Welcome to Training for Android developers. Here you'll find sets of lessons within classes that describe how to accomplish a specific task with code samples you can re-use in your app. Classes are organized into several groups you can see at the top-level of the left navigation.
This first group, Getting Started, teaches you the bare essentials for Android app development. If you're a new Android app developer, you should complete each of these classes in order:
https://developer.android.com/training/index.html
