Extending an enum via inheritance

Question

I know this rather goes against the idea of enums, but is it possible to extend enums in C#/Java? I mean \"extend\" in both the sense of adding new values to an enum, but a

Answer 1

Adding enums is a fairly common thing to do if you go back to the source code and edit, any other way (inheritance or reflection, if either is possible) is likely to come back and hit you when you get an upgrade of the library and they have introduced the same enum name or the same enum value - I have seen plenty of lowlevel code where the integer number matches to the binary encoding, where you would run into problems

Ideally code referencing enums should be written as equals only (or switches), and try to be future proof by not expecting the enum set to be const

Answer 2

If you mean extends in the Base class sense, then in Java... no.

But you can extend an enum value to have properties and methods if that's what you mean.

For example, the following uses a Bracket enum:

class Person {
    enum Bracket {
        Low(0, 12000),
        Middle(12000, 60000),
        Upper(60000, 100000);

        private final int low;
        private final int high;
        Brackets(int low, int high) {
            this.low = low;
            this.high = high;
        }

        public int getLow() {
            return low;
        }

        public int getHigh() {
            return high;
        }

        public boolean isWithin(int value) {
           return value >= low && value <= high;
        }

        public String toString() {
            return "Bracket " + low + " to " + high;
        }
    }

    private Bracket bracket;
    private String name;

    public Person(String name, Bracket bracket) {
        this.bracket = bracket;
        this.name = name;
    }

    public String toString() {
        return name + " in " + bracket;
    }        
}

Answer 3

You're going the wrong way: a subclass of an enum would have fewer entries.

In pseudocode, think:

enum Animal { Mosquito, Dog, Cat };
enum Mammal : Animal { Dog, Cat };  // (not valid C#)

Any method that can accept an Animal should be able to accept a Mammal, but not the other way around. Subclassing is for making something more specific, not more general. That's why "object" is the root of the class hierarchy. Likewise, if enums were inheritable, then a hypothetical root of the enum hierarchy would have every possible symbol.

But no, C#/Java don't allow sub-enums, AFAICT, though it would be really useful at times. It's probably because they chose to implement Enums as ints (like C) instead of interned symbols (like Lisp). (Above, what does (Animal)1 represent, and what does (Mammal)1 represent, and are they the same value?)

You could write your own enum-like class (with a different name) that provided this, though. With C# attributes it might even look kind of nice.

Answer 4

A temporary/local workaround, when you just want very local/one time usage:

enum Animals { Dog, Cat }
enum AnimalsExt { Dog = Animals.Dog, Cat= Animals.Cat,  MyOther}
// BUT CAST THEM when using:
var xyz = AnimalsExt.Cat;
MethodThatNeedsAnimal(   (Animals)xyz   );

See all answers at: Enum "Inheritance"

Answer 5

Hmmm - as far as I know, this can't be done - enumerations are written at design-time and are used as a convenience to the programmer.

I'm pretty sure that when the code is compiled, the equivalent values will be substituted for the names in your enumeration, thereby removing the concept of an enumeration and (therefore) the ability to extend it.

Answer 6

Saw a post regarding this for Java a while back, check out http://www.javaspecialists.eu/archive/Issue161.html .