So far in the book, you've mostly been navigating users from one Activity to another Activity, and this is in fact how most applications are built. However, there is another option, which is often much more flexible and allows you to build even more modular applications--navigation using Fragment instances. So far, we've only really looked at Fragments as little blocks of your application that can be assembled to form parts of a screen, but they can be so much more than that.
The tabbed Activity classes both provide a sort of navigation using the ViewPager class and the FragmentPagerAdapter class. In these cases, each of the pages that the user can swipe to is a complete Fragment, with its life cycle that is paused and resumed, stopped, and started as the user swipes the Fragment in or out of view.
If you look into the FragmentPagerAdapter class, you'll find that it doesn't add and remove the Fragment view instances directly to the ViewPager object. Instead, it uses...