Android play Text To Speech with audio focus (java)

I don't understand how to play text to speech with audio focus (there is spotify running some music when I try my app),

I don't understand the audioFocusChangeListener.

Here is my code:

public class MainActivity extends AppCompatActivity {


    AudioManager.OnAudioFocusChangeListener audioFocusChangeListener;

    public void playTTSWithAudioFocus(String toSpeak) {
        AudioManager audioManager = (AudioManager) getSystemService(Context.AUDIO_SERVICE);

        audioManager.requestAudioFocus(audioFocusChangeListener,
                // Use the music stream.
                AudioManager.STREAM_MUSIC,
                // Request temporary focus.
                AudioManager.AUDIOFOCUS_GAIN_TRANSIENT);
        textToSpeech.speak(toSpeak, TextToSpeech.QUEUE_FLUSH, null,null);

  }

    @RequiresApi(api = Build.VERSION_CODES.O)
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        playTTSWithAudioFocus("hello world");
 }
}

Please help me.