ボタンをクリックしたときに内部ストレージにリストビューとしてmp3ファイルを表示できません

Question

ボタンをクリックすると、電話の内部ストレージにすべてのmp3ファイルを表示しようとしています。しかし、私はボタンをクリックするとすぐに、ストレージのアクセス許可を求め、アクセス許可を与えた後、Unfortunately, app has stoppedと表示されます。実行時例外が発生しているようですが、これを解決する方法が見つかりません。誰も助けることができますか？ これはボタンです：

<Button 
    android:id="@+id/button4" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_alignLeft="@+id/button3" 
    android:layout_alignStart="@+id/button3" 
    android:layout_below="@+id/button3" 
    android:layout_marginTop="22dp" 
    android:onClick="onClick3" 
    android:text="@string/button_audio" /> 

これはクラスである：ここでは

package com.example.dell_1.myapp3; 

    import android.content.Intent; 
    import android.database.Cursor; 
    import android.media.MediaPlayer; 
    import android.net.Uri; 
    import android.os.Bundle; 
    import android.app.Activity; 
    import android.os.Environment; 
    import android.provider.MediaStore; 
    import android.util.Log; 
    import android.view.View; 
    import android.widget.AdapterView; 
    import android.widget.ArrayAdapter; 
    import android.widget.ListView; 
    import java.io.File; 
    import java.io.IOException; 


     public class Bacon1 extends Activity { 

     private static int RESULT_LOAD_IMAGE = 1; 
    private static int MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE = 1; 

    private String[] mAudioPath; 
    private MediaPlayer mMediaPlayer; 
    private String[] mMusicList;; 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_bacon1); 

     final Button button4 = (Button) findViewById(R.id.button4); 

     button4.setOnClickListener(
       new View.OnClickListener(){ 
        public void onClick (View v){ 
         buttonClicked(v); 

         abc(); 

        } 
       }); 
    } 


    private void buttonClicked(View view){ 

     if (ContextCompat.checkSelfPermission(this, 
       Manifest.permission.READ_EXTERNAL_STORAGE) 
       != PackageManager.PERMISSION_GRANTED) { 

      Snackbar.make(view, "Permission not Granted, Requesting permission.", Snackbar.LENGTH_LONG).show(); 
      if (ActivityCompat.shouldShowRequestPermissionRationale(this, 
        Manifest.permission.READ_EXTERNAL_STORAGE)) { 

       Snackbar.make(view, "We need permission to internal storage for displaying songs", Snackbar.LENGTH_LONG).show(); 

      } else { 

       Snackbar.make(view, "Allow myapp3 to access this device's internal storage", Snackbar.LENGTH_LONG).show(); 

       ActivityCompat.requestPermissions(this, 
         new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, 
         MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE); 

       // MY_PERMISSIONS_REQUEST_READ_CONTACTS is an 
       // app-defined int constant. The callback method gets the 
       // result of the request. 
      } 
     } 
     } 



    public void onClick2(View view) { 

     Intent i = new Intent(
       Intent.ACTION_PICK, 
       android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI); 

     startActivityForResult(i, RESULT_LOAD_IMAGE); 
    } 


    public void onClick4(View view) { 

     Intent viewIntent1 = new Intent(Intent.ACTION_VIEW); 
     File file = Environment.getExternalStorageDirectory(); 
     viewIntent1.setDataAndType(Uri.fromFile(file), "video/*"); 
     startActivity(Intent.createChooser(viewIntent1, null)); 
    } 

    public void onClick5(View view) { 

     Intent viewIntent1 = new Intent(Intent.ACTION_VIEW); 
     File file = Environment.getExternalStorageDirectory(); 
     viewIntent1.setDataAndType(Uri.fromFile(file), "zip/*"); 
     startActivity(Intent.createChooser(viewIntent1, null)); 
    } 
    public void onClick6(View view) { 

