Why would an Enum implement an Interface?

Question

I just found out that Java allows enums to implement an interface. What would be a good use case for that?

Answer 1

There is a case I often use. I have a IdUtil class with static methods to work with objects implementing a very simple Identifiable interface:

public interface Identifiable {
    K getId();
}


public abstract class IdUtil {

    public static  & Identifiable, S> T get(Class type, S id) {
        for (T t : type.getEnumConstants()) {
            if (Util.equals(t.getId(), id)) {
                return t;
            }
        }
        return null;
    }

    public static  & Identifiable, S extends Comparable> List getLower(T en) {
        List list = new ArrayList<>();
        for (T t : en.getDeclaringClass().getEnumConstants()) {
             if (t.getId().compareTo(en.getId()) < 0) {
                 list.add(t);
            }
        }
        return list;
    }
}

If I create an Identifiable enum:

    public enum MyEnum implements Identifiable {

        FIRST(1), SECOND(2);

        private int id;

        private MyEnum(int id) {
            this.id = id;
        }

        public Integer getId() {
            return id;
        }

    }

Then I can get it by its id this way:

MyEnum e = IdUtil.get(MyEnum.class, 1);