在Android <= 4.2中，ViewPager findViewById为null

Question

I use a ViewPager to have swipe tabs in my app. It works in Android versions above 4.2, but crashes in versions >= 4.0 and <= 4.2.

I get a null pointer exception when I call setAdapter on the view pager and that's because it turns out the view pager is null after findViewById. It works fine in Android versions > 4.2. I can't figure out what the issue is with other Android versions.

public class MainActivity extends FragmentActivity {

    ViewPager mViewPager;
    TabsPagerAdapter mAdapter;

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

        mViewPager = (ViewPager) findViewById(R.id.main_view_pager);
        mAdapter = new TabsPagerAdapter(getSupportFragmentManager());
        mViewPager.setAdapter(mAdapter);
    }
}

R.layout.activity_main

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_height="match_parent"
    android:layout_width="match_parent"
    android:orientation="vertical"
    android:id="@+id/profile_list_container">

    <android.support.v4.view.ViewPager
        android:id="@+id/main_view_pager"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1">
    </android.support.v4.view.ViewPager>

</LinearLayout>

Adapter

public class TabsPagerAdapter extends FragmentPagerAdapter {

    @Override
    public int getCount() {
        return 3;
    }

    @Override
    public Fragment getItem(int position) {

        switch (position) {

            case 0:
                return new ProfileListFragment();
            case 1:
                return new BoostFragment();
            case 2:
                return new StatsFragment();

        }
        return null;
    }

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

Answer 1

Probably ViewPager is not load correctly in the layout. Check it in the logcat.

Answer 2

try

mViewPager = new ViewPager(this);
linearlayout.addView (mViewPager);

since you get null ViewPager, new ViewPager(this) can make sure the problem is not related to the Class of ViewPager .

LinearLayout linearlayout = (LinearLayout)findViewById(R.id.profile_list_container); helps you make sure your activity is "connected" to the correct layout.

after you have initialized the ViewPager, remember to set the layoutParam, including width/height...etc

Answer 3

Oh wow, I figured it out. It was using a totally different layout for tablets. I had used the default master/detail template from Android Studio and had forgotten to update the layout for tablets, which explains why my views could not be found.