
private String[] getAudioList() {
    final Cursor mCursor = getContentResolver().query(
            MediaStore.Audio.Media.EXTERNAL_CONTENT_URI,
            new String[] { MediaStore.Audio.Media.DISPLAY_NAME, MediaStore.Audio.Media.DATA }, null, null,
            "LOWER(" + MediaStore.Audio.Media.TITLE + ") ASC");

    int count = mCursor.getCount();

    String[] songs = new String[count];
    String[] mAudioPath = new String[count];
    int i = 0;
    if (mCursor.moveToFirst()) {
        do {
            songs[i] = mCursor.getString(mCursor.getColumnIndexOrThrow(MediaStore.Audio.Media.DISPLAY_NAME));
            mAudioPath[i] = mCursor.getString(mCursor.getColumnIndexOrThrow(MediaStore.Audio.Media.DATA));
            i++;
        } while (mCursor.moveToNext());
    }   

    mCursor.close();

    return songs;
}

private void playSong(String path) throws IllegalArgumentException,
IllegalStateException, IOException {

Log.d("ringtone", "playSong :: " + path);

mMediaPlayer.reset();
mMediaPlayer.setDataSource(path);       
mMediaPlayer.prepare();
mMediaPlayer.start();

}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    // Respond to the action bar's Up/Home button
    case android.R.id.home:
        mSlidingMenu.toggleLeftDrawer();
        return true;
    }
    return super.onOptionsItemSelected(item);
}
}

为什么当我尝试播放某些内容时应用程序崩溃？请帮助我:(:)

Answer 1

除此之外，onItemClick() 方法中的 NullPointerException 可能会在您这样做时发生：

playSong(mAudioPath[arg2]);

原因是mAudioPath 未初始化且为空。为什么？

您已将其声明为 Maincu 类中的一个字段：

private String[] mAudioPath;

你认为你是通过调用 getAudioList() 来初始化它，但在这个方法中，你是在声明：

String[] mAudioPath = new String[count];

哪个 SHADOWS 您的全局变量 mAudioPath，因此您的本地 mAudioPath 正在被初始化，而不是全局的，然后用于播放歌曲。要修复，请在您的 getAudioList() 中更改：

String[] mAudioPath = new String[count];

到：

mAudioPath = new String[count];

Answer 2

这是因为您的mAudioPath 成员为空。请务必在调用onItemClick()之前进行设置。