从安卓中的另一个类调用mainactivity中的java方法

3 周，3 日 Questions & Answers 3132

我有一个带有文本视图和三个图像按钮的列表视图。我试图在单击其中一个图像按钮时从主页调用一个方法。但我有一个错误：

  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 '安卓.content.res.Resources 安卓.content.Context.getResources()' on a null object reference
                                                                            at 安卓.content.ContextWrapper.getResources(ContextWrapper.java:92)
                                                                            at 安卓.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:81)
                                                                            at 安卓.media.MediaPlayer.create(MediaPlayer.java:1007)
                                                                            at 安卓.media.MediaPlayer.create(MediaPlayer.java:990)
                                                                            at com.example.lama.audiotest.Main3Activity.play(Main3Activity.java:35)
                                                                            at com.example.lama.audiotest.CustomAdapter$1.onClick(CustomAdapter.java:83)
                                                                            at 安卓.view.View.performClick(View.java:5697)
                                                                            at 安卓.view.View$PerformClick.run(View.java:22526)
                                                                            at 安卓.os.Handler.handleCallback(Handler.java:739)
                                                                            at 安卓.os.Handler.dispatchMessage(Handler.java:95)
                                                                            at 安卓.os.Looper.loop(Looper.java:158)
                                                                            at 安卓.app.ActivityThread.main(ActivityThread.java:7229)
                                                                            at java.lang.reflect.Method.invoke(Native Method)
                                                                            at com.安卓.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                                                                            at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:1120)

我试着移动这条线：

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

到onCreate（）；然后它工作了，但是我想播放不同的音频，这取决于我用play方法得到的字符串。如何解决这个问题

我的主要活动：

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;
}
}

public interface ItemActionListener{ void onpause(DataModel model); void onStop(DataModel model); void onPlay(DataModel model); } private ItemActionListener onItemActionlListener;

CustomAdapter customAdapter = new CustomAdapter(); customAdapter.setOnItemActionlListener(onItemActionListener); listView.setAdapter(customAdapter); private CustomAdapter .ItemActionListener onItemActionListener=new CustomAdapter .ItemActionListener() { @Override public void onpause(DataModel model) { //Call your pause() method written in activity class pause(); } @Override public void onStop(DataModel model) { //Call your stop() method written in activity class stop(); } @Override public void onPlay(DataModel model) { //Call your play() method written in activity class play(); } }; In your adapter class pause.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View view) { if(onItemActionlListener !=null){ onItemActionlListener.onpause(dataModel); } } });

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")); }

@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); }

共 (3) 个答案

# 1 楼答案

在适配器类中，您正在创建一个新的Main3Activity实例，它是不同的实例，这就是为什么您会得到NullPointerException。下面给出了您需要做的一切

在适配器类内创建接口

为此接口创建一个setter

 public void setOnItemActionlListener(ItemActionListener onItemActionlListener) {
            this.onItemActionlListener = onItemActionlListener;
        }

在活动类中使用此接口，如下所示

# 2 楼答案

在CustomAdapter中，您正在创建一个新的Main3Activity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，您将得到一个空指针异常

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

然后，回到你的主要活动。类，将活动实例传递给CustomAdapter：

# 3 楼答案
您需要能够从适配器引用外部类

我假设适配器是在Activity类中定义的。然后，您所需要做的就是直接调用该方法，而不使用前缀，它就会找到它

或者，为了清楚起见，您可以使用：
```
Main3Activity.this.play(xxxx)
```
在监听器内部

Python中文网

有 Java 编程相关的问题?

从安卓中的另一个类调用mainactivity中的java方法

共 (3) 个答案

# 1 楼答案

# 2 楼答案

# 3 楼答案