Android development is hot, and many programmers are interested in joining the fun. However, because this technology is based on Java, you should first obtain a solid grasp of the Java language and its foundational APIs to improve your chances of succeeding as an Android app developer. After all, you will be busy learning the architecture of an Android app, the various Android-specific APIs, and Android-specific tools. If you do not already know Java fundamentals, you will probably end up with a massive headache from also having to quickly cram those fundamentals into your knowledge base.  Learn Java for Android Development, Second Edition teaches programmers of any skill level the essential Java language and foundational Java API skills that must be learned to improve the programmer’s chances of succeeding as an Android app developer. Each of the book’s 14 chapters provides an exercise section that gives you the opportunity to reinforce your understanding of the chapter’s material. Answers to the book’s more than 500 exercises are provided in an appendix. A second appendix provides a significant game-oriented Java application, which you can convert into an Android app.
Once you complete this book, you should be ready to dive into beginning Android app development.  Maybe, start that journey with Apress' Beginning Android.