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

I suggest you take the other way around approach.

Instead of extending the existing enumeration, create a larger one and create a subset of it. For example, if you had an enumeration called PET and you wanted to extend it to ANIMAL you should do this instead:

public enum ANIMAL {
    WOLF,CAT, DOG
} 
EnumSet<ANIMAL> pets = EnumSet.of(ANIMAL.CAT, ANIMAL.DOG);

Be careful, pets are not immutable collections, you might want to use Guava or Java9 for more safety.