可滚动选项卡显示一些问题

Question

我正在学习Scrollable选项卡，引用http://www.androidhive.info/2013/10/android-tab-layout-with-swipeable-views-1/，当我编写代码时，我得到了我在主要活动下面发布的日志错误。 不知道发生了什么。

日志

 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.cisystem3.scrolltab/com.example.cisystem3.scrolltab.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setHomeButtonEnabled(boolean)' on a null object reference
   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
  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:5254)
 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.setHomeButtonEnabled(boolean)' on a null object reference
 at com.example.cisystem3.scrolltab.MainActivity.onCreate(MainActivity.java:30)
 at android.app.Activity.performCreate(Activity.java:5990)at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387) 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:5254) 
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) 

MainActivity

  package com.example.cisystem3.scrolltab;

import android.support.v7.app.ActionBar;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity implements ActionBar.TabListener
        {

    private ViewPager viewPager;
    private TabsPagerAdapter mAdapter;
    private ActionBar actionBar;
    // Tab titles
    private String[] tabs = {"Top Rated", "Games", "Movies"};

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

        // Initilization
        viewPager = (ViewPager) findViewById(R.id.pager);
        actionBar = getSupportActionBar();
        mAdapter = new TabsPagerAdapter(getSupportFragmentManager());

        viewPager.setAdapter(mAdapter);
        actionBar.setHomeButtonEnabled(false);
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        // Adding Tabs
        for (String tab_name : tabs) {
            actionBar.addTab(actionBar.newTab().setText(tab_name)
                    .setTabListener(this));
        }

        /**
         * on swiping the viewpager make respective tab selected
         * */
        viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

            @Override
            public void onPageSelected(int position) {
                // on changing the page
                // make respected tab selected
                actionBar.setSelectedNavigationItem(position);
            }

            @Override
            public void onPageScrolled(int arg0, float arg1, int arg2) {
            }

            @Override
            public void onPageScrollStateChanged(int arg0) {
            }
        });
    }


            @Override
            public void onTabSelected(ActionBar.Tab tab, android.support.v4.app.FragmentTransaction ft) {

            }

            @Override
            public void onTabUnselected(ActionBar.Tab tab, android.support.v4.app.FragmentTransaction ft) {

            }

            @Override
            public void onTabReselected(ActionBar.Tab tab, android.support.v4.app.FragmentTransaction ft) {

            }
        }

清单

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.cisystem3.scrolltab">

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

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

</manifest>

式

<resources>

    <!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
        <!-- Customize your theme here. -->
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>
    </style>

</resources>