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
.