First simple Android program

app > java > com.example.myfirstapp > MainActivity: This is the main activity (the entry point for your app). When you build and run the app, the system launches an instance of this Activity and loads its layout.
app > res > layout > activity_main.xml: This XML file defines the layout for the activity’s UI. It contains a TextView element with the text “Hello world!”.
app > manifests > AndroidManifest.xml: The manifest file describes the fundamental characteristics of the app and defines each of its components.
Gradle Scripts > build.gradle: You’ll see two files with this name: one for the project and one for the “app” module. Each module has its own build.gradle file, but this project currently has just one module. You’ll mostly work with the module’s build.gradle file to configure how the Gradle tools compile and build your app. For more information about this file, see Configure Your Build.