The book describes practical techniques for refactoring and porting code written in Java mobile applications for Android to Kotlin while maintaining compatibility. A detailed comparison of these two languages ​ ​ is given, examples of translating projects from Java to Kotlin, adding Kotlin support to the Java assembly are given. Shows the transition from Java classes to Kotlin classes, from Java collections to Kotlin collections, from JavaBeans objects to values, from static methods to top-level functions.