使用AudioManager类时获取java.lang.NullPointerException

Question

尝试使用AudioManager类。但该应用程序正在给出运行时错误：尝试调用虚拟方法＆＃39; int android.media.AudioManager.requestAudioFocus（android.media.AudioManager $ OnAudioFocusChangeListener，int，int）＆＃39;在空对象引用上

 public class NumbersActivity extends AppCompatActivity {

    private MediaPlayer mediaPlayer;

    private AudioManager audioManager;



    private AudioManager.OnAudioFocusChangeListener audioFocusChangeListener=new AudioManager.OnAudioFocusChangeListener() {
        @Override
        public void onAudioFocusChange(int focusChange) {
            if(focusChange==AudioManager.AUDIOFOCUS_LOSS_TRANSIENT||focusChange==AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK)
            {
                //now we need to call pause because listener detected that some else app wants audiofocus for short time

                mediaPlayer.pause();

                //rewinding our audio back to 0 i.e start so when we get audio focus back it resumes from start
                mediaPlayer.seekTo(0);
            }

            else if (focusChange == AudioManager.AUDIOFOCUS_GAIN) {
                // The AUDIOFOCUS_GAIN case means we have regained focus and can resume playback.
                mediaPlayer.start();
            }

            else if (focusChange == AudioManager.AUDIOFOCUS_LOSS) {
                // The AUDIOFOCUS_LOSS case means we've lost audio focus completely
                //so now  Stop playback and clean up resources
                releasePlayer();
            }

        }
    };

    //this is a listener triggering when audio is completed playing in our app
    private MediaPlayer.OnCompletionListener songCompletionListener = new MediaPlayer.OnCompletionListener() {

        @Override
        public void onCompletion(MediaPlayer mediaPlayer) {
            releasePlayer();
        }
    };

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


        final ArrayList<Word> numbers = new ArrayList<Word>();

        numbers.add(..CODE TO ADD DATA HERE..);

         //created custom arrya adapter
        WordAdapter itemsAdapter = new WordAdapter(this, numbers);
        ListView listView = (ListView) findViewById(R.id.list);
        listView.setAdapter(itemsAdapter);


        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {


                releasePlayer();
                Word word = numbers.get(i);

                // now we are going to gain focus on audio
                int getAudioFocus=audioManager.requestAudioFocus(audioFocusChangeListener,
                        AudioManager.STREAM_MUSIC, AudioManager.AUDIOFOCUS_GAIN_TRANSIENT);

                if(getAudioFocus==AudioManager.AUDIOFOCUS_REQUEST_GRANTED)
                {
                    mediaPlayer = MediaPlayer.create(NumbersActivity.this, word.getAudioResourceID());
                    mediaPlayer.start();

                    //called when audio is finished playing
                    mediaPlayer.setOnCompletionListener(songCompletionListener);
                }

            }

        });
    }

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

        releasePlayer();
    }

    //all the methods that are called are below here


    //to check if mediaplayer is null so that we can play new audio and it not then make it null
    private void releasePlayer() {
        if (mediaPlayer != null) {
            mediaPlayer.release();

            mediaPlayer = null;

            // Regardless of whether or not we were granted audio focus, abandon it. This also
            // unregisters the AudioFocusChangeListener so we don't get anymore callbacks.
            audioManager.abandonAudioFocus(audioFocusChangeListener);
        }

    }


}

这里我找不到null对象引用。我是Android的初学者，所以请帮助我理解这个问题。我已尽力理解并发现错误，但我无法做到。