从其活动调用片段适配器时出现NullPointerException

Question

我创建了具有4个不同内容的片段的ViewPager。每个片段在按钮上都有一个按钮栏。当我点击其中一个按钮时，应该更新当前片段的数据。因此，我在活动中实现了一些回调方法。但不幸的是，每次我调用方法reloadArticleListFragment（）时，当我点击按钮栏中的一个按钮时，我得到一个NullPointerException。我猜目前的片段没有附加到活动中，但我不知道为什么。你有什么建议吗？

感谢您的帮助！

MainActivity.java

public class MainActivity extends FragmentActivity implements ActionBar.TabListener, DataFragment.Callbacks { 

... 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.main); 

    // setup ui and instantiate some variables (missing in the example) 

    mDataFragment = (AbstractDataFragment) mPagerAdapter.getItem(mViewPager.getCurrentItem()); 
    mAdapter = mDataFragment.getAdapter(); 
    rssService.execute(RssService.BLOG_URL); 

    sharedPreferences = PreferenceManager.getDefaultSharedPreferences(this); 
    sharedPreferences.registerOnSharedPreferenceChangeListener(new SharedPreferences.OnSharedPreferenceChangeListener() { 
     @Override 
     public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) { 
      if (key.equalsIgnoreCase("feedScope")) { 
       mPagerAdapter.setFeedScope(Integer.parseInt(sharedPreferences.getString(key, "0"))); 
       reloadArticleListFragment(); 
      } else if (key.equalsIgnoreCase("countySelection")) { 
       reloadArticleListFragment(); 
      } 
     } 
    }); 
} 

private void setupUI() { 
    mViewPager = (ViewPager) findViewById(R.id.pager); 
    mActionBar = getActionBar(); 
    mPagerAdapter = new TabsPagerAdapter(getFragmentManager(), rssService); 

    mViewPager.setAdapter(mPagerAdapter); 
    mActionBar.setHomeButtonEnabled(false); 
    mActionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); 

    // Adding Tabs 
    ... 

    mViewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() { 

     @Override 
     public void onPageSelected(int position) { 
      mActionBar.setSelectedNavigationItem(position); 
     } 

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

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

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

@Override 
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) { 
    mViewPager.setCurrentItem(tab.getPosition()); 
} 

@Override 
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft) { 

} 

@Override 
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft) { 

} 

private void reloadArticleListFragment() { 
    mPagerAdapter.setFilterType(filterType); 
    if (mDataFragment != null) { 
     mDataFragment.setFilterType(filterType); 
     mDataFragment.refreshData(false); 
    } 
} 
... 
} 

AbstractDataFragment.java

public abstract class AbstractDataFragment extends Fragment implements DataFragment { 

... 

@Override 
public void onAttach(Activity activity) { 
    super.onAttach(activity); 
    if (!(activity instanceof Callbacks)) { 
     throw new IllegalStateException("Activity must implement fragment's callbacks."); 
    } 
    callbacks = (Callbacks) activity; 
} 

@Override 
public void onDetach() { 
    super.onDetach(); 
    callbacks = sDummyCallbacks; 
} 

@Override 
public void refreshData(boolean force) { 
    if (force) { 
     if (rssService != null) { 
      rssService = new RssService(getActivity()); 
     } 
     rssService.execute(BLOG_URL); 
    } else { 
     setFilterType(filterType); 
    } 
    adapter.notifyDataSetChanged(); // NullPointerException is thrown here 
} 
} 

ArticleListFragment.java

public class ArticleListFragment extends AbstractDataFragment { 
private ListView listView; 

public static ArticleListFragment newInstance(SelectionType selectionType, FilterType filterType, int feedScope) { 
    ArticleListFragment fragment = new ArticleListFragment(); 
    Bundle args = new Bundle(); 
    args.putSerializable(ARG_SELECTION_TYPE, selectionType); 
    args.putSerializable(ARG_FILTER_TYPE, filterType); 
    args.putInt(ARG_FEED_SCOPE, feedScope); 
    fragment.setArguments(args); 
    return fragment; 
} 

@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
    View view = inflater.inflate(R.layout.article_list, container, false); 
    this.listView = (ListView) view.findViewById(R.id.listView); 
    this.listView.setEmptyView(view.findViewById(R.id.emptyView)); 
    return view; 
} 

@Override 
public void onActivityCreated(@Nullable Bundle savedInstanceState) { 
    super.onActivityCreated(savedInstanceState); 
    if (savedInstanceState != null) { 
     if (savedInstanceState.containsKey(STATE_ACTIVATED_POSITION)) { 
      setActivatedPosition(savedInstanceState.getInt(STATE_ACTIVATED_POSITION)); 
     } 
    } 
    // init image loader if it is not initialized before 
    if (!ImageLoader.getInstance().isInited()) { 
     initImageLoader(); 
    } 
    initializeAdapter(); 
    // set click listener 
    if (getDataView() != null) { 
     getDataView().setOnItemClickListener(new AdapterView.OnItemClickListener() { 
      @Override 
      public void onItemClick(AdapterView<?> parent, View view, int position, long id) { 
       getCallbacks().onItemSelected(String.valueOf(position)); 
      } 
     }); 
    } 
} 
... 
} 

//编辑：这里是堆栈跟踪

09-05 13:20:03.035 1237-1237/de.ssncomputer.retternews E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    Process: de.ssncomputer.retternews, PID: 1237 
    java.lang.RuntimeException: Unable to resume activity {de.ssncomputer.retternews/de.ssncomputer.retternews.views.activities.MainActivity}: java.lang.NullPointerException 
     at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2788) 
     at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2817) 
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2250) 
     at android.app.ActivityThread.access$800(ActivityThread.java:135) 
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) 
     at android.os.Handler.dispatchMessage(Handler.java:102) 
     at android.os.Looper.loop(Looper.java:136) 
     at android.app.ActivityThread.main(ActivityThread.java:5017) 
     at java.lang.reflect.Method.invokeNative(Native Method) 
     at java.lang.reflect.Method.invoke(Method.java:515) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
     at dalvik.system.NativeStart.main(Native Method) 
    Caused by: java.lang.NullPointerException 
     at de.ssncomputer.retternews.views.fragments.AbstractDataFragment.refreshData(AbstractDataFragment.java:202) 
     at de.ssncomputer.retternews.views.activities.MainActivity.reloadArticleListFragment(MainActivity.java:186) 
     at de.ssncomputer.retternews.views.activities.MainActivity.onResume(MainActivity.java:116) 

Answer 1

如果（rssService！= NULL）{ rssService =新RssService（getActivity（））; }

我想你想给

如果（rssService == NULL）{ rssService =新RssService（getActivity（））; }