尝试在空对象引用上调用虚拟方法'void android.app.ActionBar.setDisplayHomeAsUpEnabled（boolean）'

Question

我对此很陌生，当尝试向具有片段的活动添加导航抽屉时出现以下错误：

java.lang.RuntimeException: Unable to start activity ComponentInfo{uk.co.XXXX.EditFactFind}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference 
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325) 
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390) 
     at android.app.ActivityThread.access$800(ActivityThread.java:151) 
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303) 
     at android.os.Handler.dispatchMessage(Handler.java:102) 
     at android.os.Looper.loop(Looper.java:135) 
     at android.app.ActivityThread.main(ActivityThread.java:5257) 
     at java.lang.reflect.Method.invoke(Native Method) 
     at java.lang.reflect.Method.invoke(Method.java:372) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference 
     at uk.co.XXXX.NavigationDrawerFragment.setUp(NavigationDrawerFragment.java:135) 
     at uk.co.XXXX.EditFactFind.onCreate(EditFactFind.java:69) 
     at android.app.Activity.performCreate(Activity.java:5990) 
     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106) 
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278) 

我的第一次的Java文件是：

package XXXX; 

import android.support.v4.app.FragmentActivity; 
import android.support.v7.app.AppCompatActivity; 
import android.app.Activity; 
import android.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.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); 

     // 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>(
       getActivity(), 
       android.R.layout.simple_list_item_activated_1, 
       android.R.id.text1, 
       new String[]{ 
         getString(R.string.title_section1), 
         getString(R.string.title_section2), 
         getString(R.string.title_section3), 
         "Assets", 
         "ID", 
       })); 
     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); 
     } 
    } 

    @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) { 
      Toast.makeText(getActivity(), "Example action.", Toast.LENGTH_SHORT).show(); 
      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 android.app.ActionBar getActionBar() { 
     return ((FragmentActivity) getActivity()).getActionBar(); 

    } 

    /** 
    * 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); 
    } 
} 

我的第二点是：

package XXXX; 


import java.io.Serializable; 
import java.text.DateFormat; 
import java.text.SimpleDateFormat; 
import java.util.Calendar; 


import android.app.AlertDialog; 
import android.app.FragmentManager; 
import android.app.FragmentTransaction; 
import android.content.DialogInterface; 
import android.content.Intent; 
import android.os.Bundle; 
import android.support.v4.app.FragmentActivity; 
import android.support.v4.widget.DrawerLayout; 
import android.app.ActionBar; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.TextView; 

public class EditFactFind extends FragmentActivity implements NavigationDrawerFragment.NavigationDrawerCallbacks { 

    public static final int RESULT_DELETE = -500; 
    private boolean isInEditMode = true; 
    private boolean isAddingFactFind = true; 
    private NavigationDrawerFragment mNavigationDrawerFragment; 
    private CharSequence mTitle; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.edit_factfind); 

     final Button saveButton = (Button)findViewById(R.id.saveButton); 
     final Button cancelButton = (Button)findViewById(R.id.cancelButton); 
     final EditText titleEditText = (EditText)findViewById(R.id.titleEditText); 
     //final EditText factFindEditText = (EditText)findViewById(R.id.factFindEditText); 
     final TextView dateTextView = (TextView)findViewById(R.id.dateTextView); 
     final Button nextButton =(Button) findViewById(R.id.nextButton); 

     //Create fragment and give it an argument for the selected article 
     secA_pg1 iniSecFrag = new secA_pg1(); 
     Bundle args = new Bundle(); 
     args.putInt(secA_pg1.ARG_INDEX, 1); 
     iniSecFrag.setArguments(args); 

     FragmentTransaction initialTransaction = getFragmentManager().beginTransaction(); 

     // Replace whatever is in the fragment_container view with this fragment, 
     // and add the transaction to the back stack so the user can navigate back 
     initialTransaction.replace(R.id.fragment_container, iniSecFrag); 
     initialTransaction.addToBackStack(null); 

     //Commit the transaction 
     initialTransaction.commit(); 

     //// TODO: 10/09/2015 
     mNavigationDrawerFragment = (NavigationDrawerFragment) 
       getSupportFragmentManager().findFragmentById(R.id.navigation_drawer); 
     mTitle = getTitle(); 

     // Set up the drawer. 
     mNavigationDrawerFragment.setUp(
       R.id.navigation_drawer, 
       (DrawerLayout) findViewById(R.id.drawer_layout)); 

     Serializable extra = getIntent().getSerializableExtra("FactFind"); 
     if(extra != null) 
     { 
      FactFind factFind = (FactFind) extra; 
      titleEditText.setText(factFind.getTitle()); 
      // factFindEditText.setText(factFind.getFactFindTitle()); 

