java - 从android中的另一个类调用mainactivity中的方法

Question

我有一个带有 TextView 和三个图像按钮的 ListView 。当单击其中一个图像按钮时，我试图从主页调用一种方法。但我收到了这个错误:

  04-19 13:02:48.021 30996-30996/com.example.lama.audiotest E/AndroidRuntime: FATAL EXCEPTION: main
                                                                        Process: com.example.lama.audiotest, PID: 30996
                                                                        java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
                                                                            at android.content.ContextWrapper.getResources(ContextWrapper.java:92)
                                                                            at android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:81)
                                                                            at android.media.MediaPlayer.create(MediaPlayer.java:1007)
                                                                            at android.media.MediaPlayer.create(MediaPlayer.java:990)
                                                                            at com.example.lama.audiotest.Main3Activity.play(Main3Activity.java:35)
                                                                            at com.example.lama.audiotest.CustomAdapter$1.onClick(CustomAdapter.java:83)
                                                                            at android.view.View.performClick(View.java:5697)
                                                                            at android.view.View$PerformClick.run(View.java:22526)
                                                                            at android.os.Handler.handleCallback(Handler.java:739)
                                                                            at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                            at android.os.Looper.loop(Looper.java:158)
                                                                            at android.app.ActivityThread.main(ActivityThread.java:7229)
                                                                            at java.lang.reflect.Method.invoke(Native Method)
                                                                            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                                                                            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)

我尝试移动这一行:

mp= MediaPlayer.create(Main3Activity.this,R.raw.alfatiha);

到 onCreate();然后它起作用了，但我想根据我通过播放方法获得的字符串来播放不同的音频。怎么解决这个问题？

我的主要 Activity :

public class Main3Activity extends Activity {

private static final String TAG = Main3Activity.class.getSimpleName();
static MediaPlayer mp;

/**
 * Called when the activity is first created.
 */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main3);

    ListView listView = (ListView) findViewById(R.id.list_data);

    CustomAdapter customAdapter = new CustomAdapter();

    listView.setAdapter(customAdapter);

}

public void play(String name){
    if (name.equals("الفاتحه")){
        mp= MediaPlayer.create(Main3Activity.this,R.raw.alfatiha);
        mp.start();
    }
}

public void pause(){
        mp.pause();
}

public void stop(){
    mp.stop();
}
}

我的列表适配器:

public class CustomAdapter extends BaseAdapter {
Main3Activity  main= new Main3Activity();
private static final String TAG = CustomAdapter.class.getSimpleName();
ArrayList<DataModel> listArray;

public CustomAdapter() {
    listArray = new ArrayList<DataModel>(5);
    listArray.add(new DataModel("الفاتحه", 5, 1.8, "Java"));
    listArray.add(new DataModel("البقرة", 10, 2.8, "Python"));
    listArray.add(new DataModel("ال عمران", 15, 3.8, "Django"));

}

@Override
public int getCount() {
    return listArray.size();    // total number of elements in the list
}

@Override
public Object getItem(int i) {
    return listArray.get(i);    // single item in the list
}

@Override
public long getItemId(int i) {
    return i;                   // index number
}

@Override
public View getView(int index, View view, final ViewGroup parent) {

    if (view == null) {
        LayoutInflater inflater = LayoutInflater.from(parent.getContext());
        view = inflater.inflate(R.layout.list, parent, false);
    }

    final MediaPlayer mp;


    final DataModel dataModel = listArray.get(index);

    TextView textView = (TextView) view.findViewById(R.id.Itemname);
    textView.setText(dataModel.getName());

    ImageButton play = (ImageButton) view.findViewById(R.id.btnplay);
    ImageButton pause = (ImageButton) view.findViewById(R.id.btnpause);
    ImageButton stop = (ImageButton) view.findViewById(R.id.btnstop);

    play.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {


            main.play(dataModel.getName());
           // Toast.makeText(parent.getContext(), "you clicked play for: " + dataModel.getName(), Toast.LENGTH_SHORT).show();
        }
    });

    pause.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            main.pause();

          //  Toast.makeText(parent.getContext(), "you clicked pause for: " + dataModel.getName(), Toast.LENGTH_SHORT).show();
        }
    });
    stop.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            main.stop();
          //  Toast.makeText(parent.getContext(), "you clicked stop for: " + dataModel.getName(), Toast.LENGTH_SHORT).show();
        }
    });



    view.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Log.d(TAG, "string: " + dataModel.getName());
            Log.d(TAG, "int: " + dataModel.getAnInt());
            Log.d(TAG, "double: " + dataModel.getaDouble());
            Log.d(TAG, "otherData: " + dataModel.getOtherData());

            Toast.makeText(parent.getContext(), "view clicked: " + dataModel.getOtherData(), Toast.LENGTH_SHORT).show();
        }
    });

    return view;
}
}   

Answer 1

在您的 CustomAdapter 中，您将创建 Main3Activity 的新实例，而不是将当前的 Main3Activity 实例作为自定义适配器的参数传递。因此，当您尝试访问 Main3Activity 方法时，您会收到空指针异常。

要解决您的问题，请修改您的 CustomAdapter 构造函数以允许 Main3Activity 作为参数:

public class CustomAdapter extends BaseAdapter {
    Main3Activity  main;
    private static final String TAG = CustomAdapter.class.getSimpleName();
    ArrayList<DataModel> listArray;

    public CustomAdapter(Main3Activity main) {
        this.main = main;
        listArray = new ArrayList<DataModel>(5);
        listArray.add(new DataModel("الفاتحه", 5, 1.8, "Java"));
        listArray.add(new DataModel("البقرة", 10, 2.8, "Python"));
        listArray.add(new DataModel("ال عمران", 15, 3.8, "Django"));
    }

然后，回到 Main3Activity.class，将 Activity 实例传递给 CustomAdapter:

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main3);
    ListView listView = (ListView) findViewById(R.id.list_data);
    CustomAdapter customAdapter = new CustomAdapter(this);
    listView.setAdapter(customAdapter);
}