This book teaches Android development for the versions of Android in wide use at the time of writing. For this edition, that is Android 5.0 (Lollipop, API level 21) - Android 9.0 (Pie, API level 28). While there is still limited use of older versions of Android, we find that for most developers the amount of effort required to support those versions is not worth the reward.
If you would like information on supporting versions of Android earlier than 5.0, see earlier editions of this book. The third edition targeted Android 4.4 and up, the second edition targeted Android 4.1 and up, and the first edition targeted Android 2.3 and up.
As Android releases new versions, the techniques you learn in this book will continue ...