      DateFormat dateFormat = new SimpleDateFormat("dd-MM-yyyy HH:mm:ss"); 
      String date = dateFormat.format(factFind.getDate()); 

      dateTextView.setText(date); 

      isInEditMode = false; 
      titleEditText.setEnabled(false); 
      // factFindEditText.setEnabled(false); 
      saveButton.setText("Edit"); 

      isAddingFactFind = false; 

     } 

     cancelButton.setOnClickListener(new OnClickListener() { 

      @Override 
      public void onClick(View v) { 
       setResult(RESULT_CANCELED, new Intent()); 
       finish(); 
      } 
     }); 

     nextButton.setOnClickListener(new OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       //secA_pg1 secFrag = (secA_pg1) getFragmentManager().findFragmentById(R.id.Sec_A_pg1_fragment); 
       //Create fragment and give it an argument for the selected article 
       secA_pg2 newSecFrag = new secA_pg2(); 
       Bundle args = new Bundle(); 
       args.putInt(secA_pg2.ARG_INDEX, 2); 
       newSecFrag.setArguments(args); 

       FragmentTransaction transaction = getFragmentManager().beginTransaction(); 

       // Replace whatever is in the fragment_container view with this fragment, 
       // and add the transaction to the back stack so the user can navigate back 
       transaction.replace(R.id.fragment_container, newSecFrag); 
       transaction.addToBackStack(null); 

       //Commit the transaction 
       transaction.commit(); 
      } 
     }); 

     saveButton.setOnClickListener(new OnClickListener() { 

      @Override 
      public void onClick(View v) { 


       if(isInEditMode) 
       { 
        Intent returnIntent = new Intent(); 
        FactFind factFind = new FactFind(titleEditText.getText().toString(),Calendar.getInstance().getTime()); 
        returnIntent.putExtra("FactFind", factFind); 
        setResult(RESULT_OK, returnIntent); 
        finish(); 

       } 
       else 
       { 
        isInEditMode = true; 
        saveButton.setText("Save"); 
        titleEditText.setEnabled(true); 
        // factFindEditText.setEnabled(true); 
       } 

      } 
     }); 
    } 


    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     AlertDialog.Builder builder = new AlertDialog.Builder(this); 
     builder.setMessage(R.string.are_you_sure_you_want_to_delete_this_fact_find_it_can_t_be_undone_); 
     builder.setTitle("Confirm Delete"); 
     builder.setPositiveButton("Delete", new DialogInterface.OnClickListener() { 

      @Override 
      public void onClick(DialogInterface dialog, int which) { 
       Intent returnIntent = new Intent(); 

       setResult(RESULT_DELETE, returnIntent); 
       finish(); 

      } 
     }); 
     builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() { 

      @Override 
      public void onClick(DialogInterface dialog, int which) { 
       // TODO Auto-generated method stub 
      } 
     }); 

     builder.create().show(); 
     return true; 

    } 


    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 

     // Inflate the menu; this adds items to the action bar if it is present. 
     if (!mNavigationDrawerFragment.isDrawerOpen()) { 
      // Only show items in the action bar relevant to this screen 
      // if the drawer is not showing. Otherwise, let the drawer 
      // decide what to show in the action bar. 
      getMenuInflater().inflate(R.menu.nav_drawer, menu); 
      restoreActionBar(); 
      return true; 
     } 
     else { 
      //return super.onCreateOptionsMenu(menu); 
      getMenuInflater().inflate(R.menu.action_menu, menu); 
      return true; 
     } 
    } 
    //@Override 
    public void onSelectedFragChanged(int index) { 
     FragmentManager fragmentManager = getFragmentManager(); 
     secA_pg1 secA_pg1 = (secA_pg1) fragmentManager.findFragmentById(R.id.Sec_A_pg1_fragment); 

     secA_pg1.setSectionTitle(index); 
    } 

    public void restoreActionBar() { 
     ActionBar actionBar = getActionBar(); 
     //actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD); 
     actionBar.setDisplayShowTitleEnabled(true); 
     actionBar.setTitle(mTitle); 
    } 

    @Override 
    public void onNavigationDrawerItemSelected(int position) { 

    } 
} 

我读救援人员到场还有一些其他提到ActionBar需要成为SupportActionBar的文章，但是当我在两个Java文件上改变导入时，抛出了更多的问题。尽管如此，我还是犯了一个错误。

Answer 1

错误：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference

因为你ActionBar为空您收到此错误。

，如果你使用的是最新的appcompact库，则应该更换

public class EditFactFind extends FragmentActivity 

到

public class EditFactFind extends ActionBarActivity 

或

public class EditFactFind extends AppCompatActivity 

使用AppCompatActivity。

，改变ActionBar actionBar = getSupportActionBar();