Android FragmentActivity在getActionBar（）中返回null

Question

I have imported an Android sample project called HorizontalPaging in Android Studio and it works fine when I run it. But when I copied the code into my code, I get a NULL pointer exception in getActionBar() .

I have been reading about these problems the whole day but can't get it working. Tried copying the entire code from the sample's MainActivity into my project but no improvements so I am guessing that the problem is in some other file like manifest,styles,etc.

MainActivity.java copied from the sample project

import android.app.ActionBar;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import java.util.Locale;


public class View_Tabs extends FragmentActivity implements ActionBar.TabListener {


SectionsPagerAdapter mSectionsPagerAdapter;


ViewPager mViewPager;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // Load the UI from res/layout/activity_main.xml
    setContentView(R.layout.view_tabs);


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


    // Create the adapter that will return a fragment for each of the three primary sections
    // of the app.
    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mSectionsPagerAdapter);

    // When swiping between different sections, select the corresponding tab. We can also use
    // ActionBar.Tab#select() to do this if we have a reference to the Tab.

    mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
        @Override
        public void onPageSelected(int position) {
            actionBar.setSelectedNavigationItem(position);
        }
    });



    // For each of the sections in the app, add a tab to the action bar.
    for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
        // Create a tab with text corresponding to the page title defined by the adapter. Also
        // specify this Activity object, which implements the TabListener interface, as the
        // callback (listener) for when this tab is selected.
        actionBar.addTab(
                actionBar.newTab()
                        .setText(mSectionsPagerAdapter.getPageTitle(i))
                        .setTabListener(this));
    }

}


@Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
    // When the given tab is selected, tell the ViewPager to switch to the corresponding page.
    mViewPager.setCurrentItem(tab.getPosition());
}



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


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


public class SectionsPagerAdapter extends FragmentPagerAdapter {


    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }



    @Override
    public Fragment getItem(int position) {
        // getItem is called to instantiate the fragment for the given page.
        // Return a DummySectionFragment (defined as a static inner class
        // below) with the page number as its lone argument.
        Fragment fragment = new DummySectionFragment();
        Bundle args = new Bundle();
        args.putInt(DummySectionFragment.ARG_SECTION_NUMBER, position + 1);
        fragment.setArguments(args);
        return fragment;
    }


    @Override
    public int getCount() {
        // Show 3 total pages.
        return 3;
    }


    @Override
    public CharSequence getPageTitle(int position) {
        Locale l = Locale.getDefault();
        switch (position) {
            case 0:
                return getString(R.string.tab_list).toUpperCase(l);
            case 1:
                return getString(R.string.tab_map).toUpperCase(l);
            case 2:
                return getString(R.string.tab_list).toUpperCase(l);
        }
        return null;
    }

}

/**
 * A dummy fragment representing a section of the app, but that simply displays dummy text.
 * This would be replaced with your application's content.
 */
public static class DummySectionFragment extends Fragment {
    /**
     * The fragment argument representing the section number for this
     * fragment.
     */
    public static final String ARG_SECTION_NUMBER = "section_number";

    public DummySectionFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.list_tab_fragment, container, false);
        TextView dummyTextView = (TextView) rootView.findViewById(R.id.section_label);
        dummyTextView.setText(Integer.toString(getArguments().getInt(ARG_SECTION_NUMBER)));
        return rootView;
    }
}



}

my manifest

<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"/>

<!-- TABS: While ViewPager will work on API 4 or above, tabs require an ActionBar. ActionBar is only
 available in API 11 or above. -->
<!-- Min/target SDK versions (<uses-sdk>) managed by build.gradle -->

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >



    <meta-data android:name="com.google.android.gms.version"
        android:value="@integer/google_play_services_version" />

    <!--
    <meta-data android:name="com.google.android.gms.version"
        android:value="@integer/google_play_services_version" />
    -->

    <meta-data
        android:name="com.google.android.maps.v2.API_KEY"
        android:value="xxxxx"/>

    <activity
        android:name=".MainActivity"
        android:label="@string/app_name"
        android:screenOrientation="portrait"
        >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

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

.... 

    <activity
        android:name=".View_Tabs"
        android:label="@string/app_name"
        android:screenOrientation="portrait"
        >
    </activity>
</application>

my styles.xml

<resources>

    <!-- Base application theme. -->
    <!-- BEFORE <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">  -->
    <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
        <!-- Customize your theme here. -->
        <item name="android:windowActionBar">true</item>
    </style>

    <style name="Theme.Base" parent="android:Theme.Light" />




</resources>

my gradle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 21
    buildToolsVersion "21.1.2"

    defaultConfig {
        applicationId "xxxx"
        minSdkVersion 14
        targetSdkVersion 21
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.android.support:appcompat-v7:21.0.3'
    compile 'com.google.android.gms:play-services:6.5.87'
    //compile 'com.google.android.gms:play-services:7.0.0'
    compile files('lib/gson-2.3.jar')
}

Answer 1

You must extend ActionBarActivity instead of FragmentActivity to have Actionbar with fragments.

If you're using the v7 appcompat library, your activity should instead extend ActionBarActivity, which is a subclass of FragmentActivity (for more information, read Adding the Action Bar).

Still you can try this,

ActionBar actionBar = (ActionBarActivity)getActivity().getSupportActionBar();

More detail you can found here. http://developer.android.com/training/basics/fragments/creating.html

Answer 2

getActionBar() will return correct value only if you have the activity with a title bar. Make sure you have set a theme with titlebar for your activity in your manifest file.

Answer 3

在你的maifest文件中试试这个

android:theme="@style/AppTheme"