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 th

Answer 1

add those views to the pager adapter before accessing them.

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

    adapter = new MyPagerAdapter();
    pager = (ViewPager) findViewById(R.id.main_pager);
    pager.setAdapter(adapter);

    layout1 = (LinearLayout) findViewById(R.id.first_View);
    layout2 = (LinearLayout) findViewById(R.id.second_View);
    layout3 = (LinearLayout) findViewById(R.id.third_View);

}

in the pager adapter:

public Object instantiateItem(View collection, int position) {
    if(position == 0){
        View layout = inflater.inflate(R.layout.activity_first, null);

        ((ViewPager) collection).addView(layout);

        return layout;
    } 
    ... and so forth.

}

from here you can access them via findViewById.

Answer 2

The findViewById method must find what is in the layout (which you called in the setContentView)

Answer 3

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.-

@Override
public 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;
}

Answer 4

In Android, findViewById(R.id.some_id) works when you are finding view in the layout set.

That is, if you have set a layout say:

setContentView(R.layout.my_layout);

Views can be found only in this layout (my_layout).

In your code layout1, layout2, layout3 all are three different layouts and they are not set to the activity.