Activities and other application components, such as services, are declared in the AndroidManifest.xml file. Declaring an activity node is how we tell the OS about our Activity class and how it can be requested. For example, an application will usually indicate that at least one activity should be visible as a desktop icon and serve as the main entry point to the application.

Android Studio, now at version 3.2, is used for all the code samples shown in this book. If you have not already installed it, visit the Android Studio website (see the link in the previous tip) to install the IDE and the SDK bundle for your platform.

For this first example, we'll guide you through creating a new project. Android Studio provides a Quick Start wizard, which makes the process extremely easy. Follow these steps to get started:

1. Launch Android Studio, which brings up the Welcome to Android Studio dialog:

2. Click on the Start a new Android Studio project option.
3. Enter an application name; for this example, we used DeclareAnActivity. Click on Next:

4. In the Target Android Devices dialog, you can leave the Phone and Tablet checkbox selected with the default API 21: Android 5.0 (Lollipop) selection for the minimum SDK (for this example, it really doesn't matter which API level you choose, as activities have existed since API level 1). Click on Next:

5. In the Add an Activity to Mobile dialog, select the Empty Activity option. Click on Next:

6. In the Configure Activity dialog, you can leave the defaults as provided, but note that the default activity name is MainActivity. Click on Finish:

After finishing the wizard, Android Studio will create the project files. For this recipe, the two files that we will examine are MainActivity.java (which corresponds to the activity name mentioned in step 6) and AndroidManifest.xml.

If you take a look at the MainActivity.java file, you will realize that it's pretty basic. This is because we chose the Empty Activity option (in step 5). Now, look at the AndroidManifest.xml file. This is where we actually declare the activity. Within the <application> element is the <activity> element:

<activity android:name=".MainActivity" android:label="@string/app_name"> <intent-filter> <action android:name="android.intent.action.MAIN"/> <category android:name=
 "android.intent.category.LAUNCHER"/> </intent-filter> </activity>

Declaring an activity is a simple matter of declaring the <activity> element and specifying the name of the activity class with the android:name attribute. By adding the <activity> element to the Android Manifest, we are specifying our intention to include this component in our application. Any activities (or any other component for that matter) that are not declared in the manifest will not be available to the application. Attempting to access or utilize an undeclared component will result in an exception being thrown at runtime.

In the preceding code, there is another attribute: android:label. This attribute indicates the title shown on the screen, as well as the icon if this is the Launcher activity.