java.lang.NullPointerException：尝试在空对象引用上调用虚方法'ActionBar.setNavigationMode（int）'

Question

I am getting this error at runtime.

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setNavigationMode(int)' on a null object reference

    public class MainActivity <T extends Fragment> extends ActionBarActivity implements ListView.OnItemClickListener
    {  
    @Override
    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    ActionBar actionBar = getSupportActionBar();

    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
    actionBar.setDisplayShowTitleEnabled(false);

    ActionBar.Tab tab;
    tab = actionBar.newTab()
            .setText("Categories")
            .setTabListener(new TabListener<CategoriesFragment>(
                    this, "Categories", CategoriesFragment.class));
    actionBar.addTab(tab);

    tab = actionBar.newTab()
            .setText("Acceuil")
            .setTabListener(new TabListener<AcceuilFragment>(
                    this, "Acceuil", AcceuilFragment.class));
    actionBar.addTab(tab);
    tab = actionBar.newTab()
            .setText("Topics")
            .setTabListener(new TabListener<TopicsFragment>(
                    this, "Topics", TopicsFragment.class));
    actionBar.addTab(tab);

    tab = actionBar.newTab()
            .setText("Tendances")
            .setTabListener(new TabListener<TendancesFragment>(
                    this, "Tendances", TendancesFragment.class));
    actionBar.addTab(tab);

    setRef();

    //Set the custom toolbar
    if (toolbar != null)
    {
        toolbar.setTitle(R.string.app_name);
        setSupportActionBar(toolbar);
    }

    drawerToggle = new ActionBarDrawerToggle(
            this,
            drawerLayout,
            toolbar,
            R.string.open,
            R.string.close
    )

    {
        public void onDrawerClosed(View view)
        {
            Log.d("MainActivity", "OnDrawerClosed");
            super.onDrawerClosed(view);
            invalidateOptionsMenu();
        }

        public void onDrawerOpened(View drawerView)
        {
            Log.d("MainActivity", "OnDrawerOpened");
            super.onDrawerOpened(drawerView);
            invalidateOptionsMenu();
        }
    };

    drawerLayout.setDrawerListener(drawerToggle);

    getSupportActionBar().setDisplayHomeAsUpEnabled(true);

    // use this setting to improve performance if you know that changes
    // in content do not change the layout size of the RecyclerView
    mRecyclerView.setHasFixedSize(true);

    // use a linear layout manager
    mLayoutManager = new LinearLayoutManager(this);
    mRecyclerView.setLayoutManager(mLayoutManager);
    mRecyclerView.setItemAnimator(new DefaultItemAnimator());

    // specify an adapter (see also next example)
    mAdapter = new MyAdapter(myDataset);
    mRecyclerView.setAdapter(mAdapter);


}

@Override
protected void onPostCreate(Bundle savedInstanceState) {
    super.onPostCreate(savedInstanceState);
    drawerToggle.syncState();
}

private void setRef()
{
    // create reference to the xml views
    mRecyclerView = (android.support.v7.widget.RecyclerView) findViewById(R.id.my_recycler_view);
    fab = (Button) findViewById(R.id.addButton);
    drawerLayout = (android.support.v4.widget.DrawerLayout) findViewById(R.id.drawer_layout);
    drawerLayout.setStatusBarBackground(R.color.primarydark);

    toolbar = (android.support.v7.widget.Toolbar) findViewById(R.id.toolbar);
    leftDrawerList = (ListView) findViewById(R.id.list_drawer);

    View list_header = getLayoutInflater().inflate(R.layout.drawerlist_header, null);
    leftDrawerList.addHeaderView(list_header);
    navigationDrawerAdapter = new ArrayAdapter<String>(MainActivity.this, android.R.layout.simple_expandable_list_item_1, leftSliderData);
    leftDrawerList.setAdapter(navigationDrawerAdapter);
    leftDrawerList.setOnItemClickListener(this);
}

