具有Uri的setDataSource導致NullPointerException

Question

好吧，這是button的onClick method ：

public void aggiungi (View v){

    Intent intent = new Intent(Intent.ACTION_GET_CONTENT);
    intent.setType("audio/*");
    startActivityForResult(Intent.createChooser(intent, "Select"), PICK_AUDIO_REQUEST_CODE);

}

在外部，我已經處理了輸出：

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {

    if (requestCode == PICK_AUDIO_REQUEST_CODE){
        if (resultCode == RESULT_OK){
            Uri uri_to_use = data.getData();
            setAudioToButton(uri_to_use);
        }
    }

}

這是名為setAudioToButton的method

public void setAudioToButton(final Uri audio){
    PercentRelativeLayout percentRelativeLayout = (PercentRelativeLayout)findViewById(R.id.main_layout);
    PercentRelativeLayout.LayoutParams layoutParams = new PercentRelativeLayout.LayoutParams(43, 12);
    layoutParams.addRule(PercentRelativeLayout.ALIGN_PARENT_RIGHT);
    layoutParams.addRule(PercentRelativeLayout.BELOW, R.id.quattro);
    Button button = new Button(this);
    button.setLayoutParams(layoutParams);
    PercentRelativeLayout.LayoutParams layoutParams1 = (PercentRelativeLayout.LayoutParams)button.getLayoutParams();
    PercentLayoutHelper.PercentLayoutInfo percentLayoutInfo = layoutParams1.getPercentLayoutInfo();
    percentLayoutInfo.heightPercent = 0.12f;
    percentLayoutInfo.widthPercent = 0.43f;
    percentLayoutInfo.topMarginPercent = 0.063f;
    percentLayoutInfo.leftMarginPercent = 0.007f;
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if (mediaPlayer != null){mediaPlayer.stop();}
            try {mediaPlayer.setDataSource(getBaseContext(), audio);} catch (IOException error){error.printStackTrace();}
            try {
                mediaPlayer.prepare();
            } catch (IOException e) {
                e.printStackTrace();
            }
            mediaPlayer.start();
        }
    });
    button.setText("start");
    percentRelativeLayout.addView(button);
}

問題是，當我單擊新按鈕使其發出聲音時，我有一個NullPointerException ：

08-22 02:32:43.164 11880-11880/com.example.utente.usefulsounds E/AndroidRuntime: FATAL EXCEPTION: main
                                                                             Process: com.example.utente.usefulsounds, PID: 11880
                                                                             Theme: themes:{}
                                                                             java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.setDataSource(android.content.Context, android.net.Uri)' on a null object reference
                                                                                 at com.example.utente.usefulsounds.MainActivity$1.onClick(MainActivity.java:182)
                                                                                 at android.view.View.performClick(View.java:5204)
                                                                                 at android.view.View$PerformClick.run(View.java:21158)
                                                                                 at android.os.Handler.handleCallback(Handler.java:739)
                                                                                 at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                 at android.os.Looper.loop(Looper.java:148)
                                                                                 at android.app.ActivityThread.main(ActivityThread.java:5461)
                                                                                 at java.lang.reflect.Method.invoke(Native Method)
                                                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

我想知道為什么會這樣，我聽不懂

**

編輯：

**

我把它放在onCreate method

if (ContextCompat.checkSelfPermission(getApplicationContext(), android.Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED){
        ActivityCompat.requestPermissions(this, new String[]{android.Manifest.permission.READ_EXTERNAL_STORAGE}, READ_EXTERNAL_STORAGE_REQUEST_CODE);
    }

但這沒用

現在，我嘗試了另一種方式：將Uri傳遞給MediaPlayer ，我傳遞了filedescriptor ：

 button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if (mediaPlayer != null){mediaPlayer.stop();}
            File file = new File(audio.getPath());
            FileInputStream fileInputStream;
            try {
                fileInputStream = new FileInputStream(file);
                try {mediaPlayer.setDataSource(fileInputStream.getFD());} catch (IOException error){error.printStackTrace();}
            }catch (IOException ioe){button.setText("FILE ERROR");}

            try {
                mediaPlayer.prepare();
            } catch (IOException e) {
                e.printStackTrace();
            }
            mediaPlayer.start();
        }
    });

該按鈕顯示為“選擇”文本，因此該file沒有任何問題（否則應該是“文件錯誤”），但是當我單擊該按鈕時，我仍然遇到由mediaPlayer的NullPointerException引起的崩潰。 為什么這個？ 我聽不懂

Answer 1

這是答案

不必執行setDataSource() method ，只需使用create() method ：)

例如。

 MediaPlayer.create(getApplicationContext(), yourAudioUri).start();

編碼愉快，玩得開心:)