NullPointerException异常增加动作条

Question

这里是我的代码

public class Income extends Fragment implements ActionBar.TabListener { 

    private String[] tabs = { "My Income", "Add Income" }; 
    private ViewPager viewPager; 
    private ActionBar actionBar; 

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

     viewPager = (ViewPager) root.findViewById(R.id.vpIncome); 
     viewPager.setAdapter(new MyPagerAdapter(getChildFragmentManager())); 

     return root; 
    } 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 

     actionBar = getActivity().getActionBar(); 
     actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); 

     for (String tab_name : tabs) { 
      actionBar.addTab(actionBar.newTab().setText(tab_name) 
        .setTabListener(this)); 
     } 

    } 

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

     @Override 
     public Fragment getItem(int pos) { 
      switch(pos) { 

       case 0: return new IncomeList(); 
       case 1: return new IncomeAdd(); 
       //case 2: return ThirdFragment.newInstance("ThirdFragment, Instance 1"); 
       default: return new IncomeList(); 
      } 
     } 

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

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

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

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

的应用程序将没有任何问题的运行没有这个代码上面

final ActionBar actionBar = getActivity().getActionBar(); 
     actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); 

     for (String tab_name : tabs) { 
      actionBar.addTab(actionBar.newTab().setText(tab_name) 
        .setTabListener(this)); 
     } 

应用程序可以运行，我可以选择导航抽屉阶层的收入和在IncomeList和IncomeAdd之间执行滑动操作。但是，当我添加上述（这是我所需要的，因为我需要的选项卡）的代码，应用程序会抛出NullPointerException

忘了加上（几乎）完整的错误信息是： 显示java.lang.NullPointerException 在COM。 code.imin.mymoneymanaged.Income.onTabSelected（Income.java:86）

这里是我的NavigationDrawer这将调用收入类代码：

import android.support.v4.app.FragmentManager; 
import android.support.v4.app.FragmentTransaction; 
import android.support.v7.app.ActionBarActivity; 
import android.app.Activity; 
import android.support.v7.app.ActionBar; 
import android.support.v4.app.Fragment; 
import android.support.v4.app.ActionBarDrawerToggle; 
import android.support.v4.view.GravityCompat; 
import android.support.v4.widget.DrawerLayout; 
import android.content.SharedPreferences; 
import android.content.res.Configuration; 
import android.os.Bundle; 
import android.preference.PreferenceManager; 
import android.text.format.Time; 
import android.view.LayoutInflater; 
import android.view.Menu; 
import android.view.MenuInflater; 
import android.view.MenuItem; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.AdapterView; 
import android.widget.ArrayAdapter; 
import android.widget.ListView; 
import android.widget.Toast; 

/** 
* Fragment used for managing interactions for and presentation of a navigation drawer. 
* See the <a href="https://developer.android.com/design/patterns/navigation-drawer.html#Interaction"> 
* design guidelines</a> for a complete explanation of the behaviors implemented here. 
*/ 
public class NavigationDrawerFragment extends Fragment { 

    /** 
    * Remember the position of the selected item. 
    */ 
    private static final String STATE_SELECTED_POSITION = "selected_navigation_drawer_position"; 

    /** 
    * Per the design guidelines, you should show the drawer on launch until the user manually 
    * expands it. This shared preference tracks this. 
    */ 
    private static final String PREF_USER_LEARNED_DRAWER = "navigation_drawer_learned"; 

    /** 
    * A pointer to the current callbacks instance (the Activity). 
    */ 
    private NavigationDrawerCallbacks mCallbacks; 

    /** 
    * Helper component that ties the action bar to the navigation drawer. 
    */ 
    private ActionBarDrawerToggle mDrawerToggle; 

    private DrawerLayout mDrawerLayout; 
    private ListView mDrawerListView; 
    private View mFragmentContainerView; 

    private int mCurrentSelectedPosition = 0; 
    private boolean mFromSavedInstanceState; 
    private boolean mUserLearnedDrawer; 

    public NavigationDrawerFragment() { 
    } 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 

     // Retain this fragment across configuration changes - added to make sure data in fragment retains after orientation changes 
     setRetainInstance(true); 

     // Read in the flag indicating whether or not the user has demonstrated awareness of the 
     // drawer. See PREF_USER_LEARNED_DRAWER for details. 
     SharedPreferences sp = PreferenceManager.getDefaultSharedPreferences(getActivity()); 
     mUserLearnedDrawer = sp.getBoolean(PREF_USER_LEARNED_DRAWER, false); 

     if (savedInstanceState != null) { 
      mCurrentSelectedPosition = savedInstanceState.getInt(STATE_SELECTED_POSITION); 
      mFromSavedInstanceState = true; 
     } 

     // Select either the default item (0) or the last selected item. 
     selectItem(mCurrentSelectedPosition); 
    } 

    @Override 
    public void onActivityCreated (Bundle savedInstanceState) { 
     super.onActivityCreated(savedInstanceState); 
     // Indicate that this fragment would like to influence the set of actions in the action bar. 
     setHasOptionsMenu(true); 
    } 

