As others have exemplified: it gets really hard to talk about this stuff outside of formal models. Especially if your goal is to talk about equivalency.
At this point I start trying to turn toward System F, but that's my hammer for this particular nail and I don't know an equivalent one in the object side of things. I'm certain you can express higher-level things in System F which are equivalent to interfaces. I know there's a translation of typeclasses to System F—it's called Haskell, har har—although you have to recognize that the "search" component of typeclasses will be lost.
At this point I start trying to turn toward System F, but that's my hammer for this particular nail and I don't know an equivalent one in the object side of things. I'm certain you can express higher-level things in System F which are equivalent to interfaces. I know there's a translation of typeclasses to System F—it's called Haskell, har har—although you have to recognize that the "search" component of typeclasses will be lost.