Oracle. (n.d.). Lesson: Interfaces and Inheritance. Retrieved from https://docs.oracle.com/javase/tutorial/java/IandI/index.html
Tutorialspoint.com. (n.d.). Data Structures and Algorithms Recursion Basics. Retrieved from https://www.tutorialspoint.com/data_structures_algorithms/recursion_basics.htm
Teukolsky, R. (2015). AP Computer Science A. Barrons Educational Series.