    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
      Bundle savedInstanceState) { 
     mDrawerListView = (ListView) inflater.inflate(
       R.layout.fragment_navigation_drawer, container, false); 
     mDrawerListView.setOnItemClickListener(new AdapterView.OnItemClickListener() { 
      @Override 
      public void onItemClick(AdapterView<?> parent, View view, int position, long id) { 
       selectItem(position); 
      } 
     }); 
     mDrawerListView.setAdapter(new ArrayAdapter<String>(
       getActionBar().getThemedContext(), 
       android.R.layout.simple_list_item_activated_1, 
       android.R.id.text1, 
       new String[]{ 
         getString(R.string.menu_dashboard), 
         getString(R.string.menu_expenses), 
         getString(R.string.menu_income), 
         getString(R.string.menu_moneyjar), 
         getString(R.string.menu_goal), 
         getString(R.string.menu_report), 
         getString(R.string.menu_settings), 
       })); 
     mDrawerListView.setItemChecked(mCurrentSelectedPosition, true); 
     return mDrawerListView; 
    } 

    public boolean isDrawerOpen() { 
     return mDrawerLayout != null && mDrawerLayout.isDrawerOpen(mFragmentContainerView); 
    } 

    /** 
    * Users of this fragment must call this method to set up the navigation drawer interactions. 
    * 
    * @param fragmentId The android:id of this fragment in its activity's layout. 
    * @param drawerLayout The DrawerLayout containing this fragment's UI. 
    */ 
    public void setUp(int fragmentId, DrawerLayout drawerLayout) { 
     mFragmentContainerView = getActivity().findViewById(fragmentId); 
     mDrawerLayout = drawerLayout; 

     // set a custom shadow that overlays the main content when the drawer opens 
     mDrawerLayout.setDrawerShadow(R.drawable.drawer_shadow, GravityCompat.START); 
     // set up the drawer's list view with items and click listener 

     ActionBar actionBar = getActionBar(); 
     actionBar.setDisplayHomeAsUpEnabled(true); 
     actionBar.setHomeButtonEnabled(true); 

     // ActionBarDrawerToggle ties together the the proper interactions 
     // between the navigation drawer and the action bar app icon. 
     mDrawerToggle = new ActionBarDrawerToggle(
       getActivity(),     /* host Activity */ 
       mDrawerLayout,     /* DrawerLayout object */ 
       R.drawable.ic_drawer,    /* nav drawer image to replace 'Up' caret */ 
       R.string.navigation_drawer_open, /* "open drawer" description for accessibility */ 
       R.string.navigation_drawer_close /* "close drawer" description for accessibility */ 
     ) { 
      @Override 
      public void onDrawerClosed(View drawerView) { 
       super.onDrawerClosed(drawerView); 
       if (!isAdded()) { 
        return; 
       } 

       getActivity().supportInvalidateOptionsMenu(); // calls onPrepareOptionsMenu() 
      } 

      @Override 
      public void onDrawerOpened(View drawerView) { 
       super.onDrawerOpened(drawerView); 
       if (!isAdded()) { 
        return; 
       } 

       if (!mUserLearnedDrawer) { 
        // The user manually opened the drawer; store this flag to prevent auto-showing 
        // the navigation drawer automatically in the future. 
        mUserLearnedDrawer = true; 
        SharedPreferences sp = PreferenceManager 
          .getDefaultSharedPreferences(getActivity()); 
        sp.edit().putBoolean(PREF_USER_LEARNED_DRAWER, true).apply(); 
       } 

       getActivity().supportInvalidateOptionsMenu(); // calls onPrepareOptionsMenu() 
      } 
     }; 

     // If the user hasn't 'learned' about the drawer, open it to introduce them to the drawer, 
     // per the navigation drawer design guidelines. 
     if (!mUserLearnedDrawer && !mFromSavedInstanceState) { 
      mDrawerLayout.openDrawer(mFragmentContainerView); 
     } 

     // Defer code dependent on restoration of previous instance state. 
     mDrawerLayout.post(new Runnable() { 
      @Override 
      public void run() { 
       mDrawerToggle.syncState(); 
      } 
     }); 

     mDrawerLayout.setDrawerListener(mDrawerToggle); 
    } 

    private void selectItem(int position) { 
     mCurrentSelectedPosition = position; 
     if (mDrawerListView != null) { 
      mDrawerListView.setItemChecked(position, true); 
     } 
     if (mDrawerLayout != null) { 
      mDrawerLayout.closeDrawer(mFragmentContainerView); 
     } 
     if (mCallbacks != null) { 
      mCallbacks.onNavigationDrawerItemSelected(position); 
     } 

