java - Android MediaPlayer 在它自己的类中导致应用程序崩溃

Question

我为 MediaPlayer 创建了一个类，以便我可以从多个类访问它，因为我需要在一次 Activity 中启动媒体并在另一个 Activity 中停止它。 当尝试调用 mediaPlayer 时，我收到错误。 我不明白我哪里出了问题，因为我不完全理解这个错误。 在为 MediaPlayer 创建类之前，它正在工作，但它不允许我做我想做的事情。 有人可以建议我如何解决我的问题或访问 MediaPlayer 的不同方式。提前致谢。

MainActivity 类是我调用媒体播放器的地方。

public class MainActivity extends AppCompatActivity {
    Music gm = new Music();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Display the Main Activity.
        setContentView(R.layout.activity_main);

        gm.menuSound(true);
    }
}

<小时/>

MediaPlayer 类是创建媒体播放器以及启动和停止媒体播放器的位置。

public class Music extends gameActivity{
    Context context;

    MediaPlayer menu;
    MediaPlayer soundtrack;
    MediaPlayer death;
    MediaPlayer start;

    public void menuSound(boolean x) {
        if(menu == null) {
            menu = MediaPlayer.create(context, R.raw.menumusic);
            menu.setVolume(100, 100);
        }

        if(x)
            menu.start();
        if(!x)
            menu.stop();
    }
}

<小时/>

--------- beginning of crash E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.sam.myfirstapp, PID: 2395 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.sam.myfirstapp/com.example.sam.myfirstapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6077) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference at android.media.MediaPlayer.create(MediaPlayer.java:931) at android.media.MediaPlayer.create(MediaPlayer.java:914) at com.example.sam.myfirstapp.Music.menuSound(Music.java:21) at com.example.sam.myfirstapp.MainActivity.onCreate(MainActivity.java:18) at android.app.Activity.performCreate(Activity.java:6664) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)  at android.app.ActivityThread.-wrap12(ActivityThread.java)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)  at android.os.Handler.dispatchMessage(Handler.java:102)  at android.os.Looper.loop(Looper.java:154)  at android.app.ActivityThread.main(ActivityThread.java:6077)  at java.lang.reflect.Method.invoke(Native Method)  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) 

Answer 1

错误是由于上下文为空，您尚未在音乐类中初始化上下文。

public class MainActivity extends AppCompatActivity { 
Music gm ;

@Override 
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState); 

    // Display the Main Activity. 
    setContentView(R.layout.activity_main); 
      gm = new Music(this); // pass context here to music  
    gm.menuSound(true); 
}

和音乐课是这样的:

public class Music extends gameActivity{ 
Context context;

MediaPlayer menu;
MediaPlayer soundtrack;
MediaPlayer death;
MediaPlayer start;

 public Music(Context context){
       this.context =context; // initialising context here
      }

public void menuSound(boolean x) {
    if(menu == null) { 
        menu = MediaPlayer.create(context, R.raw.menumusic); // here context was causing null pointer exception as context was null
        menu.setVolume(100, 100); 
    } 

    if(x) 
        menu.start(); 
    if(!x) 
        menu.stop(); 
}