What makes an activity appear in this app drawer is based upon stuff found in the AndroidManifest.xml file, and we will see how that works later in the book. While the user thinks of those as “running an app”, in reality those icons pass control to an activity inside of the app, one designated as being something that should appear in a “launcher” or home screen. The user’s home screen will often have an “app drawer” or similar thing with a bunch of icons. In a simple app with one activity, such as this sample app, that activity will serve as the entry point for the app itself. So, while it is easy to think of activities as being equivalent to the screen, just remember that this is a simplification, and that reality is more complicated (as reality often is). However, bear in mind that on some devices, the user will be able to work with more than one activity at a time, such as split-screen mode on a phone or multi-window mode on a Chrome OS device. Normally, an activity will take up most of the screen, leaving space for things like a status bar (the strip across the top with the clock, battery icon, etc.) and a navigation bar (the strip across the bottom with buttons for going back, going to the home screen, etc.) It represents a chunk of your user interface and, in many cases, a discrete entry point into your app (i.e., a way for other apps to link to your app). You can think of an activity as being the Android analogue for the window in a desktop application or the page in a classic Web app. The building block of the user interface is the activity. This class represents an “activity”, one of the core components in an Android app.

Ignoring the test code for a while, our one-and-only source file in our project implements a MainActivity class, either in Java or Kotlin.

Android studio language activities