java – Swipe Tab – NullPointerException – Android

I followed this tutorial: #127 Android scrollable tabs Part 1: Android tutorial for beginners. I created three clips with three layouts, but I got java.lang.nullpointerexception. I don't know what went wrong

MainActivity.java：

public class MainActivity extends FragmentActivity implements TabListener {

ViewPager viewPager;
ActionBar actionBar;

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

    viewPager = (ViewPager) findViewById(R.id.pager);
    viewPager.setAdapter(new MyAdapter(getSupportFragmentManager()));
    viewPager.setOnPagechangelistener(new ViewPager.OnPagechangelistener() {

        @Override
        public void onPageSelected(int position) {
            // TODO Auto-generated method stub
            actionBar.setSelectedNavigationItem(position);
        }
        @Override
        public void onPageScrolled(int position, float arg1, int pixels) {
            // TODO Auto-generated method stub  
        }
        @Override
        public void onPageScrollStateChanged(int position) {
            // TODO Auto-generated method stub
        }
    });

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

    ActionBar.Tab tab1 = actionBar.newTab();
    tab1.setText("Tab 1");
    tab1.setTabListener(this);

    ActionBar.Tab tab2 = actionBar.newTab();
    tab2.setText("Tab 2");
    tab2.setTabListener(this);

    ActionBar.Tab tab3 = actionBar.newTab();
    tab3.setText("Tab 3");
    tab3.setTabListener(this);

    actionBar.addTab(tab1);
    actionBar.addTab(tab2);
    actionBar.addTab(tab3);
}

@Override
public void onTabReselected(Tab tab, FragmentTransaction ft) {
    // TODO Auto-generated method stub      
}
@Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
    // TODO Auto-generated method stub
    viewPager.setCurrentItem(tab.getPosition());
}
@Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
    // TODO Auto-generated method stub  
}
    }

    class MyAdapter extends FragmentPagerAdapter {

public MyAdapter(FragmentManager fm) {
    super(fm);
    // TODO Auto-generated constructor stub
}
@Override
public Fragment getItem(int position) {
    // TODO Auto-generated method stub
    Fragment fragment = null;

    switch(position)
    {
    case 0:
        fragment = new FragmentA();
        break;      
    case 1:
        fragment = new FragmentB();
        break;  
    case 2:
        fragment = new FragmentC();
        break;
    }
    return fragment;
}
@Override
public int getCount() {
    // TODO Auto-generated method stub
    return 3;
}
   }

Here's everything, actionbars, viewpager and adapter. Can someone tell me what went wrong?

Of logcat:

05-05 16:38:22.670: E/AndroidRuntime(13118): FATAL EXCEPTION: main
05-05 16:38:22.670: E/AndroidRuntime(13118): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.kiko.swipetabs/com.kiko.swipetabs.MainActivity}: java.lang.NullPointerException
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2074)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2099)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.ActivityThread.access$600(ActivityThread.java:138)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1205)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.os.Handler.dispatchMessage(Handler.java:99)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.os.Looper.loop(Looper.java:137)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.ActivityThread.main(ActivityThread.java:4929)
05-05 16:38:22.670: E/AndroidRuntime(13118): at java.lang.reflect.Method.invokeNative(Native Method)
05-05 16:38:22.670: E/AndroidRuntime(13118): at java.lang.reflect.Method.invoke(Method.java:511)
05-05 16:38:22.670: E/AndroidRuntime(13118): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:798)
05-05 16:38:22.670: E/AndroidRuntime(13118): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:565)
05-05 16:38:22.670: E/AndroidRuntime(13118): at dalvik.system.NativeStart.main(Native Method)
05-05 16:38:22.670: E/AndroidRuntime(13118): Caused by: java.lang.NullPointerException
05-05 16:38:22.670: E/AndroidRuntime(13118): at com.kiko.swipetabs.MainActivity.onCreate(MainActivity.java:44)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.Activity.performCreate(Activity.java:5254)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1082)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2038)
05-05 16:38:22.670: E/AndroidRuntime(13118): ... 11 more

In fact, the person in the video released the code on guthub. This is the code:

public class MainActivity extends FragmentActivity {

ViewPager viewPager=null;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    viewPager= (ViewPager) findViewById(R.id.pager);
    FragmentManager fragmentManager=getSupportFragmentManager();
    viewPager.setAdapter(new MyAdapter(fragmentManager));
}

}

class MyAdapter extends FragmentStatePagerAdapter
{

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

@Override
public Fragment getItem(int i) {
    Fragment fragment=null;
//        Log.d("VIVZ", "get Item is called "+i);
    if(i==0)
    {
        fragment=new FragmentA();
    }
    if(i==1)
    {
        fragment=new FragmentB();
    }
    if(i==2)
    {
        fragment=new FragmentC();
    }
    return fragment;
}

@Override
public int getCount() {
//        Log.d("VIVZ", "get Count is called");
    return 3;
}

@Override
public CharSequence getPageTitle(int position) {
    if(position==0)
    {
        return "Tab 1";
    }
    if(position==1)
    {
        return "Tab 2";
    }
    if(position==2)
    {
        return "Tab 3";
    }
    return null;
}
}

This code is valid, but there is no actionbar here, only fragments are sliding

resolvent:

Getactionbar() seems to return null. This happens if the topic specifies that the activity should not have an action bar

Try calling

getWindow().requestFeature(Window.FEATURE_ACTION_BAR);

Before setcontentview()