Java is often recommended and used for mobile app development. But some time ago the language Kotlin was released and thus there is now an alternative to Java. Kotlin is a statically typed programming language, which can be used for several applications and uses the Java Virtual Machine. Two examples of possible uses would be the development of server-side or mobile applications. In addition, it is similar in syntax to a scripting language. In this work, advantages and disadvantages and the elementary differences between Java and Kotlin are shown by code examples. Additionally, a general insight about object-oriented programming is provided. In general, it can be said that there is no winner or loser in this comparison, because both languages offer certain advantages and disadvantages. In the end, it is up to the developer which programming language is used. For example, both languages can be used in a project, because Kotlin is interoperable with Java.