 Solution

Throw an informative exception, such as NoSuchElementException, and leave the set unchanged. This makes a violated nonempty precondition visible at the point of misuse. Returning a distinguished integer would be ambiguous because every integer is a legitimate set member.

A different object interface could return an optional integer, representing failure explicitly, but that changes the declared return type. For the supplied integer-returning operation, a checked failure is useful and consistent with the stated precondition.