Probably the question is not related but why this annotation is named as @FunctionalInterface? Why it wasn't named as @Functional? Or in the future we could get something like @FunctionalClass?
Because this annotation goes into java.lang and the probability of collision with an existing type named Functional is extremely high, wherease with FunctionalInterface is much lower. We like our users and we don't want to break their code for no good reason.
Functional is not only an english word, but it is also a term from mathematics (http://en.wikipedia.org/wiki/Functional_analysis). Guaranteed that it collides with something.
