Can enums be subclassed to add new elements?
No, you can't do this in Java. Aside from anything else, d
would then presumably be an instance of A
(given the normal idea of "extends"), but users who only knew about A
wouldn't know about it - which defeats the point of an enum being a well-known set of values.
If you could tell us more about how you want to use this, we could potentially suggest alternative solutions.
Enums represent a complete enumeration of possible values. So the (unhelpful) answer is no.
As an example of a real problem take weekdays, weekend days and, the union, days of week. We could define all days within days-of-week but then we would not be able to represent properties special to either weekdays and weekend-days.
What we could do, is have three enum types with a mapping between weekdays/weekend-days and days-of-week.
public enum Weekday {
MON, TUE, WED, THU, FRI;
public DayOfWeek toDayOfWeek() { ... }
}
public enum WeekendDay {
SAT, SUN;
public DayOfWeek toDayOfWeek() { ... }
}
public enum DayOfWeek {
MON, TUE, WED, THU, FRI, SAT, SUN;
}
Alternatively, we could have an open-ended interface for day-of-week:
interface Day {
...
}
public enum Weekday implements Day {
MON, TUE, WED, THU, FRI;
}
public enum WeekendDay implements Day {
SAT, SUN;
}
Or we could combine the two approaches:
interface Day {
...
}
public enum Weekday implements Day {
MON, TUE, WED, THU, FRI;
public DayOfWeek toDayOfWeek() { ... }
}
public enum WeekendDay implements Day {
SAT, SUN;
public DayOfWeek toDayOfWeek() { ... }
}
public enum DayOfWeek {
MON, TUE, WED, THU, FRI, SAT, SUN;
public Day toDay() { ... }
}