@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id)
{
    Toast.makeText(getApplicationContext(), "Clicked on " + leftSliderData[position - 1], Toast.LENGTH_LONG).show();
}

public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {
    // Check if the fragment is already initialized
    if (mFragment == null) {
        // If not, instantiate and add it to the activity
        mFragment = Fragment.instantiate(mActivity, mClass.getName());
        ft.add(android.R.id.content, mFragment, mTag);
    } else {
        // If it exists, simply attach it in order to show it
        ft.attach(mFragment);
    }
}

public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft) {
    if (mFragment != null) {
        // Detach the fragment, because another one is being attached
        ft.detach(mFragment);
    }
}

public void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft) {
    // User selected the already selected tab. Usually do nothing.
    }
}

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
tools:context=".MainActivity">

<!-- Setting this one to true makes the status bar
    to have a non-transparent shitty background -->

<!--
     As the main content view, the view below consumes the entire
     space available using match_parent in both dimensions.
-->

<FrameLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="false">

    <include layout="@layout/actionbarlayout"></include>

    <include layout="@layout/content_view"></include>

    <Button
        android:id="@+id/addButton"
        android:text="+"
        android:textSize="25sp"
        android:textColor="@android:color/white"
        android:textAlignment="center"
        android:layout_marginRight="15dp"
        android:layout_marginBottom="15dp"
        android:background="@drawable/circlebtn"
        android:layout_width="56dp"
        android:layout_height="56dp"
        android:stateListAnimator="@animator/anim"
        android:elevation="4dp"
        style="?android:buttonStyleSmall"
        android:layout_gravity="right|bottom" >
    </Button>

</FrameLayout>

<com.androprogrammer.test.materialapp1.ScrimInsetsFrameLayout
    android:id="@+id/container"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    android:elevation="8dp"
    android:layout_gravity="start"
    app:insetForeground="#4000">

    <ListView
        android:id="@+id/list_drawer"
        android:layout_width="@dimen/drawer_width"
        android:layout_height="match_parent"
        android:choiceMode="singleChoice"
        android:background="@android:color/white"
        android:fitsSystemWindows="true">
    </ListView>

</com.androprogrammer.test.materialapp1.ScrimInsetsFrameLayout>
</android.support.v4.widget.DrawerLayout>

<resources>

<!-- Base application theme. -->
<style name="AppTheme" parent="AppTheme.Base"/>

<style name="AppTheme.Base" parent="Theme.AppCompat.Light">
    <item name="colorPrimary">@color/primary</item>
    <item name="colorPrimaryDark">@color/primarydark</item>
    <item name="colorAccent">@color/primary</item>
    <item name="colorControlHighlight">@color/primary</item>
    <item name="android:windowNoTitle">true</item>
    <item name="windowActionBar">false</item>
    <item name="drawerArrowStyle">@style/DrawerArrowStyle</item>
    <item name="windowActionModeOverlay">true</item>

</style>

<style name="DrawerArrowStyle" parent="Widget.AppCompat.DrawerArrowToggle">
    <item name="spinBars">true</item>
    <item name="color">@android:color/white</item>
</style>
</resources>

<?xml version="1.0" encoding="utf-8"?>
<resources>
<style name="AppTheme" parent="AppTheme.Base">
    <item name="android:windowDrawsSystemBarBackgrounds">true</item>
    <item name="android:statusBarColor">@android:color/transparent</item>
    <item name="android:windowContentTransitions">true</item>
    <item name="android:windowAllowEnterTransitionOverlap">true</item>
    <item name="android:windowAllowReturnTransitionOverlap">true</item>
    <item name="android:windowSharedElementEnterTransition">@android:transition/move</item>
    <item name="android:windowSharedElementExitTransition">@android:transition/move</item>

</style>
</resources>

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.androprogrammer.test.materialapp1" >

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

</manifest>

Help Please :)

Answer 1

Try to change the parent style

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">

and ActionBarActivity is deprecated, you should use AppCompatActivity

Answer 2

In your style.xml you have to have the actionbar enabled. If the actionbar is disabled (set to false) then it will not be added in to the activity at runtime (ie getActionBar() returns null ).