尝试在空对象引用调用虚拟方法无效android.app.ActionBar.setDisplayHomeAsUpEnabled（布尔）' [英] Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
本文介绍了尝试在空对象引用调用虚拟方法无效android.app.ActionBar.setDisplayHomeAsUpEnabled（布尔）'的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我是新来的这一点，并有尝试将抽屉式导航栏添加到活动与片段时，出现以下错误：
I'm new to this and have the following error when trying to add a Navigation Drawer to an activity with Fragments:
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)
我的第一的Java文件是：
My 1st Java file is:
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) {
}
}
我通过中提到的动作条必须SupportActionBar一些其他的文章阅读，但抛出了更多的问题，当我改变了进口了两个Java文件。很可能，我犯了一个错误那里也不过。
I read through a few other articles that mentioned the ActionBar needs to be SupportActionBar but that threw up even more problems when I changed the imports over on both Java files. Quite possible that I've made a mistake there as well though.
推荐答案
错误：
产生的原因：显示java.lang.NullPointerException：尝试调用虚拟
法无效android.app.ActionBar.setDisplayHomeAsUpEnabled（布尔）'
在一个空对象引用
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
你是因为你的动作条为null收到此错误。
you are getting this error because your ActionBar is null.
您应该更改
public class EditFactFind extends FragmentActivity
到
public class EditFactFind extends ActionBarActivity
或
public class EditFactFind extends AppCompatActivity
使用 AppCompatActivity 如果您使用的是最新的appcompact库。
Use AppCompatActivity if you are using latest appcompact library.
，改变动作条动作条= getSupportActionBar（）;
这篇关于尝试在空对象引用调用虚拟方法无效android.app.ActionBar.setDisplayHomeAsUpEnabled（布尔）'的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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