This book is designed for those developers who are new to Kotlin who wish to develop projects with Spring and Android. Spring for Android provides a functional REST client that supports marshaling objects from JSON. Developers depend on other language platforms, such as PHP and Python for REST API, but Spring comes with Java/Kotlin and a rich content that helps developers to use REST API with the maximum security. There are some dependencies in the application code and Spring removes these dependencies. Nowadays, Java is being replaced by Kotlin, which is lighter and requires fewer lines of code to finish the job.