Null pointer Exception - findViewById()

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.

^{Emphasis added}

For those cases within an Activity class.

Activity.findViewById(int id)

Finds a view that was identified by the id attribute from the XML that was processed in onCreate(Bundle).

Otherwise, such as an Fragment, Adapter, a View from a LayoutInflater, etc.

View.findViewById(int id)

Look for a child view with the given id. If this view has the given id, return this view.

Either case,

Returns
The view if found or null otherwise.

Now, re-check your XML files. Make sure you put the right value into setContentView or inflater.inflate.

In the case of an Activity, call findViewById after setContentView.

Then, make sure there is a View you are looking for with android:id="@+id/..." in that layout. Make sure the + is at @+id, which will add the resource to the R.id values to ensure you can find it from Java.

The views you're trying to get are not defined in your activity_main layout. You need to programmatically inflate the views you're trying to add to the pager.-

@Overridepublic Object instantiateItem(ViewGroup collection, int position) {    LinearLayout l = null;    if (position == 0) {        l = (LinearLayout) View.inflate(this, R.layout.activity_first, null);    }    if (position == 1) {        l = (LinearLayout) View.inflate(this, R.layout.activity_second, null);    }    if (position == 2) {        l = (LinearLayout) View.inflate(this, R.layout.activity_third, null);    }    collection.addView(l, position);    return l;}