Android .setViewPager 返回 NullPointerException

Question

所以我遇到了这个问题。我正在尝试使用一些选项卡制作简单的操作栏。不知何故，我得到了 NullPointerException，我无法弄清楚如何解决它。这里有一些代码：

MainActivity.java

public class MainActivity extends FragmentActivity {
    ViewPager Tab;
    TabPagerAdapter TabAdapter;
    ActionBar actionBar;
    PagerSlidingTabStrip tabs;

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

        tabs = (PagerSlidingTabStrip) findViewById(R.id.tabs);
        Tab = (ViewPager) findViewById(R.id.pager);
        TabAdapter = new TabPagerAdapter(getSupportFragmentManager());

        Tab.setAdapter(TabAdapter);
        tabs.setViewPager(Tab); //line 27 where the error appears

        tabs.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
            @Override
            public void onPageSelected(int position) {

                actionBar = getActionBar();
                actionBar.setSelectedNavigationItem(position);
            }
        });
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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" >

    <com.astuetz.PagerSlidingTabStrip
        android:id="@+id/tabs"
        android:layout_width="match_parent"
        android:layout_height="48dip" />

    <android.support.v4.view.ViewPager
        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity" />

</RelativeLayout>

这里也是错误日志：

01-05 08:30:20.576: E/AndroidRuntime(19913): FATAL EXCEPTION: main
01-05 08:30:20.576: E/AndroidRuntime(19913): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.hiddensofia/com.example.hiddensofia.MainActivity}: java.lang.NullPointerException
01-05 08:30:20.576: E/AndroidRuntime(19913):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2463)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2520)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at android.app.ActivityThread.access$600(ActivityThread.java:162)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1366)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at android.os.Handler.dispatchMessage(Handler.java:99)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at android.os.Looper.loop(Looper.java:158)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at android.app.ActivityThread.main(ActivityThread.java:5751)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at java.lang.reflect.Method.invokeNative(Native Method)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at java.lang.reflect.Method.invoke(Method.java:511)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1083)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:850)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at dalvik.system.NativeStart.main(Native Method)
01-05 08:30:20.576: E/AndroidRuntime(19913): Caused by: java.lang.NullPointerException
01-05 08:30:20.576: E/AndroidRuntime(19913):    at com.astuetz.PagerSlidingTabStrip.notifyDataSetChanged(PagerSlidingTabStrip.java:200)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at com.astuetz.PagerSlidingTabStrip.setViewPager(PagerSlidingTabStrip.java:182)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at com.example.hiddensofia.MainActivity.onCreate(MainActivity.java:27)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at android.app.Activity.performCreate(Activity.java:5165)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1103)
01-05 08:30:20.576: E/AndroidRuntime(19913):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2419)

这是我的适配器：

TabPagerAdapter.java

public class TabPagerAdapter extends FragmentStatePagerAdapter {
    public TabPagerAdapter(FragmentManager fm) {
        super(fm);
        // TODO Auto-generated constructor stub
    }

    @Override
    public Fragment getItem(int i) {
        switch (i) {
        case 0:
            //Fragement for Stories Tab
            return new Stories();
        case 1:
           //Fragment for Legends Tab
            return new Legends();
        case 2:
            //Fragment for Multimedia Tab
            return new Multimedia();

        case 3:
            //Fragment for Books Tab
            return new Books();
        }
        return null;

    }

    @Override
    public int getCount() {
        // TODO Auto-generated method stub
        return 4; //No of Tabs
    }
}

提前感谢您的帮助！

Answer 1

public CharSequence getPageTitle(int position) {

            if (position == 0)
            {
                return "Stories";
            }
            if (position == 1)
            {
                return "Legends";
            }
            if (position == 2)
            {
                return "Multimedia";
            }
            if (position == 3)
            {
                return "Books";
            } 
            return null;
        }

Answer 2

您需要在您的PagerAdapter 中覆盖getPageTitle() 并提供您的PagerSlidingTabStrip 可以获得的标题。