Activities and fragments are two of the most important concepts for an Android developer to master. In fact, in just about any Android-related programming interview, a common starting question is to ask a candidate to describe and outline activities, fragments, and their respective life cycles!

Broadly speaking, most Android apps consist of a series of connected screens that a user is able to navigate through. Obviously, more complex apps can be offered additional functionality such as background services, cloud messaging, broadcast receivers, and so on. However, the core UI will usually be centered around navigating through a series of connected activities or fragments nested within an activity.

A simple example of this basic UI in, for instance, a cooking-recipe application, would be a screen displaying a list of available recipes (Recipe List Activity) and another screen displaying details of each recipe (Recipe Detail Activity...