     Fragment fragment = null; 
     FragmentTransaction transaction = getFragmentManager().beginTransaction(); 
     switch (position) { 
      case 0: 
       fragment = new Dashboard(); 
       transaction.replace(R.id.container, fragment, "fragmentDashboard"); 
       transaction.addToBackStack(null); 
       transaction.commit(); 
       break; 
      case 1: 
       fragment = new Expenses(); 
       transaction.replace(R.id.container, fragment, "fragmentExpenses"); 
       transaction.addToBackStack(null); 
       transaction.commit(); 
       break; 
      case 2: 
       fragment = new Income(); 
       transaction.replace(R.id.container, fragment, "fragmentIncome"); 
       transaction.addToBackStack(null); 
       transaction.commit(); 
       break; 
      case 3: 
       fragment = new MoneyJar(); 
       transaction.replace(R.id.container, fragment, "fragmentMoneyJar"); 
       transaction.addToBackStack(null); 
       transaction.commit(); 
       break; 
      case 4: 
       fragment = new Goal(); 
       transaction.replace(R.id.container, fragment, "fragmentGoal"); 
       transaction.addToBackStack(null); 
       transaction.commit(); 
       break; 
      case 5: 
       fragment = new Report(); 
       transaction.replace(R.id.container, fragment, "fragmentReport"); 
       transaction.addToBackStack(null); 
       transaction.commit(); 
       break; 
      case 6: 
       fragment = new Settings(); 
       transaction.replace(R.id.container, fragment, "fragmentSettings"); 
       transaction.addToBackStack(null); 
       transaction.commit(); 
       break; 
      default: 
       break; 
     } 
    } 

    @Override 
    public void onAttach(Activity activity) { 
     super.onAttach(activity); 
     try { 
      mCallbacks = (NavigationDrawerCallbacks) activity; 
     } catch (ClassCastException e) { 
      throw new ClassCastException("Activity must implement NavigationDrawerCallbacks."); 
     } 
    } 

    @Override 
    public void onDetach() { 
     super.onDetach(); 
     mCallbacks = null; 
    } 

    @Override 
    public void onSaveInstanceState(Bundle outState) { 
     super.onSaveInstanceState(outState); 
     outState.putInt(STATE_SELECTED_POSITION, mCurrentSelectedPosition); 
    } 

    @Override 
    public void onConfigurationChanged(Configuration newConfig) { 
     super.onConfigurationChanged(newConfig); 
     // Forward the new configuration the drawer toggle component. 
     mDrawerToggle.onConfigurationChanged(newConfig); 
    } 

    @Override 
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) { 
     // If the drawer is open, show the global app actions in the action bar. See also 
     // showGlobalContextActionBar, which controls the top-left area of the action bar. 
     if (mDrawerLayout != null && isDrawerOpen()) { 
      inflater.inflate(R.menu.global, menu); 
      showGlobalContextActionBar(); 
     } 
     super.onCreateOptionsMenu(menu, inflater); 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     if (mDrawerToggle.onOptionsItemSelected(item)) { 
      return true; 
     } 

     if (item.getItemId() == R.id.action_example) { 
      Time today = new Time(Time.getCurrentTimezone()); 
      today.setToNow(); 

      int selectedDay = Integer.parseInt(today.format("%d")); 
      int selectedMonth = Integer.parseInt(today.format("%m")) - 1; 
      int selectedYear = Integer.parseInt(today.format("%Y")); 
      ExpensesAdd dfExpensesAdd = ExpensesAdd.newInstance(selectedYear,selectedMonth,selectedDay); 
      dfExpensesAdd.show(getActivity().getFragmentManager(),"fragmentDialog"); 
      return true; 
     } 

     return super.onOptionsItemSelected(item); 
    } 

    /** 
    * Per the navigation drawer design guidelines, updates the action bar to show the global app 
    * 'context', rather than just what's in the current screen. 
    */ 
    private void showGlobalContextActionBar() { 
     ActionBar actionBar = getActionBar(); 
     actionBar.setDisplayShowTitleEnabled(true); 
     actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD); 
     actionBar.setTitle(R.string.app_name); 
    } 

    private ActionBar getActionBar() { 
     return ((ActionBarActivity) getActivity()).getSupportActionBar(); 
    } 

    /** 
    * Callbacks interface that all activities using this fragment must implement. 
    */ 
    public static interface NavigationDrawerCallbacks { 
     /** 
     * Called when an item in the navigation drawer is selected. 
     */ 
     void onNavigationDrawerItemSelected(int position); 
    } 
} 

Answer 1

viewPager是在你的代码无效，你定义内viewPager对象在onCreateView

View root = inflater.inflate(R.layout.income,container, false); 

    ViewPager viewPager = (ViewPager) root.findViewById(R.id.vpIncome); 
    viewPager.setAdapter(new MyPagerAdapter(getChildFragmentManager())); 

但你必须尝试访问类ViewPager对象，你不初始化：

private ViewPager viewPager; 

，你使用这个对象在onTabSelected

@Override 
    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) { 
     // on tab selected 
     // show respected fragment view 
     viewPager.setCurrentItem(tab.getPosition()); 
    } 

为解决这一问题，使用类ViewPager onCreateView，请使用以下代码

 View root = inflater.inflate(R.layout.income,container, false); 

     viewPager = (ViewPager) root.findViewById(R.id.vpIncome); 
     viewPager.setAdapter(new MyPagerAdapter(getChildFragmentManager())); 

Answer 2

尝试移动您的代码以获取actionBar到onActivityCreated方法并查看是否有效。