Book excerpt: 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.