Question

有人可以向我解释下面这个神秘的logcat消息，并解释如何解决下面代码中的问题。谢谢你，节日快乐：）

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.SearchView.setOnQueryTextListener(android.support.v7.widget.SearchView$OnQueryTextListener)' on a null object reference
    at com.meguillaume.cloudmusic.MainActivity.onCreateOptionsMenu(MainActivity.java:183)
    at android.app.Activity.onCreatePanelMenu(Activity.java:3142)
    at android.support.v4.app.FragmentActivity.onCreatePanelMenu(FragmentActivity.java:360)
    at android.support.v7.view.WindowCallbackWrapper.onCreatePanelMenu(WindowCallbackWrapper.java:88)
    at android.support.v7.app.AppCompatDelegateImplBase$AppCompatWindowCallbackBase.onCreatePanelMenu(AppCompatDelegateImplBase.java:331)
    at android.support.v7.view.WindowCallbackWrapper.onCreatePanelMenu(WindowCallbackWrapper.java:88)
    at android.support.v7.app.ToolbarActionBar.populateOptionsMenu(ToolbarActionBar.java:454)
    at android.support.v7.app.ToolbarActionBar$1.run(ToolbarActionBar.java:61)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6077)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

public class MainActivity extends AppCompatActivity implements SearchView.OnQueryTextListener {
    private static final String TAG = "MainActivity";

    private TracksAdapter mAdapter;
    //List is actually an interfact (Hold down command and left click to view.) and arraylist is
    //actually a class that implements that interface.
    private List<Track> mTracks;
    private TextView mSelectedTitle;
    private ImageView mSelectedThumbnail;
    private MediaPlayer mMediaPlayer;
    private ImageView mPlayerStateButton;
    private SearchView mSearchView;
    private List<Track> mPreviousTracks;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        mMediaPlayer = new MediaPlayer();
        mMediaPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);
        mMediaPlayer.setOnPreparedListener(new MediaPlayer.OnPreparedListener() {
            @Override
            public void onPrepared(MediaPlayer mp) {
                toggleSongState();
            }
        });

        mMediaPlayer.setOnCompletionListener(new MediaPlayer.OnCompletionListener() {
            @Override
            public void onCompletion(MediaPlayer mp) {
                mPlayerStateButton.setImageResource(R.drawable.ic_play);
            }
        });

        toolbar = (Toolbar) findViewById(R.id.player_toolbar);
        mSelectedTitle = (TextView) findViewById(R.id.selected_title);
        mSelectedThumbnail = (ImageView) findViewById(R.id.selected_thumbnail);

        mPlayerStateButton = (ImageView) findViewById(R.id.player_state);
        mPlayerStateButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                toggleSongState();
            }
        });

        RecyclerView recyclerView = (RecyclerView) findViewById(R.id.songs_list);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        mTracks = new ArrayList<Track>(); //BC of the list interface array list can implement here
        mAdapter = new TracksAdapter(this, mTracks);
        mAdapter.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Track selectedTrack = mTracks.get(position);

                mSelectedTitle.setText(selectedTrack.getTitle());
                Picasso.with(MainActivity.this).load(selectedTrack.getAvatarURL()).into(mSelectedThumbnail);

                if (mMediaPlayer.isPlaying()) {
                    mMediaPlayer.stop();
                }
                mMediaPlayer.reset();
                try {
                    mMediaPlayer.setDataSource(selectedTrack.getStreamURL() + "?client_id=" + SoundCloudService.CLIENT_ID);
                    mMediaPlayer.prepareAsync();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        });
        recyclerView.setAdapter(mAdapter);


        SoundCloudService service = SoundCloud.getService();
        service.getRecentSongs(new SimpleDateFormat("yyyy-MM-dd hh:mm:ss").format(new Date()), new Callback<List<Track>>() {
            @Override
            public void success(List<Track> tracks, Response response) {
                updateTracks(tracks);
            }

            @Override
            public void failure(RetrofitError error) {
                Log.d(TAG, "Error is " + error);
            }
        });
    }

    private void updateTracks(List<Track> tracks) {
        mTracks.clear();
        mTracks.addAll(tracks);
        mAdapter.notifyDataSetChanged();
    }

    private void toggleSongState() {
        if (mMediaPlayer.isPlaying()) {
            mMediaPlayer.pause();
            mPlayerStateButton.setImageResource(R.drawable.ic_play);
        } else {
            mMediaPlayer.start();
            mPlayerStateButton.setImageResource(R.drawable.ic_pause);
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();

        if (mMediaPlayer != null) {
            if (mMediaPlayer.isPlaying()) {
                mMediaPlayer.stop();
            }
            mMediaPlayer.release();
            mMediaPlayer = null;
        }
    }

    @Override
    public boolean onQueryTextSubmit(String query) {
        mSearchView.clearFocus();
        SoundCloud.getService().searchSongs(query, new Callback<List<Track>>() {
            @Override
            public void success(List<Track> tracks, Response response) {
                updateTracks(tracks);
            }

            @Override
            public void failure(RetrofitError error) {

            }
        });
        return true;
    }

    @Override
    public boolean onQueryTextChange(String newText) {
        return false;
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        mSearchView = (SearchView) menu.findItem(R.id.search_view).getActionView();
        mSearchView.setOnQueryTextListener(this);
        MenuItemCompat.setOnActionExpandListener(menu.findItem(R.id.search_view), new MenuItemCompat.OnActionExpandListener() {

            @Override
            public boolean onMenuItemActionExpand(MenuItem item) {
                mPreviousTracks = new ArrayList<Track>(mTracks);
                return true;
            }

            @Override
            public boolean onMenuItemActionCollapse(MenuItem item) {
                updateTracks(mPreviousTracks);
                return true;
            }
        });
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.search_view) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

Answer 1

程序文件的第183行附近有一行调用

mSearchView.setOnQueryTextListener(this);

但mSerachView仍为null（这可能意味着之前的行，关于menu.findItem（），返回null）

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    mSearchView = (SearchView) menu.findItem(R.id.search_view).getActionView();
    mSearchView.setOnQueryTextListener(this);  // <--- This line gave you the NullPointerException
    MenuItemCompat.setOnActionExpandListener(menu.findItem(R.id.search_view), new MenuItemCompat.OnActionExpandListener() {

        @Override
        public boolean onMenuItemActionExpand(MenuItem item) {
            mPreviousTracks = new ArrayList<Track>(mTracks);
            return true;
        }

        @Override
        public boolean onMenuItemActionCollapse(MenuItem item) {
            updateTracks(mPreviousTracks);
            return true;
        }
    });
    return true;
}

java.lang.NullPointerException：尝试调用虚方法

1 个答案: