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第一步布局文件  
   第二步权限    
       
   第三步 手机设置权限第四步  类package com.example.myapplication;import android.content.Context;import android.media.MediaPlayer;import android.os.Environment;import android.widget.Toast;import java.io.IOException;public class MP3Player {
       private MediaPlayer mp;    private String path;    private int length;    public MP3Player(Context ctx) {
           mp = new MediaPlayer();        // 获取内部存储器绝对路径/sdcard/Download/ANewDay.mp3        path = Environment.getExternalStorageDirectory().getAbsolutePath();        path="/sdcard/Music/";        Toast.makeText(ctx, path, Toast.LENGTH_LONG).show();    }    public int getPosition() {
           // 获取当前位置        return mp.getCurrentPosition();    }    public int getLength() {
           return length;    }    public void init(String fileName) {
           path = path + "/" + fileName;        try {
               // 存储在SD卡或其他文件路径下的媒体文件            mp.setDataSource(path);            // 音乐文件准备            mp.prepare();            // 音乐文件长度            length = mp.getDuration();        } catch (IOException e) {
               e.printStackTrace();        }    }    public void play() {
           if (mp.isPlaying()) {
               mp.stop();        }        else {
               mp.start();        }    }    public void pause() {
           mp.pause();    }    public void stop() {
           mp.stop();    }    public void destroy() {
           mp.release();    }}第五步主代码package com.example.myapplication;import androidx.appcompat.app.AppCompatActivity;import android.os.Bundle;import android.os.Environment;import android.util.Log;import android.view.View;import android.widget.AdapterView;import android.widget.ArrayAdapter;import android.widget.ListView;import android.widget.TextView;import java.io.File;import java.util.ArrayList;import java.util.List;public class MainActivity extends AppCompatActivity {
       private MP3Player mp3Player;    @Override    protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        System.out.println("11111111");        System.out.println(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_MUSIC));        ///mnt/sdcard/Music        File file=new File("/sdcard/Music");        final File[] files=file.listFiles();        if (files == null){
               Log.e("error","空目录");        }        List
   
     s = new ArrayList<>();        for(int i =0;i
    
      arrayAdapter= new ArrayAdapter
     
       (                MainActivity.this, android.R.layout.simple_list_item_1,s);        ListView listView = (ListView) findViewById(R.id.lv);        listView.setAdapter(arrayAdapter);        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
               @Override            public void onItemClick(AdapterView
       parent, View view, int position, long id) {
                   System.out.println(id);                System.out.println(files[(int)id].getAbsolutePath());                mp3Player = new MP3Player(MainActivity.this);                mp3Player.init(files[(int)id].getName());                mp3Player.play();            }        });    }}
     
    
   

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