A button is an Android widget, and widgets are the user interface elements that you use in your application.
The onCreate() method is invoked when the activity is started. The first thing you should do is chain upward to the superclass, so the stock Android activity initialization can be done.
All widgets extend the View base class. You usually build the user interface out of a hierarchy of views.
In Android, a button click causes onClick() to be invoked in the OnClickListener instance configured for the button.
