Question

我编写了一个测验，现在我在FOR-query下设置了一个媒体播放器，每次用户点击正确的答案时，都会播放声音。现在在同一个Activity中，我希望bttOFF会将Activity的声音静音，我该怎么做？我用mp.setVolume（0,0）设置了一个onClickListener;但是应用程序在重启时崩溃了。谢谢你的期待！：D

         public class QuizActivity extends AppCompatActivity {

private ActionBarDrawerToggle mToggle;

private QuestionLibrary mQuestionLibrary = new QuestionLibrary();

private TextView mScoreView;
private TextView mQuestionView;
private Button mButtonChoice1;
private Button mButtonChoice2;
private Button mButtonChoice3;
private String mAnswer;
private int mScore = 0;
private int mQuestionNumber = 0;
Dialog dialog;
Dialog dialog2;
TextView closeButton;
TextView closeButton2;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_quiz);

    final MediaPlayer mp = new MediaPlayer();

    //Dialog 1
    createDialog();
    Button dialogButton = (Button) findViewById(R.id.dialogbtn);
    dialogButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            dialog.show();

        }
    });

    closeButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            dialog.dismiss();
        }
    });
    //end Dialog 1

    //Dialog 2
    createDialog2();
    Button dialogButton2 = (Button) findViewById(R.id.dialogbtn2);
    dialogButton2.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            dialog2.show();

        }
    });


    closeButton2.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            dialog2.dismiss();
        }
    });
    //end Dialog 2

    Button bttON = (Button)findViewById(R.id.bttON);

    bttON.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            mp.setVolume(0,0);


        }


    });




    TextView shareTextView = (TextView) findViewById(R.id.share);
    shareTextView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent myIntent = new Intent(Intent.ACTION_SEND);
            myIntent.setType("text/plain");
            myIntent.putExtra(Intent.EXTRA_SUBJECT, "Hello!");
            myIntent.putExtra(Intent.EXTRA_TEXT, "My highscore in Quizzi is very high! I bet you can't beat me except you are cleverer than me. Download the app now! https://play.google.com/store/apps/details?id=amapps.impossiblequiz");
            startActivity(Intent.createChooser(myIntent, "Share with:"));
        }
    });

    mQuestionLibrary.shuffle();


    setSupportActionBar((Toolbar) findViewById(R.id.nav_action));
    DrawerLayout mDrawerLayout = (DrawerLayout) findViewById(R.id.drawerLayout);
    mToggle = new ActionBarDrawerToggle(this, mDrawerLayout, R.string.open, R.string.close);
    mDrawerLayout.addDrawerListener(mToggle);
    mToggle.syncState();
    getSupportActionBar().setDisplayHomeAsUpEnabled(true); // Able to see the Navigation Burger "Button"

    ((NavigationView) findViewById(R.id.nv1)).setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(MenuItem menuItem) {
            switch (menuItem.getItemId()) {
                case R.id.nav_stats:
                    startActivity(new Intent(QuizActivity.this, Menu2.class));
                    break;
                case R.id.nav_about:
                    startActivity(new Intent(QuizActivity.this, Menu3.class));
                    break;
            }

            return true;
        }
    });

    mScoreView = (TextView) findViewById(R.id.score_score);
    mQuestionView = (TextView) findViewById(R.id.question);
    mButtonChoice1 = (Button) findViewById(R.id.choice1);
    mButtonChoice2 = (Button) findViewById(R.id.choice2);
    mButtonChoice3 = (Button) findViewById(R.id.choice3);

    final List<Button> choices = new ArrayList<>();
    choices.add(mButtonChoice1);
    choices.add(mButtonChoice2);
    choices.add(mButtonChoice3);

    updateQuestion();



    for (final Button choice : choices) {
        choice.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View view) {
                if (choice.getText().equals(mAnswer)) {


                    try {
                        mp.reset();
                        AssetFileDescriptor afd;
                        afd = getAssets().openFd("sample.mp3");
                        mp.setDataSource(afd.getFileDescriptor(),afd.getStartOffset(),afd.getLength());
                        mp.prepare();
                        mp.start();
                    } catch (IllegalStateException e) {
                        e.printStackTrace();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                    updateScore();
                    updateQuestion();
                    Toast.makeText(QuizActivity.this, "Correct", Toast.LENGTH_SHORT).show();



                } else {
                    Toast.makeText(QuizActivity.this, "Wrong... Try again!", Toast.LENGTH_SHORT).show();

                    Intent intent = new Intent(QuizActivity.this, Menu2.class);
                    intent.putExtra("score", mScore); // pass score to Menu2
                    startActivity(intent);
                }
            }
        });
    }

}

private void updateQuestion() {
    if (mQuestionNumber < mQuestionLibrary.getLength()) {
        mQuestionView.setText(mQuestionLibrary.getQuestion(mQuestionNumber));
        mButtonChoice1.setText(mQuestionLibrary.getChoice1(mQuestionNumber));
        mButtonChoice2.setText(mQuestionLibrary.getChoice2(mQuestionNumber));
        mButtonChoice3.setText(mQuestionLibrary.getChoice3(mQuestionNumber));
        mAnswer = mQuestionLibrary.getCorrectAnswer(mQuestionNumber++);
    } else {
        Toast.makeText(QuizActivity.this, "Last Question! You are very intelligent!", Toast.LENGTH_SHORT).show();
        Intent intent = new Intent(QuizActivity.this, Menu2.class);
        intent.putExtra("score", mScore);
        startActivity(intent);
    }
}

private void updateScore() {
    mScoreView.setText(String.valueOf(++mScore));

    SharedPreferences mypref = getPreferences(MODE_PRIVATE);
    int highScore = mypref.getInt("highScore", 0);

    if (mScore > highScore) {
        SharedPreferences.Editor editor = mypref.edit();
        editor.putInt("highScore", mScore);
        editor.apply();
    }
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    return mToggle.onOptionsItemSelected(item) || super.onOptionsItemSelected(item);
}

private void createDialog() {
    dialog = new Dialog(this);
    dialog.setTitle("Tutorial");
    dialog.setContentView(R.layout.popup_menu1_1);
    closeButton = (TextView) dialog.findViewById(R.id.closeTXT);
}


private void createDialog2() {
    dialog2 = new Dialog(this);
    dialog2.setTitle("Settings");
    dialog2.setContentView(R.layout.popup_menu1_2);
   closeButton2 = (TextView) dialog2.findViewById(R.id.closeTXT2);

}

logcat的：

    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                                                                     at shapy.appz.QuizActivity.onCreate(QuizActivity.java:96)

Answer 1

您从findViewById和closeButton

获取closeButton2

Answer 2

如MediaPlayer类的文档[此处]（https://developer.android.com/reference/android/media/MediaPlayer.html#setVolume(float，float）所示，所需参数为浮点数。你可以尝试这样做：

public void onClick(View v) {
    mp.setVolume( 0.0, 0.0 );
}

看看是否能解决你的问题。

<强> setVolume

在API级别1中添加

void setVolume (float leftVolume, float rightVolume)

设置此播放器的音量。建议使用此API来平衡应用程序中音频流的输出。除非您正在编写用于控制用户设置的应用程序，否则应优先使用此API，以设置特定类型的所有流的卷的setStreamVolume(int, int, int)。请注意，传递的卷值是0.0到1.0范围内的原始标量。 UI控件应该以对数方式进行缩放。

<强>参数

leftVolume  float: left volume scalar

rightVolume float: right volume scalar

实现静音按钮时应用程序崩溃

2 个答案: