内片段ViewPager导致NullPointerException异常

Question

我试图使用ViewPager一个Fragment内得到

03-01 15:23:05.375 3937-3937/com.example.android.navigationdrawerexample E/AndroidRuntime﹕ FATAL EXCEPTION: main 
java.lang.NullPointerException 
     at com.example.android.navigationdrawerexample.SpaFragment.onCreateView(SpaFragment.java:50) 
     at android.app.Fragment.performCreateView(Fragment.java:1695) 

代码：

public class SpaFragment extends Fragment { 

    public SpaFragment(){} 
    private SpaList mSpa; 
    private TabHost tabs; 

    private FragmentActivity mFragmentActivity; 
    private View rootView; 

    public void setSpa(SpaList s) 
    { 
     mSpa=s; 
    } 


    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
          Bundle savedInstanceState) { 
     rootView = inflater.inflate(R.layout.fragment_spa, container, false); 

     ViewPager pager = (ViewPager) rootView.findViewById(R.id.pager); 
     pager.setAdapter(new MyPagerAdapter(mFragmentActivity.getSupportFragmentManager())); 

     return rootView; 
    } 

    @Override 
    public void onAttach(Activity activity) { 
     mFragmentActivity = (FragmentActivity) activity; 
     Log.d("ACTIVITY on attach"+mFragmentActivity,"\n"); 
     super.onAttach(activity); 
    } 

    private class MyPagerAdapter extends FragmentPagerAdapter { 

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


     @Override 
     public CharSequence getPageTitle(int position) { 
      return "Title " + position; 
     } 

     @Override 
     public android.support.v4.app.Fragment getItem(int pos) { 
      switch(pos) { 

       case 0: return FirstFragment.newInstance("FirstFragment, Instance 1"); 
       case 1: return SecondFragment.newInstance("SecondFragment, Instance 1"); 
       default: return FirstFragment.newInstance("ThirdFragment, Default"); 
      } 
     } 

     @Override 
     public int getCount() { 
      return 2; 
     } 
    } 
} 

Answer 1

您正在运行到一个NullPointerException因为你是上调用的方法当时为NULL的对象。

这可能是因为你的

@Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
          Bundle savedInstanceState) { 
     // ... 
    } 

方法是之前叫你

@Override 
    public void onAttach(Activity activity) { } 

方法，因此，你的变量mFragmentActivity是NULL。在这种情况下，你的代码

String.valueOf(mFragmentActivity); 

将抛出一个NullPointerException。