Kotlin programming has been recognized as the main language for deploying Android projects, so this language has tremendous potential. In order to understand the main principles and best applications of Kotlin programming, further research is needed. Principles, Policies, and Applications of Kotlin Programming provides complete knowledge of Kotlin programming. It presents the latest technology of Vert.X, Restful API programming application by Kotlin, as well as authentication techniques for Restful APIs, which are new and important content in backend deployment for software. Covering topics such as array processing, graphic user interface, and string processing, this premier reference source is a valuable resource for software developers, programmers, students and educators of higher education, researchers, and academicians. Kotlin is a form of programming language designed and standardized according to its own rule system, it can be programmed stand-alone or integrated with the Java programming language. Kotlin has now become a popular language for Android programming, increasingly interested in the programming community. There are many advantages that the Kotlin programming language offers such as the ability to overcome the weaknesses of Java. Currently, Java is one of the preferred programming languages when developers implement Android projects, but it still has certain limitations. One of the biggest lingering problems in Java is the lack of extensibility and inability to support functional programming.