A Modularity Bug in Java 8.

We demonstrate a modularity bug in the interface system of Java 8 on the practical example of a textbook design of a modular interface for vector spaces. Our example originates in our teaching of modular object-oriented design in Java 8 to undergraduate students, simply following standard programming practices and mathematical definitions. The bug shows up as a compilation error and should be fixed with a language extension due to the importance of best practices (design fidelity).