Can enums be subclassed to add new elements?

Question

I want to take an existing enum and add more elements to it as follows:

enum A {a,b,c}  enum B extends A {d}  /*B is {a,b,c,d}*/ 

Is this possible in Java?

Answer 1

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.

Answer 2

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() { ... } }