The most obvious are lambdas (anything in this package, basically): https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html https://d

The most obvious are lambdas (anything in this package, basically): https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html https://docs.oracle.com/javase/8/docs/api/java/util/function... Iterable/Iterator in general might also be problematic, since they might yield an infinite sequence of values (trying to equate them would loop indefinitely... obviously you can write a concrete class that is guaranteed to iterate a finite number of times, but this means that you should then implement an Equality interface only for them) Also, anything that is thus closely related to Streams, references to stuff in the outside world or things that aren't easily Serializable (and thus are probably not Value Objects), like File Handles/Sockets (you might compare them by reference, obviously) Also, Float and Doubles should arguably not implement an Equality interface. Rust for example does not have them implement Eq, but rather PartialEq due to NaN. Any Product (composite) or Sum type containing any such value might also not make sense to implement value equality PS: I think that Tikhon forgot to link the document he mentioned, this should be it: http://abel.math.harvard.edu/~mazur/preprints/when_is_one.pdf http://abel.math.harvard.edu/~mazur/preprints/when_is_one.pd...