尝试在空对象引用上调用虚拟方法'void android.media.MediaPlayer.setLooping（boolean）'

Question

我有个问题要解决：

我正在研究Android Studio中的一个程序，该程序实质上循环播放故事板列表（如幻灯片显示），并且正在使用viewPager组件来执行此操作。这有点酷，因为我不需要按钮。我可以滑动（尽管我正在设置），但是，我也尝试将声音中的幻灯片（在本例中为情节提要）链接起来。问题是该应用程序崩溃了，并且在logcat中给了我一些空错误。这是我到目前为止所做的。 。 。

我已经创建了slideAdapter活动：

package com.example.******.*************;
import android.content.Context;
import android.support.v4.view.PagerAdapter;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.LinearLayout;

public class SlideAdapter extends PagerAdapter {

    Context context;
    LayoutInflater inflater;

    // List of Images

    public int[] lst_storyboard = {
            R.drawable.imgmob02, R.drawable.imgmob03, R.drawable.imgmob04,
            R.drawable.imgmob05, R.drawable.imgmob06, R.drawable.imgmob07,
            R.drawable.imgmob08, R.drawable.imgmob09, R.drawable.imgmob010,
            R.drawable.imgmob011,R.drawable.imgmob012,R.drawable.imgmob013
    };

    // List of mp3 raw audio

    public int[] lst_audio_eng = {
            R.raw.audio_eng_01, R.raw.audio_eng_02, R.raw.audio_eng_03,
            R.raw.audio_eng_04, R.raw.audio_eng_05, R.raw.audio_eng_06,
            R.raw.audio_eng_07, R.raw.audio_eng_08, R.raw.audio_eng_09,
            R.raw.audio_eng_010, R.raw.audio_eng_011, R.raw.audio_eng_012
    };

    public SlideAdapter(Context context) {
        this.context = context;
    }

    @Override
    public int getCount() {
        return lst_storyboard.length;
    }

    @Override
    public boolean isViewFromObject(View view, Object object) {
        return (view==(LinearLayout)object);
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        inflater = (LayoutInflater) context.getSystemService(context.LAYOUT_INFLATER_SERVICE);
        View view = inflater.inflate(R.layout.activity_slide_adapter,container,false);
        LinearLayout layoutslide = (LinearLayout) view.findViewById(R.id.slidelinearlayout);
        ImageView imgslide = (ImageView)  view.findViewById(R.id.slideimg);

        imgslide.setImageResource(lst_storyboard[position]);


        container.addView(view);
        return view;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        container.removeView((LinearLayout)object);
    }

}

这是slideactivity.xml：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:gravity="center"
    android:id="@+id/slidelinearlayout">

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/slideimg"
        android:contentDescription="@string/todo" />
</LinearLayout>

这是mainActivity文件：

    package com.example.******.***********;

import android.media.MediaPlayer;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import static com.example.gman5541.childrensbookconcept2.SlideAdapter.lst_audio_eng;

public class MainActivity extends AppCompatActivity {
    private ViewPager viewPager;
    private SlideAdapter myadapter;

    private MediaPlayer mp;
    private MediaPlayer BG;


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        myadapter = new SlideAdapter(this);
        viewPager = (ViewPager) findViewById(R.id.viewpager);

        viewPager.addOnPageChangeListener(player);

        viewPager.setAdapter(myadapter);
        viewPager.setCurrentItem(0);

        BG = MediaPlayer.create(getApplicationContext(), R.raw.audio_eng_01);
        BG.setLooping(true);
        BG.setVolume(100, 100);
        BG.start();
        mp = MediaPlayer.create(MainActivity.this,lst_audio_eng[0]);
        mp.start();
    }




    @Override
    protected void onPause() {
        super.onPause();
        BG.pause();
    }

    @Override
    protected void onResume() {
        super.onResume();
        BG.start();
    }

    public ViewPager.OnPageChangeListener player = new ViewPager.OnPageChangeListener() {

        @Override
        public void onPageSelected(int arg1)
        {
            mp = MediaPlayer.create(MainActivity.this, lst_audio_eng[arg1]);
            mp.start();
        }

        @Override
        public void onPageScrolled(int arg0, float arg1, int arg2)
        {
        }

        @Override
        public void onPageScrollStateChanged(int arg0)
        {
        }
    };

}

这是xml文件：

   <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.gman5541.childrensbookconcept2.MainActivity">

    <android.support.v4.view.ViewPager
        android:id="@+id/viewpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

    </android.support.v4.view.ViewPager>

</LinearLayout>

这是我从logcat调试中得到的：

--------- beginning of crash
2018-11-25 18:40:07.511 15354-15354/com.example.gman5541.childrensbookconcept2 E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.gman5541.childrensbookconcept2, PID: 15354
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.gman5541.childrensbookconcept2/com.example.gman5541.childrensbookconcept2.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.setLooping(boolean)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.setLooping(boolean)' on a null object reference
        at com.example.gman5541.childrensbookconcept2.MainActivity.onCreate(MainActivity.java:31)
        at android.app.Activity.performCreate(Activity.java:7009)
        at android.app.Activity.performCreate(Activity.java:7000)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1214)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2731)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6494) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807) 

尽管我也正在尝试使用带有“声音”开关的“播放”按钮来打开和关闭声音，但现在，我只是想将一个列表（故事板）与另一个列表（音频）进行匹配。如果我注释掉MediaPlayer行，则该程序有效。请帮忙。