Null object 在 RecyclerView 适配器中的 MediaPlayer.start() 参考

Question

我的目标是根据每个适配器 position 播放音频。

适配器从 Firebase 实时数据库中获取MediaPlayer的Uri ，格式类似于R.raw.songfile (no extension) 。 但是当我将String (FirebaseRT-DB.getValue) 转换为Uri并将其用于MediaPlayer时，它崩溃了。

而且我也测试了Uri output ，但只使用了Toast 。它按我预期的方式工作。

Toast.makeText(con, "Audio is playing.\n" + Uri.parse(lessonModels.get(pos).getAudioPath()), Toast.LENGTH_SHORT).show();

Toast Screenshot 1 Toast Screenshot 2 (它基于适配器位置)

Logcat 错误：

2022-08-15 12:05:36.746 6494-6494/com.kurokumo.redacted E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.kurokumo.redacted, PID: 6494
    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.start()' on a null object reference
        at com.kurokumo.redacted.adapters.LessonDetailsAdapter$ViewHolder.playAudio(LessonDetailsAdapter.java:109)
        at com.kurokumo.redacted.adapters.LessonDetailsAdapter$ViewHolder.access$200(LessonDetailsAdapter.java:58)
        at com.kurokumo.redacted.adapters.LessonDetailsAdapter$ViewHolder$1.onClick(LessonDetailsAdapter.java:95)
        at android.view.View.performClick(View.java:6597)
        at android.view.View.performClickInternal(View.java:6574)
        at android.view.View.access$3100(View.java:778)
        at android.view.View$PerformClick.run(View.java:25885)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

我很困惑，为什么是null..

以下是代码：

Query.java （从 FirebaseRTDB 获取音频路径）

for (DataSnapshot dataSnapshot : snapshot.child(g_selected_lesson).getChildren()){
   final String getAudioPath = dataSnapshot.child("audioPath").getValue(String.class);

   lessonModels.add(new LessonDetailsModel(getAudioPath));
}

课程详情Model.java

public class LessonDetailsModel {
   private final String audioPath;

   public LessonDetailsModel(String audioPath) {
      this.audioPath = audioPath;
   }
   public String getAudioPath() {
      return audioPath;
   }
}

LessonDetailsAdapter.java (ViewHolder)

public void onBindViewHolder(@NonNull LessonDetailsAdapter.ViewHolder holder, int position) {
    holder.setData(position);
}

public class ViewHolder extends RecyclerView.ViewHolder { //======== crashed in this line
   private final ImageButton playBtn;
   Context context = itemView.getContext();
   MediaPlayer mediaPlayer;

   public ViewHolder(@NonNull View itemView) {
      super(itemView);
      playBtn = itemView.findViewById(R.id.audioPlayBtn);
   }

   private void setData(final int position){
      mediaPlayer = new MediaPlayer();

      if (!lessonModels.get(position).getAudioPath().isEmpty()){
            playBtn.setVisibility(View.VISIBLE);
      }
      playBtn.setOnClickListener(new View.OnClickListener() {
         @Override
         public void onClick(View v) {
            playAudio(context, position); //======================== crashed in this line
         }
      });
   }

   private void playAudio(Context con, int pos) {
      if (mediaPlayer != null && mediaPlayer.isPlaying()){
         mediaPlayer.stop();
         mediaPlayer.reset();
         mediaPlayer.release();
         mediaPlayer = null;
      }
  
      mediaPlayer = MediaPlayer.create(
         con, Uri.parse(lessonModels.get(pos).getAudioPath()) // using Uri.parse to convert the String
      );
      mediaPlayer.start(); //======================================= crashed in this line
      Toast.makeText(con, "Audio is playing.\n" + lessonModels.get(pos).getAudioPath(), Toast.LENGTH_SHORT).show();
    }
}

哦，我还测试了我是否只使用了一个音频文件，并且效果很好。 （除了 static 噪音... ）

mediaPlayer = MediaPlayer.create(con, R.raw.audioFile);