That was my immediate thought - this is closer to multiple inheritance, as it introduces the diamond problem: https://en.wikipedia.org/wiki/Multiple_inheritance

That was my immediate thought - this is closer to multiple inheritance, as it introduces the diamond problem: https://en.wikipedia.org/wiki/Multiple_inheritance#The_diamond_problem https://en.wikipedia.org/wiki/Multiple_inheritance#The_diamo... According to that page, Java forces programmers to resolve the ambiguity by making it a compiler error. A reasonable approach, I think. I've done more OO development in C++, and I've found this technique useful - putting a default implementation in a class which was mostly an interface because only a small number of subclasses needed specialized behavior.