Tip: An easy way to add import packages to your project is to press Ctrl-Shift-O (Cmd-Shift-O, on Mac). 발췌: http://developer.android.com/resources/tutorials/hello-world.html

Notice that the class is based on the Activity class. An Activity is a single application entity that is used to perform actions. An application may have many separate activities, but the user interacts with them one at a time. The onCreate() method is called by the Android system when your Activity starts — it is where you should perform all initialization and UI setup. An activity is not requi..