Java Android接口回调引发空指针异常

我的所有活动/片段等都有一个asyntask，但现在我正在为每个活动实现一个接口，但是我的接口回调总是空的，我不知道为什么。每个调用asyncTask的活动都实现接口

实现接口并调用异步任务的我的类

public class MainActivity extends Activity implements MainActivityAsyncInterface, OnClickListener, UserPictureDialogInterface {

private DrawerLayout            moodydrawerLayout;

private HashMap<String, String> organizedCourses    = new HashMap<String, String>();

// ManSession Manager Class
ManSession                      session;

private long                    startTime;
private long                    endTime;
private ModDevice               md;

private float                   screenX;

private float                   screenY;

private int                     shotType            = ShowcaseView.TYPE_ONE_SHOT;

private MoodleUser              currentUser;

private String                  url;

private String                  token;

private String                  userId;

private static long             backPressed;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // The following line triggers the initialization of ACRA
.......

    session = new ManSession(getApplicationContext());
    url = session.getValues(ModConstants.KEY_URL, null);
    token = session.getValues(ModConstants.KEY_TOKEN, null);
    userId = session.getValues(ModConstants.KEY_ID, null);

    new DataAsyncTask(this,).execute(url, token, EXAMPLE.CORE_USER_GET_USERS_BY_ID, userId, MainActivity.class.getSimpleName());

    populateLeft();
    populateRight();
    receiveNotification();
    initDemoOverlay();
    drawerLayoutListener();
    warningMessage(checkConnection(), Toast.LENGTH_LONG, null, getString(R.string.no_internet));

    ChangeLogListView sad = new ChangeLogListView(getApplicationContext());

}

public class DataAsyncTask extends AsyncTask<Object, Void, Object> {
Object                              jObj    = null;
public MainActivityAsyncInterface   mainActivityInterface;
private ProgressDialog              dialog;
private CountDownTimer              cvt     = createCountDownTimer();
private Context                     context;
private MoodleServices              webService;
private String                      parentActivity;
private String                      fillTheSpace;

public DataAsyncTask(Context context) {
    this.context = context;
    dialog = new ProgressDialog(context);
}

@Override
protected void onPreExecute() {
    super.onPreExecute();
    cvt.start();
}

@Override
protected Object doInBackground(Object... params) {
    String urlString = (String) params[0];
    String token = (String) params[1];
    webService = (MoodleServices) params[2];
    Object webServiceParams = params[3];
    parentActivity = (String) params[4];

        case EXAMPLE:
            InputStream inputStream = new URL(urlString).openStream();
            Drawable drawable = Drawable.createFromStream(inputStream, null);
            inputStream.close();
            return drawable;

        default:
            return null;

        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return null;
}

/**
 * <p>
 * Method that parses a supposed id list object
 * </p>
 *
 * @param Object
 *            ids - The object to be parsed to Long[].
 * @return resultList - The ids List
 */
private Long[] parseIds(Object ids) {

    Long[] resultList = null;

    try {
        resultList = (Long[]) ids;
    } catch (Exception e) {
        resultList = new Long[1];

        resultList[0] = (Long) ids;
    }

    return resultList;
}

@Override
protected void onPostExecute(Object obj) {
    cvt.cancel();

    if (dialog != null && dialog.isShowing())   
        dialog.dismiss();

    switch (webService) {
    case EXAMPLE:
        if (parentActivity.equalsIgnoreCase(MainActivity.class.getSimpleName()))
            mainActivityInterface.userAsyncTaskResult(obj); \\This the line 173 and the obj != null and mainActivityInterface is null

        if (parentActivity.equalsIgnoreCase(UserDetailsActivity.class.getSimpleName()))
            fillTheSpace = "TODO - Interface for each parent class";

        if (parentActivity.equalsIgnoreCase(FragTopicsPreview.class.getSimpleName()))
            fillTheSpace = "TODO - Interface for each parent class";

        if (parentActivity.equalsIgnoreCase(FragTopics.class.getSimpleName()))
            fillTheSpace = "TODO - Interface for each parent class";
        break;

    case EXAMPLE2:
        if (parentActivity.equalsIgnoreCase(MainActivity.class.getSimpleName()))
            mainActivityInterface.userAsyncTaskResult(obj);

        if (parentActivity.equalsIgnoreCase(UserDetailsActivity.class.getSimpleName()))
            fillTheSpace = "TODO - Interface for each parent class";

        if (parentActivity.equalsIgnoreCase(FragTopicsPreview.class.getSimpleName()))
            fillTheSpace = "TODO - Interface for each parent class";

        if (parentActivity.equalsIgnoreCase(FragTopics.class.getSimpleName()))
            fillTheSpace = "TODO - Interface for each parent class";
        break;

    default:
        break;
    }
}

private CountDownTimer createCountDownTimer() {
    return new CountDownTimer(250, 10) {
        @Override
        public void onTick(long millisUntilFinished) {

        }

        @Override
        public void onFinish() {
            dialog = new ProgressDialog(context);
            dialog.setMessage("Loading...");
            dialog.setCancelable(false);
            dialog.setCanceledOnTouchOutside(false);
            dialog.show();
        }
    };
}

---

根据我的经验，接口不应该在我的代码中发送nullPointer异常，接口也不需要初始化，但在这一点上，我将所有选项都放在了表上。

这是因为您从未将侦听器传递到任务中

您只发送构造函数中的上下文，但它应该如下所示：

public DataAsyncTask(Context context , MainActivityAsyncInterface mainActivityInterface) {
    this.context = context;
    this.mainActivityInterface = mainActivityInterface;
    dialog = new ProgressDialog(context);
}

在活动中，将此添加到开始任务的位置

  YourAcivity.this. 

更新： 回答你的问题,

您可以创建BaseActivity，它将扩展活动并实现侦听器。

---

然后，所有活动都必须重写侦听器函数。

如果

MainActivityAsyncInterface

是活动实现的接口，则需要将其传递给异步任务。当前，

mainActivityInterface

从未初始化，并且始终为空，因此您会得到异常

您可以在构造函数中传递引用

public DataAsyncTask(Context context, MainActivityAsyncInterface mainActivityInterface) {
    this.context = context;
    this.mainActivityInterface = mainActivityInterface;
    dialog = new ProgressDialog(context);
}

---

哪一行是onPostExecute（DataAsyncTask.java:173）？我对标记行mainActivityInterface.userAsyncTaskResult（obj）的代码有评论\\这是第173行和obj！=null且mainActivityInterface为null您没有初始化mainActivityInterface，如果您在哪里？您从未将

mainActivityInterface

设置为值，因此它为null。值？！mainActivityInterface这是一个接口，你建议初始化接口吗？！像mainActivityInterface=toSomething？我已经看到了这种可能性，但问题是我会有6个以上的接口，每个活动的每个接口，所以唯一的解决方案是有6个构造函数，接口有一个泛型类型吗？你必须先了解，除非在某个地方初始化侦听器，否则无法使用它。@firetrap您似乎只在界面中使用了

userAsyncTaskResult

方法。它应该是定义该方法的接口。您可以创建扩展活动的BaseActivity并实现侦听器，然后您的所有活动都将扩展该类。没关系，异步任务类无权访问您的活动，除非发送您的侦听器，即您的活动。这是。

public DataAsyncTask(Context context, MainActivityAsyncInterface mainActivityInterface) {
    this.context = context;
    this.mainActivityInterface = mainActivityInterface;
    dialog = new ProgressDialog(context);
}