This post describes a new feature in Java 8 called virtual extension methods (formerly called default methods, or defender methods). In the example provided, an interface has one method, which is implemented by a class. Then a second method is added, but instead of forcing the class to immediately implement the interface's new method, a "virtual extension method" is created within the interface as an implementation of this new method, thus the class doesn't need to implement the new method right away.
But say that the programmer decides to create a virtual extension method in an interface and never plans to have subclasses implement its own version. Wouldn't that be a mixin? It seems like one to me, but I've never heard it referred to as such. Is there some fundamental difference that I'm not understanding?