App Basics

Your GeoQuiz application will consist of an activity and a layout:

An activity is an instance of Activity, a class in the Android SDK. An activity is responsible for managing user interaction with a screen of information.

You write subclasses of Activity to implement the functionality that your app requires. A simple application may need only one subclass; a complex application can have many.

GeoQuiz is a simple app and will start off with a single Activity subclass named MainActivity. MainActivity will manage the user interface, or UI, shown in Figure 1.1.
A layout defines a set of UI objects and the objects’ positions on the screen. A layout is made up of definitions written in XML. Each definition is used to create an object that appears onscreen, like a button or some text.

GeoQuiz will include a layout file named activity_main.xml. The XML in this file will define the UI shown in Figure 1.1.

The relationship between MainActivity and activity_main.xml is diagrammed in Figure 1.2.

Figure 1.2 MainActivity manages what activity_main.xml defines

With those ideas in mind, let’s build an app.

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