PagerSlidingTabStrip NullPointerException when setting ViewPager

Question

I'm trying to implement PagerSlidingTabStrip in my app. I did everything I'm supposed to do:

Added it to my XML:

<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" >

<android.support.v4.view.ViewPager
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

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

</RelativeLayout>

Created PagerSlidingTabStrip and tried to set ViewPager:

PagerSlidingTabStrip slidingTabStrip = (PagerSlidingTabStrip) findViewById(R.id.slidingTabs);
slidingTabStrip.setViewPager(vPager);

But when I run the app, it crashes and this is the logcat output:

04-17 20:56:55.956: E/AndroidRuntime(1587): FATAL EXCEPTION: main
04-17 20:56:55.956: E/AndroidRuntime(1587): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.matejhacin.beautifulvoicerecorder/com.matejhacin.beautifulvoicerecorder.MainActivity}: java.lang.NullPointerException
04-17 20:56:55.956: E/AndroidRuntime(1587):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at android.os.Handler.dispatchMessage(Handler.java:99)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at android.os.Looper.loop(Looper.java:137)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at android.app.ActivityThread.main(ActivityThread.java:5103)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at java.lang.reflect.Method.invokeNative(Native Method)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at java.lang.reflect.Method.invoke(Method.java:525)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at dalvik.system.NativeStart.main(Native Method)
04-17 20:56:55.956: E/AndroidRuntime(1587): Caused by: java.lang.NullPointerException
04-17 20:56:55.956: E/AndroidRuntime(1587):     at com.astuetz.PagerSlidingTabStrip.notifyDataSetChanged(PagerSlidingTabStrip.java:200)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at com.astuetz.PagerSlidingTabStrip.setViewPager(PagerSlidingTabStrip.java:182)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at com.matejhacin.beautifulvoicerecorder.MainActivity.onCreate(MainActivity.java:39)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at android.app.Activity.performCreate(Activity.java:5133)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-17 20:56:55.956: E/AndroidRuntime(1587):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)

Answer 1

The error isn't handled nicely in this library. If you look at the source of PagerAdapter you will see a method getPageTitle: https://github.com/android/platform_frameworks_support/blob/master/v4/java/android/support/v4/view/PagerAdapter.java#L306-Lundefined

Notice that by default it returns null. This is why you are seeing a null pointer exception. The solution is to override this method in your implementation of PagerAdapter

@Override
public CharSequence getPageTitle(int position) {
  // return the title of the tab
}

If you don't actually want a string title, and you want the tab to use a custom view, such as an image, then your PagerAdapter has to implement PagerSlidingTabStrip.TabCustomViewProvider and override getPageTabCustomView

@Override
public View getPageTabCustomView(int position) {
  // return a custom view
}

EDIT: In order to use the custom tab view, you will need to use this fork: https://github.com/astuetz/PagerSlidingTabStrip/pull/32

Answer 2

Caused by: java.lang.NullPointerException
at com.astuetz.PagerSlidingTabStrip.notifyDataSetChanged(PagerSlidingTabStrip.java:200)

You're throwing a NullPointerException on line 200 in PagerSlidingTabStrip.

This indicates that PagerAdapter.getPageTitle is returning null.

Answer 3

You should override your adapters getPageTitle method because it is returning null

@Override
public CharSequence getPageTitle(int position)
{

return tabTitles[position];
}