This thesis explains the basics of programming for the Android operative system. Android is an operation system specifically made for mobile devices, tablets, and other smaller portable devices. The main part of this thesis refers to the principles of programming applications in the Android ecosystem. The technologies that are available for use of creating applications, the history, and operating principles of Android and other characteristics of the operation system will be mentioned and explained. In the practical part of this thesis the use of Java programming language and extensible markup language (XML) will be explained in the use case of creating an application that determines who pays for the coffee. The operation and functionalities of the android application are explained in detail. In this application it is possible to add people into the application and group them into one or more groups. Subsequently it is possible to use the main algorithm of the application to determine who pays for the coffee in the specific group. The algorithm is relatively simple and checks how much each person paid in a given moment and uses that information to determine who should pay for the next coffee. Ways of future expansion and further work are also mentioned for the aforementioned application. It is possible to expand the application in the future from the local level to the online level. That way many users could track and use the application.