在ViewPager中使用Android ArrayAdapter會導致NullPointerException

Question

我到處都是草叢，但是我還沒有解決問題。

我在活動內使用ViewPager。 ViewPager包含3個片段：一個是普通片段，第二個片段包含自定義ListView，第三個片段是MapFragment（SupportMapFragment）。

我只使用支持片段。

問題是在活動嘗試填充設置適配器和地圖上標記的片段后，將調用ViewPager適配器（FragmentPagerAdapter）的getItem（）方法。

因此，我想在初始化ViewPager之前使用片段的單例模式來初始化片段：

在活動onCreate（）中：

setContentView(R.layout.act_discover_small);
profiloFragment = new ProfiloFragment();
mapFragment = new MappaMonumentiFragment();
elencoFragment = new ElencoMonumentiFragment();
initViewPager();

initViewPager（）方法為：

private void initViewPager() {
    mPagerAdapter = new PagerAdapter(getSupportFragmentManager());
    mViewPager = (SmartViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mPagerAdapter);
    mViewPager.setOffscreenPageLimit(3);
    mViewPager.setOnPageChangeListener(new OnPageChangeListener() {
        @Override
        public void onPageScrolled(int arg0, float arg1, int arg2) {
            if (!beginSwipe)
                elencoFragment.List.finishRefresh();
            beginSwipe = true;
        }

        @Override
        public void onPageScrollStateChanged(int arg0) {

        }

        @Override
        public void onPageSelected(int arg0) {
            beginSwipe = false;
            lockDrawer(arg0);
            if (arg0 == 0) {
                swipe_left_button.setText("show map");
            } else {
                swipe_left_button.setText("show list");
            }
        }
    });
}

這是ViewPager適配器

class PagerAdapter extends FragmentPagerAdapter {
    public PagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public int getCount() {
        return 3;
    }

    @Override
    public Fragment getItem(int i) {
        Log.i(TAG, "getItem viewpager n." + i);
        switch (i) {
        case 0:
            if (profiloFragment == null)
                profiloFragment = new ProfiloFragment();
            return profiloFragment;
        case 1:
            if (elencoFragment == null)
                elencoFragment = new ElencoMonumentiFragment();
            return elencoFragment;
        case 2:
            if (mapFragment == null)
                mapFragment = new MappaMonumentiFragment();
            return mapFragment;
        }
        return null;
    }
}

嘗試使用此方法中的元素填充第二個片段的ArrayAdapter時，我得到一個NullPointerException（在AsyncTask的onPostExecute（）中執行）：

    public void populateElencoMonumenti(List<SmartPlace> placesList) {
    if (placesList != null) {
        places = (ArrayList<SmartPlace>) placesList;
        //before: adapter.clear(); //NullPointerException at this line
                    //after:
                    if (adapter == null) {
            adapter = new ElencoMonumentiAdapter(ctx, act, app);
        }


        adapter.addAll(places);
        List.setAdapter(adapter);

    } else {
        adapter.clear();
        List.setAdapter(adapter);
    }
}

如果在調用adapter.clear（）之前登錄（適配器== null），則返回true，因此它為null。

如果我嘗試檢查它是否為空並重新初始化，則在適配器的構造函數中得到一個NPE：

   public ElencoMonumentiAdapter(Context ctx, Discover homAct, SmartApplication app) {
    // super(ctx, R.layout.list_row_monument);
    super(ctx, R.layout.list_row_monument, new ArrayList<SmartPlace>()); //NPE here
    Log.e(TAG, "null ctx? " + (ctx == null));
    this.app = app;
    this.ctx = ctx;
}

stacktrace是：

12-18 11:13:07.285: E/AndroidRuntime(20865): FATAL EXCEPTION: main
12-18 11:13:07.285: E/AndroidRuntime(20865): Process: com.Thinple.MiWalks.ui, PID: 20865
12-18 11:13:07.285: E/AndroidRuntime(20865): java.lang.NullPointerException
12-18 11:13:07.285: E/AndroidRuntime(20865):    at android.widget.ArrayAdapter.init(ArrayAdapter.java:310)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:153)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at com.Thinple.MiWalks.ui.adapters.ElencoMonumentiAdapter.<init>(ElencoMonumentiAdapter.java:41)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at com.Thinple.MiWalks.ui.fragments.ElencoMonumentiFragment.populateElencoMonumenti(ElencoMonumentiFragment.java:129)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at com.Thinple.MiWalks.ui.Discover.notifyFragments(Discover.java:328)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at com.Thinple.MiWalks.ui.Discover.populatePlaces(Discover.java:509)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at com.Thinple.MiWalks.threading.tasks.GetPlacesFromDBtask.onPostExecute(GetPlacesFromDBtask.java:53)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at com.Thinple.MiWalks.threading.tasks.GetPlacesFromDBtask.onPostExecute(GetPlacesFromDBtask.java:1)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at android.os.AsyncTask.finish(AsyncTask.java:632)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at android.os.AsyncTask.access$600(AsyncTask.java:177)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at android.os.Handler.dispatchMessage(Handler.java:102)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at android.os.Looper.loop(Looper.java:136)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at android.app.ActivityThread.main(ActivityThread.java:5017)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at java.lang.reflect.Method.invokeNative(Native Method)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at java.lang.reflect.Method.invoke(Method.java:515)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
12-18 11:13:07.285: E/AndroidRuntime(20865):    at dalvik.system.NativeStart.main(Native Method)

我真的找不到解決方案。

預先感謝您的答復。

Answer 1

更改

super(ctx, R.layout.list_row_monument, new ArrayList<SmartPlace>());

new ArrayList<SmartPlace>()是新的arraysit，但是其空沒有填充任何數據

至

super(ctx, R.layout.list_row_monument, list);

列表可能是ArrayList<SmartPlace>() ，該列表傳遞給適配器類的構造函數

看一下公共構造函數

http://developer.android.com/reference/android/widget/ArrayAdapter.html