Android : Java.NullPointerException error on my app

Question

i'm new here but I read every day questions on this forum. I'm building a music player based on a side drawer on the left (using the library "simple side drawer), when I touch the up button (on the action bar) then the left drawer open. On the left drawer there is a listview (simple) with all your's songs. When I tap on a item then that should start playing...but the app crash. This is the error

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)

I don't know why, this is the code:

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);
}
}

why the app crash when I try to play something ? Please Help me :( :)

Answer 1

This is because your mAudioPath member is null. Be sure to set it before calling onItemClick() .

Answer 2

All other things aside, your NullPointerException from onItemClick() method happens probably when you do:

playSong(mAudioPath[arg2]);

The reason for this is that mAudioPath is not initialised and null. Why?

You have declared it as a field in your class Maincu as:

private String[] mAudioPath;

You think you are initialising it by calling getAudioList(), but inside this method, you are declaring:

String[] mAudioPath = new String[count];

which SHADOWS your global variable mAudioPath, so your local mAudioPath is being initialised, instead of global, which is then used to play the song. To fix, in your getAudioList(), change:

String[] mAudioPath = new String[count];

to:

mAudioPath = new String[count];