Null pointer Exception - findViewById()

Question

Can anyone help me to find out what can be the issue with this program. In the onCreate() method the findViewById() returns null for all ids and this causes a null pointer exception later. I can not figure out why the findViewById() can not find the view. Any suggestions?

This is the main code:

public class MainActivity extends Activity {      ViewPager pager;     MyPagerAdapter adapter;     LinearLayout layout1, layout2, layout3;      @Override     protected void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_main);          layout1 = (LinearLayout) findViewById(R.id.first_View);         layout2 = (LinearLayout) findViewById(R.id.second_View);         layout3 = (LinearLayout) findViewById(R.id.third_View);          adapter = new MyPagerAdapter();         pager = (ViewPager) findViewById(R.id.main_pager);         pager.setAdapter(adapter);     }      private class MyPagerAdapter extends PagerAdapter     {          @Override         public int getCount() {              return 3;         }          @Override         public Object instantiateItem(ViewGroup collection, int position) {              LinearLayout l = null;              if (position == 0 )             {                 l = layout1;             }             if (position == 1)             {                 l = layout2;             }              if (position == 2)             {                 l = layout3;             }                 collection.addView(l, position);                 return l;         }          @Override         public boolean isViewFromObject(View view, Object object) {             return (view==object);         }           @Override          public void destroyItem(ViewGroup collection, int position, Object view) {                  collection.removeView((View) view);          }     } } 

And the related XML files:

activity_main layout:

<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"                 android:orientation="vertical"                 android:layout_width="fill_parent"                 android:layout_height="fill_parent"                 android:background="#a4c639">       <android.support.v4.view.ViewPager                         android:layout_width="match_parent"                          android:layout_height="match_parent"                          android:id="@+id/main_pager"/> </LinearLayout> 

activity_first layout:

<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:id="@+id/first_View">  <TextView     android:layout_width="wrap_content"     android:layout_height="wrap_content"     android:text="@string/hello_world" />  <Button     android:id="@+id/button1"     style="?android:attr/buttonStyleSmall"     android:layout_width="wrap_content"     android:layout_height="wrap_content"     android:text="Button" />  </LinearLayout> 

activity_second layout:

 <?xml version="1.0" encoding="utf-8"?>  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"  android:layout_width="match_parent" android:layout_height="match_parent" android:id="@+id/second_View">  <TextView     android:layout_width="wrap_content"     android:layout_height="wrap_content"     android:text="@string/hello_world" />  </LinearLayout> 

And the activity_third layout:

<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:id="@+id/third_View">  <TextView     android:layout_width="wrap_content"     android:layout_height="wrap_content"     android:text="@string/hello_world" />  </LinearLayout> 

Answer 1

findViewById() returns a View if it exists in the layout you provided in setContentView(), otherwise it returns null and that's what happening to you. Note that if you don't setContentView(), and don't have a valid view to findViewById() on, findViewById() will always return null until you call setContentView().

This also means variables in the top-level trigger an NPE, because they're called before onCreate(), and by extension, before setContentView(). See also the activity lifecycle

Example if you setContentView(R.layout.activity_first); and then call findViewById(R.id.first_View); it will return a View which is your layout.

But if you call findViewById(R.id.second_View); before setContentView(), it will return null since there is not a view in your activity_first.xml layout called @+id/second_View.