> the set of all public methods could be seen as 1, but not the only, interpretation of the functional interface.
could you describe another interpretation? "the set of public methods"¹ is the only thing i can think of when i hear about an object's "functional interface". it could be a terminology issue though – i'm reading "functional interface" in a general sort of way, but i could imagine it having some specific definition in OO theory.
---
¹ or recognized messages, in a more smalltalk-ish context
PS. if you're interested in weird perspectives on this, you might find Coinduction (and "codata") interesting. from wikipedia:
> Informally, rather than defining a function by pattern-matching on each of the inductive constructors, one defines each of the "destructors" or "observers" over the function result.
it's a deep rabbit hole, but i remember it being applied OOP-ish things – "destructors" would roughly correspond to exposed methods.
Edit: To answer your question, the set of all public methods could be seen as 1, but not the only, interpretation of the functional interface.