Error:Caused by: java.lang.NullPointerException: Attempt to invoke
virtual method 'void android.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
尝试使用getSupportActionBar（）也无法解决异常 请帮我解决这个问题
我尝试使用v7版本的ActionBarActivity，用AppCompactActivity扩展该类仍然无法解析。
在仿真器和真实设备上执行相同的问题。 在打开应用程序后，它的显示很不幸＆＃34; app＆＃34;停止了工作。
package pizzandburger.hfad.com.pizzaandburger;
import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.content.Intent;
import android.content.res.Configuration;
import android.os.Bundle;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.ShareActionProvider;
public class MainActivity extends Activity {
private String[] titles;
private ListView drawerList;
private DrawerLayout drawerLayout;
private ActionBarDrawerToggle drawerToggle;
private ShareActionProvider shareActionProvider;
private int currentPosition=0;
private class DrawerItemClickListener implements ListView.OnItemClickListener{
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
selectItem(position);
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
titles=getResources().getStringArray(R.array.items);
drawerList=(ListView)findViewById(R.id.drawer);
drawerLayout=(DrawerLayout)findViewById(R.id.drawer_layout);
drawerList.setAdapter(new ArrayAdapter<String>(this,android.R.layout.simple_list_item_activated_1,titles));
drawerList.setOnItemClickListener(new DrawerItemClickListener());
if(savedInstanceState!=null){
currentPosition=savedInstanceState.getInt("position");
setActionBarTitle(currentPosition);
}
else{
selectItem(0);
}
drawerToggle=new ActionBarDrawerToggle(this,drawerLayout,R.string.open_drawer,R.string.close_drawer){
@Override
public void onDrawerClosed(View view){
super.onDrawerClosed(view);
invalidateOptionsMenu();
}
@Override
public void onDrawerOpened(View drawerView){
super.onDrawerOpened(drawerView);
invalidateOptionsMenu();
}
};
drawerLayout.setDrawerListener(drawerToggle);
getActionBar().setDisplayHomeAsUpEnabled(true);
getActionBar().setHomeButtonEnabled(true);
getFragmentManager().addOnBackStackChangedListener(new FragmentManager.OnBackStackChangedListener() {
@Override
public void onBackStackChanged() {
FragmentManager fragmentManager=getFragmentManager();
Fragment fragment=fragmentManager.findFragmentByTag("visible_fragment");
if(fragment instanceof TopFragment){
currentPosition=0;
}
else if(fragment instanceof PizzaFragment){
currentPosition=1;
}
else if(fragment instanceof BurgerFragment){
currentPosition=2;
}
else if(fragment instanceof OtherFragment){
currentPosition=3;
}
else if(fragment instanceof BeveragesFragment){
currentPosition=4;
}
setActionBarTitle(currentPosition);
drawerList.setItemChecked(currentPosition,true);
}
});
}
private void selectItem(int position){
Fragment fragment;
currentPosition=position;
switch (position){
case 1:
fragment=new PizzaFragment();
break;
case 2:
fragment=new BurgerFragment();
break;
case 3:
fragment=new OtherFragment();
break;
case 4:
fragment=new BeveragesFragment();
break;
default:
fragment= new TopFragment();
break;
}
FragmentTransaction ft=getFragmentManager().beginTransaction();
ft.replace(R.id.content_frame, fragment, "visible_fragment");
ft.addToBackStack(null);
ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
ft.commit();
setActionBarTitle(position);
drawerLayout.closeDrawer(drawerList);
}
@Override
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
drawerToggle.onConfigurationChanged(newConfig);
}
@Override
protected void onPostCreate(Bundle savedInstanceState){
super.onPostCreate(savedInstanceState);
drawerToggle.syncState();
}
@Override
public void onSaveInstanceState(Bundle outState){
super.onSaveInstanceState(outState);
outState.putInt("position",currentPosition);
}
@Override
public boolean onPrepareOptionsMenu(Menu menu){
boolean drawerOpen=drawerLayout.isDrawerOpen(drawerList);
menu.findItem(R.id.action_share).setVisible(!drawerOpen);
return super.onPrepareOptionsMenu(menu);
}
private void setActionBarTitle(int position){
String title;
if(position==0){
title=getResources().getString(R.string.app_name);
}
else{
title=titles[position];
}
/*******************************/
/**** here its giving error *****/
/*******************************/
getActionBar().setTitle(title);
}
@Override
public boolean onCreateOptionsMenu(Menu menu){
getMenuInflater().inflate(R.menu.menu_main, menu);
MenuItem menuItem=menu.findItem(R.id.action_share);
shareActionProvider=(ShareActionProvider)menuItem.getActionProvider();
setIntent("this is some simple text for sharing through shareActionProvider");
return super.onCreateOptionsMenu(menu);
}
@Override
public boolean onOptionsItemSelected(MenuItem menuitem){
if(drawerToggle.onOptionsItemSelected(menuitem)){
return true;
}
switch (menuitem.getItemId()){
case R.id.action_settings:
return true;
default:
return super.onOptionsItemSelected(menuitem);
}
}
private void setIntent(String text){
Intent intent=new Intent(Intent.ACTION_SEND);
intent.setType("text/plain");
intent.putExtra(Intent.EXTRA_TEXT,text);
shareActionProvider.setShareIntent(intent);
}
}
这是我的styles.xml
<style name="AppTheme.NoActionBar">
<item name="windowActionBar">false</item>
<item name="windowNoTitle">true</item>
<item name="android:windowDrawsSystemBarBackgrounds">true</item>
<item name="android:statusBarColor">@android:color/transparent</item>
</style>
这是我的activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/drawer_layout"
>
<FrameLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/content_frame"></FrameLayout>
<ListView
android:layout_width="240dp"
android:layout_height="match_parent"
android:id="@+id/drawer"
android:layout_gravity="start"
android:choiceMode="singleChoice"
android:divider="@android:color/transparent"
android:dividerHeight="0dp"
android:background="#ffffff"></ListView>
</android.support.v4.widget.DrawerLayout>
答案 0 :(得分：1)
一些前提：
import android.support.v7.app.AppCompatActivity并且您的活动已延长。requestWindowFeature(Window.FEATURE_ACTION_BAR);
或者你的风格：
<item name="android:windowActionBar">true</item> 的问候，
答案 1 :(得分：0)
它是样式表中的问题 那说
"<style name="AppTheme.NoActionBar">"
定义一些主题，然后摆脱空指针异常
答案 2 :(得分：0)
如果您将INTENT与AppCompatActivity一起使用，则需要访问相同的相关线程：
YourClass mContext;
mContext.runOnUiThread(new Runnable() {
@Override
public void run() {
mContext.getSupportActionBar().hide();
}
});
YourClass是扩展AppCompActivity的一种：
public class YourClass extends AppCompatActivity {
}