     Intent viewIntent1 = new Intent(Intent.ACTION_VIEW); 
     File file = Environment.getExternalStorageDirectory(); 
     viewIntent1.setDataAndType(Uri.fromFile(file), "text/*"); 
     startActivity(Intent.createChooser(viewIntent1, null)); 
    } 

    public void onClick7(View view) { 

     Intent viewIntent1 = new Intent(Intent.ACTION_VIEW); 
     File file = Environment.getExternalStorageDirectory(); 
     viewIntent1.setDataAndType(Uri.fromFile(file), "application/*"); 
     startActivity(Intent.createChooser(viewIntent1, null)); 
    } 


    @Override 
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) { 
      switch (requestCode) { 
       case 1: { 
        // If request is cancelled, the result arrays are empty. 
        if (grantResults.length > 0 
          && grantResults[0] == PackageManager.PERMISSION_GRANTED) { 
         Toast.makeText(Bacon1.this, "WRITE_CONTACTS granted", Toast.LENGTH_SHORT) 
          .show(); 

        } else { 

         Toast.makeText(Bacon1.this, "WRITE_CONTACTS Denied", Toast.LENGTH_SHORT) 
           .show(); 

        } 
        return; 
       } 

       // other 'case' lines to check for other 
       // permissions this app might request 
      } 
    } 

    public void abc(){ 
     mMediaPlayer = new MediaPlayer(); 

     ListView mListView = (ListView) findViewById(R.id.list); 

     mMusicList = getAudioList(); 

     ArrayAdapter<String> mAdapter = new ArrayAdapter<String>(this, 
       android.R.layout.simple_list_item_1, mMusicList); 
     mListView.setAdapter(mAdapter); 

     mListView.setOnItemClickListener(new AdapterView.OnItemClickListener() { 

      @Override 
      public void onItemClick(AdapterView<?> arg0, View arg1, int arg2, 
            long arg3) { 
       try { 
        playSong(mAudioPath[arg2]); 
       } catch (IllegalArgumentException e) { 
        e.printStackTrace(); 
       } catch (IllegalStateException e) { 
        e.printStackTrace(); 
       } catch (IOException e) { 
        e.printStackTrace(); 
       } 
      } 
     }); 
    } 

    private String[] getAudioList() { 
     final Cursor mCursor = getContentResolver().query(
       MediaStore.Audio.Media.EXTERNAL_CONTENT_URI, 
       new String[] { MediaStore.Audio.Media.DISPLAY_NAME, MediaStore.Audio.Media.DATA }, null, null, 
       "LOWER(" + MediaStore.Audio.Media.TITLE + ") ASC"); 

     int count = mCursor.getCount(); 

     String[] songs = new String[count]; 
     String[] mAudioPath = new String[count]; 
     int i = 0; 
     if (mCursor.moveToFirst()) { 
      do { 
       songs[i] = mCursor.getString(mCursor.getColumnIndexOrThrow(MediaStore.Audio.Media.DISPLAY_NAME)); 
       mAudioPath[i] = mCursor.getString(mCursor.getColumnIndexOrThrow(MediaStore.Audio.Media.DATA)); 
       i++; 
      } while (mCursor.moveToNext()); 
     } 

     mCursor.close(); 

     return songs; 
    } 

    private void playSong(String path) throws IllegalArgumentException, 
      IllegalStateException, IOException { 

     Log.d("ringtone", "playSong :: " + path); 

     mMediaPlayer.reset(); 
     mMediaPlayer.setDataSource(path); 
     mMediaPlayer.prepare(); 
     mMediaPlayer.start(); 
    } 

    } 

は、Androidマニフェストファイルである。ここ

<?xml version="1.0" encoding="utf-8"?> 
<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
    package="com.example.dell_1.myapp3"> 

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/> 
    <uses-permission android:name="android.permission.WRITE_MEDIA_STORAGE" /> 
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/> 
    <uses-permission android:name="android.permission.INTERNET"/> 
    <application 
     android:allowBackup="true" 
     android:icon="@mipmap/ic_launcher" 
     android:label="@string/app_name" 
     android:roundIcon="@mipmap/ic_launcher_round" 
     android:supportsRtl="true" 
     android:theme="@style/AppTheme"> 


