If want to make an activity to full screen in some cases like playing video in full screen mode, playing game etc., you can do that by following one of the methods.
One way is using Theme.Holo.Light.NoActionBar.Fullscreen value in AndroidManifest.xml file. (Use the appropriate theme name whichever you used in your application)
Another way is, do this through code in the activity. In your activity do the following before calling setContentView() function
Happy coding! 🙂