Is modifying the key set of a map okay, or is it an abuse of a map in Java with undefined behavior?

I noticed that we can manipulate the set of keys in a map in Java.

For example, here we copy a map, then effectively remove some of the entries by modifying the set of keys with a call to Set:retainAll.

Map< DayOfWeek , String > dailyAssignment = new EnumMap<>( DayOfWeek.class ) ;
dailyAssignment.put( DayOfWeek.MONDAY, "Alice" ) ;
dailyAssignment.put( DayOfWeek.TUESDAY, "Bob" ) ;
dailyAssignment.put( DayOfWeek.SATURDAY, "Bob" ) ;
dailyAssignment.put( DayOfWeek.SUNDAY, "Carol" ) ;

List< DayOfWeek > weekend = List.of( DayOfWeek.SATURDAY , DayOfWeek.SUNDAY ) ;

Map< DayOfWeek , String > weekendAssignment = new EnumMap<>( dailyAssignment ) ;
weekendAssignment.keySet().retainAll( weekend ) ;

Dump to console.

System.out.println( "dailyAssignment.toString(): " + dailyAssignment ) ;
System.out.println( "weekendAssignment.toString(): " + weekendAssignment ) ;

This seems to work, as seen in Java 12 when run live at IdeOne.com.

>dailyAssignment.toString(): {MONDAY=Alice, TUESDAY=Bob, SATURDAY=Bob, SUNDAY=Carol}

>weekendAssignment.toString(): {SATURDAY=Bob, SUNDAY=Carol}

➥ While it seems to work, is this a proper use of a key set in a map? The Javadoc for EnumMap::keySet method does not address mutability of the returned key set.

Yes, mutating the Set of keys in a Map is a supported feature.

Look the Javadoc of Map rather than its implementation EnumMap. The Map::keySet documentation specifically states that altering the set of keys is an acceptable way to mutate the map.

To quote Map::keySet Javadoc: …The set supports element removal…

>Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.