Android studio 小白开发求助帖

Question

如图，按照书上的代码编写了一个音乐播放器，但是加载到手机后播放不了音乐（模拟机上也不行），一开始是出现下图这种情况，无法build.按照网上的说法,把那个targetsdkversion 改成26之后，能够build .但是播放不了音乐。求助啊求助！！谢谢！万分感谢！

Answer 1

package com.example.chinayexin.player;import android.app.Activity;
import android.content.Intent;
import android.content.pm.ActivityInfo;
import android.database.Cursor;
import android.media.MediaPlayer;
import android.net.Uri;
import android.os.Bundle;
import android.provider.MediaStore;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.view.WindowManager;
import android.widget.Button;
import android.widget.CheckBox;
import android.widget.TextView;
import android.widget.Toast;import java.io.IOException;public class MainActivity extends AppCompatActivity implements
    MediaPlayer.OnPreparedListener,
    MediaPlayer.OnErrorListener,
    MediaPlayer.OnCompletionListener{    Uri uri;
    TextView txvName ,txvUri;
    boolean isVideo = false;
    Button btnPlay,btnStop;
    CheckBox ckbLoop;
    MediaPlayer mper = new MediaPlayer();
    Toast tos;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);        //设置屏幕不转向
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_NOSENSOR);
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);        getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);        txvName =(TextView) findViewById(R.id.txvName);
        txvUri = (TextView) findViewById(R.id.txvUri);
        btnPlay=(Button) findViewById(R.id.btnPlay);
        btnStop= (Button) findViewById(R.id.btnStop);
        ckbLoop=(CheckBox) findViewById(R.id.ckbLoop);        uri= Uri.parse("android.resource://"+getPackageCodePath()+"/"+R.raw.music);
        txvName.setText("music.mp3");
        txvUri.setText("程序内音乐："+uri.toString());        mper=new MediaPlayer();
        mper.setOnPreparedListener(this);
        mper.setOnErrorListener(this);
        mper.setOnCompletionListener(this);
        tos=Toast.makeText(this,"",Toast.LENGTH_SHORT);        prepareMusic();
    }    void prepareMusic() {
        btnPlay.setText("播放");
        btnPlay.setEnabled(false);
        btnStop.setEnabled(false);
        try{
            mper.reset();
            mper.setDataSource(this,uri);
            mper.setLooping( ckbLoop.isChecked());
            mper.prepareAsync();
        } catch (IOException e) {
          tos.setText("指定音乐错误"+e.toString());
          tos.show();
        }
    }
    public void onPick(View view) {
        Intent it = new Intent(Intent .ACTION_GET_CONTENT);
        if(view.getId()==R.id.btnPickAudio){
            it.setType("audio/*");
            startActivityForResult(it,100);
        }
      else{
          it.setType("Vedio/*");
          startActivityForResult(it,101);
        }
    }
    protected void onActivityResult(int requestCode,int resultCode,Intent data){
        super.onActivityResult(requestCode,resultCode,data);
        if(resultCode== Activity.RESULT_OK){
            isVideo= (requestCode==101);
            uri= convertUri(data.getData());
            txvName.setText(uri.getLastPathSegment());
            txvUri.setText("文件位置："+uri.getPath());            if(!isVideo)prepareMusic();
        }
    }    private Uri convertUri(Uri uri) {
        if(uri.toString().substring(0,7).equals("content")){
            String[] colName = {MediaStore.MediaColumns.DATA};
            Cursor cursor = getContentResolver()
                    .query(uri,colName,null,null,null);
            cursor.moveToFirst();
            uri=Uri.parse("file://"+cursor.getString(0));
            cursor.close();
        }
        return uri ;
    }    @Override
    public void onCompletion(MediaPlayer mp) {
        mper.seekTo(0);
        btnPlay.setText("播放");
        btnStop.setEnabled(false);
    }    @Override
    public boolean onError(MediaPlayer mp, int what, int extra) {
        tos.setText("发生错误，停止播放");
        tos.show();        return true;
    }    @Override
    public void onPrepared(MediaPlayer mp) {
        btnPlay.setEnabled(true);
    }    public void onMpPlay(View view) {
        if(mper.isPlaying()){
            mper.pause();
            btnPlay.setText("继续");
        }
        else{
            mper.start();
            btnPlay.setText("暂停");
            btnStop.setEnabled(true);
        }    }    public void onMpStop(View view) {
        mper.pause();
        mper.seekTo(0);
        btnPlay.setText("播放");
        btnStop.setEnabled(false);
    }    public void onMpLoop(View view) {
        if (ckbLoop.isChecked())
            mper.setLooping(true);
        else
            mper.setLooping(false);
    }    public void onMpForward(View view) {
        if(!btnPlay.isEnabled())return;
        int len=mper.getDuration();
        int pos=mper.getCurrentPosition();
        pos+=10000;
        if(pos>len)pos=len;
        mper.seekTo(pos);
        tos.setText("快进10秒："+pos/1000+"/"+len/1000);
        tos.show();
    }    public void onMpBackward(View view) {
        if(!btnPlay.isEnabled())return;
        int len=mper.getDuration();
        int pos=mper.getCurrentPosition();
        pos-=10000;
        if(pos<0)pos=0;
        mper.seekTo(pos);
        tos.setText("快退10秒"+pos/1000+"/"+len/1000);
        tos.show();
    }
    @Override
    protected void onPause() {        super.onPause();
        if(mper.isPlaying()){
            btnPlay.setText("继续");
            mper.pause();
        }
    }
    @Override
    protected void onDestroy() {
        mper.release();
        super.onDestroy();
    }
}

Answer 2

提示很明显，在APP的 build.grade 文件里面的 com.android.support:media2:28.0.0 找不到，或许你可以把 28.0.0 版本号降低一点 27.0.2 或者其它