android - Null pointer Exception - findViewById()

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:

                android:orientation="vertical"
                android:layout_width="fill_parent"
                android:layout_height="fill_parent"
                android:background="#a4c639">


                            android:layout_width="match_parent" 
                        android:layout_height="match_parent" 
                        android:id="@+id/main_pager"/>

activity_first layout:

android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/first_View">


    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello_world" />

    android:id="@+id/button1"
    style="?android:attr/buttonStyleSmall"
    android:layout_width="wrap_content"

    android:layout_height="wrap_content"
    android:text="Button" />

activity_second layout:

 
 

android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/second_View">

    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello_world" />

And the activity_third layout:

android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/third_View">


    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello_world" />

Answer

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.

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); it will return null since there is not a view in your activity_first.xml layout called @+id/second_View.