<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /> 
<uses-permission 
android:name="android.permission.WRITE_MEDIA_STORAGE" /> 
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/> 
<uses-permission android:name="android.permission.INTERNET"/> 

     <activity 
      android:name=".apples" 
      android:label="@string/app_name" 
      android:theme="@style/AppTheme.NoActionBar"> 
      <intent-filter> 
       <action android:name="android.intent.action.MAIN" /> 

       <category android:name="android.intent.category.LAUNCHER" /> 
      </intent-filter> 
     </activity> 
     <activity 
      android:name=".Bacon1" 
      android:label="@string/title_activity_bacon1" /> 
     <activity 
      android:name=".SDCard" 
      android:label="@string/title_activity_sdcard" /> 
     <activity 
      android:name=".Songmanager" 
      android:label="@string/title_activity_songmanager"></activity> 
    </application> 

</manifest> 

はLogcatです：

07-02 03:40:13.655 13229-13229/com.example.dell_1.myapp3 E/ExtMediaPlayer-JNI: env->IsInstanceOf fails 
07-02 03:40:13.655 13229-13229/com.example.dell_1.myapp3 E/MediaPlayer-JNI: JNIMediaPlayerFactory: bIsQCMediaPlayerPresent 0 
07-02 03:40:13.655 13229-13229/com.example.dell_1.myapp3 E/ExtMediaPlayer-JNI: env->IsInstanceOf fails 
07-02 03:40:13.655 13229-13229/com.example.dell_1.myapp3 E/MediaPlayer-JNI: JNIMediaPlayerFactory: bIsQCMediaPlayerPresent 0 
07-02 03:40:13.700 13229-13229/com.example.dell_1.myapp3 E/AndroidRuntime: FATAL EXCEPTION: main 
                      Process: com.example.dell_1.myapp3, PID: 13229 
                      java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference 
                       at com.example.dell_1.myapp3.Bacon1.abc(Bacon1.java:158) 
                       at com.example.dell_1.myapp3.Bacon1$1.onClick(Bacon1.java:48) 
                       at android.view.View.performClick(View.java:5609) 
                       at android.view.View$PerformClick.run(View.java:22263) 
                       at android.os.Handler.handleCallback(Handler.java:751) 
                       at android.os.Handler.dispatchMessage(Handler.java:95) 
                       at android.os.Looper.loop(Looper.java:154) 
                       at android.app.ActivityThread.main(ActivityThread.java:6077) 
                       at java.lang.reflect.Method.invoke(Native Method) 
                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) 

Answer 1

01を追加する必要がありますマニフェストファイルに を追加します。あなたは、実行時に許可を求めてい

：あなたが欲しい、これまで何

if (ContextCompat.checkSelfPermission(thisActivity,Manifest.permission.READ_EXTERNAL_STORAGE)!= PackageManager.PERMISSION_GRANTED) { 


if (ActivityCompat.shouldShowRequestPermissionRationale(thisActivity, 
     Manifest.permission.READ_EXTERNAL_STORAGE)) { 

    // Show an explanation to the user *asynchronously* -- don't block 
    // this thread waiting for the user's response! After the user 
    // sees the explanation, try again to request the permission. 

} else { 

    // No explanation needed, we can request the permission. 

    ActivityCompat.requestPermissions(thisActivity, 
      new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, 
      REQUEST_PERMISSION); 

    // MY_PERMISSIONS_REQUEST_READ_CONTACTS is an 
    // app-defined int constant. The callback method gets the 
    // result of the request. 
} 

}

と比べて（もしユーザー壮大許可）：

@Override 
public void onRequestPermissionsResult(final int requestCode, @NonNull final String[] permissions, @NonNull final int[] grantResults) { 
super.onRequestPermissionsResult(requestCode, permissions, grantResults); 
if (requestCode == REQUEST_PERMISSION) { 
    if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) { 
     // Permission granted. 
    } else { 
     // User refused to grant permission. 
    } 
} 
} 

ますcaここにもっと読む： https://developer.android.com/training/permissions/requesting.html