问安卓MP3播放器，播放/暂停按钮不工作

EN

    ***************** Main Activity.java ************************

package live.regionradio.regionradio;

import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.media.MediaPlayer;
import java.io.IOException;
import android.media.AudioManager;


public class MainActivity extends AppCompatActivity {

    static FloatingActionButton btnPlayPause;

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

        btnPlayPause = (FloatingActionButton) findViewById(R.id.fab);
        btnPlayPause.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

            }
        });

        String url = "https://ia802508.us.archive.org/5/items/testmp3testfile/mpthreetest.mp3";
        if (Player.player == null)
            new Player();
        Player.player.playStream(url);
    }

    public static void flipPlayPauseButton (boolean isPlaying) {
        if (isPlaying) {
            btnPlayPause.setImageResource(android.R.drawable.ic_media_pause);
        }
        else {
            btnPlayPause.setImageResource(android.R.drawable.ic_media_play);
        }
    }
    @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);
        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.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
      }


    }

***************** Player.java ************************

    package live.regionradio.regionradio;
    import android.media.MediaPlayer;
    import android.media.AudioManager;
    import android.util.Log;
    import java.io.IOException;


    public class Player {

        MediaPlayer mediaPlayer = new MediaPlayer();
        public static Player player;
        String url = "";

        public Player() {
            this.player = this;
        }

        public void playStream(String url) {
            if (mediaPlayer != null) {
                try {
                    mediaPlayer.stop();
                } catch (Exception e) {

                }
                mediaPlayer = null;
            }

            mediaPlayer = new MediaPlayer();
            mediaPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);
            try {
                mediaPlayer.setDataSource(url);
                mediaPlayer.setOnPreparedListener(new MediaPlayer.OnPreparedListener() {
                    private MediaPlayer mp;


                    @Override
                    public void onPrepared(MediaPlayer mp) {
                        this.mp = mp;
                        playPlayer();
                    }
                });
                mediaPlayer.setOnCompletionListener(new MediaPlayer.OnCompletionListener() {
                    private MediaPlayer mp;

                    @Override
                    public void onCompletion(MediaPlayer mp) {
                        this.mp = mp;
                        MainActivity.flipPlayPauseButton(false);
                    }
                });
                mediaPlayer.prepareAsync();
            } catch (IOException e) {
                e.printStackTrace();
            }

        }

        public void pausePlayer () {
            try {
                mediaPlayer.pause();
                MainActivity.flipPlayPauseButton(false);
            }
            catch (Exception e) {
                Log.d("EXCEPTION", "Failed to pause media player.");
            }
        }

        public void playPlayer () {
            try {
                mediaPlayer.start();
                MainActivity.flipPlayPauseButton(true);
            }
            catch (Exception e) {
                Log.d("EXCEPTION", "Failed to play media player.");
            }
        }

        public void togglePlayer () {
            try  {
                if (mediaPlayer.isPlaying())
                    pausePlayer();
                else
                    playPlayer();
            }
            catch   (Exception e){
                Log.d("Exception", "failed to toggle media player.");
            }
        }

    }