Question

当我从另一个活动回到FragmentActivity时，我的STACK_TRACE。我搜索了很多如何解决这个问题，我找到了一个线索，我需要保存片段的状态。但是在那篇线索文章中说，当我使用FragmentPagerAdapter时，我不需要保存状态。

所以，我很困惑。我是否正确使用FragmentPagerAdapter？

请帮助我......

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test20140320/com.example.test20140320.MainActivity}: java.lang.NullPointerException
   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2308)
   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2362)
   at android.app.ActivityThread.access$700(ActivityThread.java:168)
   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1329)
   at android.os.Handler.dispatchMessage(Handler.java:99)
   at android.os.Looper.loop(Looper.java:137)
   at android.app.ActivityThread.main(ActivityThread.java:5493)
   at java.lang.reflect.Method.invokeNative(Native Method)
   at java.lang.reflect.Method.invoke(Method.java:525)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1209)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1025)
   at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
   at com.example.test20140320.MainActivity_Fragment_Tab_02.<init>(MainActivity_Fragment_Tab_02.java:98)
   at java.lang.Class.newInstanceImpl(Native Method)
   at java.lang.Class.newInstance(Class.java:1130)
   at android.support.v4.app.Fragment.instantiate(Fragment.java:402)
   at android.support.v4.app.FragmentState.instantiate(Fragment.java:97)
   at android.support.v4.app.FragmentManagerImpl.restoreAllState(FragmentManager.java:1801)
   at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:213)
   at com.example.test20140320.MainActivity.onCreate(MainActivity.java:47)
   at android.app.Activity.performCreate(Activity.java:5372)
   at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1104)
   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2270)
   ... 11 more
java.lang.NullPointerException
   at com.example.test20140320.MainActivity_Fragment_Tab_02.<init>(MainActivity_Fragment_Tab_02.java:98)
   at java.lang.Class.newInstanceImpl(Native Method)
   at java.lang.Class.newInstance(Class.java:1130)
   at android.support.v4.app.Fragment.instantiate(Fragment.java:402)
   at android.support.v4.app.FragmentState.instantiate(Fragment.java:97)
   at android.support.v4.app.FragmentManagerImpl.restoreAllState(FragmentManager.java:1801)
   at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:213)
   at com.example.test20140320.MainActivity.onCreate(MainActivity.java:47)
   at android.app.Activity.performCreate(Activity.java:5372)
   at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1104)
   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2270)
   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2362)
   at android.app.ActivityThread.access$700(ActivityThread.java:168)
   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1329)
   at android.os.Handler.dispatchMessage(Handler.java:99)
   at android.os.Looper.loop(Looper.java:137)
   at android.app.ActivityThread.main(ActivityThread.java:5493)
   at java.lang.reflect.Method.invokeNative(Native Method)
   at java.lang.reflect.Method.invoke(Method.java:525)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1209)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1025)
   at dalvik.system.NativeStart.main(Native Method)

at com.example.test20140320.MainActivity_Fragment_Tab_02.<init>(MainActivity_Fragment_Tab_02.java:98)

我的代码：

public MainActivity_Fragment_Tab_02()
    {
        super();
        mContext = getActivity().getApplicationContext(); //This is line 98
        tab2 = this;
    }

我认为原因是savedInstanceState，但不知道如何，如何以及如何......

以下是我使用FragmentPagerAdapter的内容。我认为这是最值得怀疑的部分。

public class SectionsPagerAdapter extends FragmentPagerAdapter 
    {

        public SectionsPagerAdapter(FragmentManager fm)
        {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) 
        {
            // getItem is called to instantiate the fragment for the given page.
            // Return a DummySectionFragment (defined as a static inner class
            // below) with the page number as its lone argument.


            Fragment fragment = new DummySectionFragment();
            Bundle args = new Bundle();
            args.putInt(DummySectionFragment.ARG_SECTION_NUMBER, position + 1);
            fragment.setArguments(args);

            switch(position)
            {
            case 0:
                Fragment fragment_tab1 = new MainActivity_Fragment_Tab_02(getApplicationContext());
                fragment_tab1.setRetainInstance(true);

                return fragment_tab1;
            case 1:
                Fragment fragment_tab2 = new MainActivity_Fragment_Tab_01(getApplicationContext());
                fragment_tab2.setRetainInstance(true);
                return fragment_tab2;
            case 2:
                Fragment fragment_tab3 =  new MainActivity_Fragment_Tab_03(getApplicationContext());
                fragment_tab3.setRetainInstance(true);
                return fragment_tab3;

            }
            return fragment;
        }

        @Override
        public int getCount() {
            // Show 3 total pages.
            return 3;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            Locale l = Locale.getDefault();
            switch (position) {
            case 0:
                return getString(R.string.title_section1).toUpperCase(l);
            case 1:
                return getString(R.string.title_section2).toUpperCase(l);
            case 2:
                return getString(R.string.title_section3).toUpperCase(l);
            }
            return null;
        }
    }

此代码是否泄漏？我正确使用FragmentPagerAdapter吗？ OTL .... 我该怎么解决它....？请帮帮我！

P.S。首先创建fragmentActivity是成功的。

Answer 1

移动

mContext = getActivity().getApplicationContext();

到onAttached或onActivityCreated

Answer 2

您的代码存在的问题是您的活动正在崩溃其他地方，因此您无法在片段类中获取活动参考。通过堆栈跟踪，整个日志，在此处发布的日志之前的某个位置，将是导致活动停止的其他异常。

为了在片段类中使用活动引用，在片段中的mActivity变量上创建，在使用之前检查NP if(mActivity!=null)。在片段类的mActivity方法中初始化onAttached

@Override
public void onAttach(Activity activity) {
    // TODO Auto-generated method stub
    super.onAttach(activity);
    mActivity = activity;
}

重新回到FragmentActivity时如何避免NullPointerException？

2 个答案: