Question

我在应用程序中调用player.pause();方法时收到此错误。我正在使用ImageButton暂停音乐。我正在调用song_list类的方法来暂停轨道。但是，发生以下异常：

尝试在空对象引用上调用虚方法'void android.media.MediaPlayer.pause（）'

我的主要活动：

public class MusicKom extends AppCompatActivity {

    private static String TAG = "PermissionDemo";
    public static final int REQUEST_READ_STORAGE = 112;
    private ImageButton btplay;
    private ImageButton pausemusic;

    public void playbutton(int playbuttonid){

        btplay=(ImageButton)findViewById(R.id.play);
        btplay.setBackgroundResource(playbuttonid);
    }

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

        ViewPager viewPager = (ViewPager) findViewById(R.id.viewpager);

        // Create an adapter that knows which fragment should be shown on each page
        final viewpageadapter adapter = new viewpageadapter(getSupportFragmentManager());

        viewPager.setAdapter(adapter);

        Button bt1 = (Button)findViewById(R.id.song_list);
        Button bt2 = (Button)findViewById(R.id.album_list);
        Button bt3 = (Button)findViewById(R.id.artist_list);
        pausemusic = (ImageButton)findViewById(R.id.play);


        bt1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ViewPager viewPager = (ViewPager) findViewById(R.id.viewpager);
                viewPager.setCurrentItem(0);
            }
        });

        bt2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ViewPager viewPager = (ViewPager) findViewById(R.id.viewpager);
                viewPager.setCurrentItem(1);
            }
        });

        bt3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ViewPager viewPager = (ViewPager) findViewById(R.id.viewpager);
                viewPager.setCurrentItem(2);
            }
        });

        pausemusic.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
               songs_list sl=new songs_list();
               sl.pauseplay();
               pausemusic.setBackgroundResource(R.drawable.stop);
            }
        });
    }
}

我的song_list片段：

public class songs_list extends Fragment {

    // Adapter that exposes data from a Cursor to a ListView widget.
    private MediaCursorAdapter mediaAdapter = null;
    public MediaPlayer player;
    private String currentFile="";
    private boolean isStarted = true;
    private TextView selelctedFile = null;
    private View v=null;
    private String selectedtitem=null;
    private int fileposition;

    @Override
    public View onCreateView(LayoutInflater inflater, final ViewGroup container, Bundle savedInstanceState) {

        player = new MediaPlayer();

        // Use cursor to get data from external resource

        final Cursor cursor = getActivity().getContentResolver().query(MediaStore.Audio.Media.EXTERNAL_CONTENT_URI, null, null, null, null);


        // check Cursor has data or not

        if (null != cursor) {
            cursor.moveToFirst();

            mediaAdapter = new MediaCursorAdapter(getActivity().getApplicationContext(), R.layout.list_layout, cursor);
            v=inflater.inflate(R.layout.fragment_songs_list, container, false);

            ListView lv = (ListView)v.findViewById(android.R.id.list);


            lv.setAdapter(mediaAdapter);



            lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
                @Override
                public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

                    selectedtitem = (String) view.getTag();
                    startPlay(selectedtitem);
                }
            });


        }


        return v;
    }

    private void startPlay(String file) {

        player.stop();
        player.reset();

        try {
            player.setDataSource(file);
            player.prepare();
            player.start();
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        } catch (IllegalStateException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        ((MusicKom) getActivity()).playbutton(R.drawable.play);
    }

    public void pauseplay(){
        Log.i(TAG, "pauseplay: ");
        player.pause();
    }
}

Answer 1

如何在不实际启动活动的情况下调用其他活动方法。在您的情况下，未加载另一个活动，因此不会创建媒体播放器对象。

请发布整个代码以获取更多信息。

Answer 2

您正在使用Fragment作为常规Java类。在实例化Fragment时不会调用片段onCreateView()。只有在显示具有有效FragmentTranscation的片段时才会调用它。见Fragment Life cycle。

如果您希望在必须致电pauseplay();时显示片段，请注释，我将添加相关代码。

否则，您应该使用常规java类从任何地方创建和播放/暂停媒体播放器。类似的东西：

// package com.example.package_name;

import android.media.MediaPlayer;

public class SoundPlayer {

    private MediaPlayer mediaPlayer;

    // rawSongId is in form of 'R.id.song_name'
    public SoundPlayer(Context context, int rawSongId){
        mediaPlayer = MediaPlayer.create(context, rawSongId);
    }

    public SoundPlayer(Context context, String dataSource){
        // handle media player with your data source
    }

    public void start(){
        if(mediaPlayer != null) {
            mediaPlayer.start();
        }
    }

    public void pause(){
        if(mediaPlayer != null) {
            mediaPlayer.pause();
        }
    }

    public void stop(){
        if(mediaPlayer != null) {
            mediaPlayer.stop(); // reset(); ? 
        }
    }

}

现在，而不是

songs_list sl = new songs_list();
sl.pauseplay();

你可以，

String dataSource = ""; // generate your dataSource here

// initialize MediaPlayer with dataSource
SoundPlayer soundPlayer = new SoundPlayer(this, dataSource);

// Use its methods
soundPlayer.start();
soundPlayer.pause();
soundPlayer.stop();

请尝试使用正确的camelCase coding conventions。您的pauseplay()应为pausePlay();或pauseOrPlay()。

调用mediaplayer方法时出现空对象引用异常

2 个答案: