How to nest an Enum inside the value of an Enum

Question

I\'d like to know if it is possible in Java to nest Enums.

Here what i\'d like to be able to do :

Have an enum Species made of CAT and DOG wich would grant m

Answer 1

Wouldn't you be better doing this with inheritance? Cat and Dog would be subclasses of Species. Persion would be a subclass of Cat and Pitbull would be a subclass of Dog.

What are trying to achieve by wanting to use Enums?

Answer 2

one way to nest enums:

package nested2;
import java.util.*;
interface HasChildren {
    Set<Enum<? extends HasChildren>> children();
    Class parent();
    Enum<? extends HasChildren> p();
}
enum Citrus implements HasChildren {
    lemon, lime, orange;
    Enum<? extends HasChildren> p;
    Class parent = getDeclaringClass();
    Class children;
    public Enum<? extends HasChildren> p() {
        return p;
    }
    public Class parent() {
        return parent;
    }
    public Set<Enum<? extends HasChildren>> children() {
        return children != null ? EnumSet.allOf(children) : null;
    }
    enum Orange implements HasChildren {
        navel, valencia, blood;
        Enum p;
        Class parent = getDeclaringClass();
        Class children;
        public Enum<? extends HasChildren> p() {
            return p;
        }
        public Class<Enum<? extends HasChildren>> parent() {
            return parent;
        }
        public Set<Enum<? extends HasChildren>> children() {
            return children != null ? EnumSet.allOf(children) : null;
        }
        enum Navel implements HasChildren {
            washinton, lateLane, caraCaraPink;
            Enum<? extends HasChildren> p;
            Class parent = getDeclaringClass();
            Class children;
            public Enum<? extends HasChildren> p() {
                return p;
            }
            public Class parent() {
                return parent;
            }
            public Set<Enum<? extends HasChildren>> children() {
                return null;
            }
        }
        static {
            navel.children = Navel.class;
            for (Navel n : Navel.values())
                n.p = orange;
            navel.p = navel;
        }
        enum Blood implements HasChildren {
            moro, taroco;
            Enum<? extends HasChildren> p;
            Class parent = getDeclaringClass();
            Class children;
            public Enum<? extends HasChildren> p() {
                return p;
            }
            public Class parent() {
                return parent;
            }
            public Set<Enum<? extends HasChildren>> children() {
                return null;
            }
        }
        static {
            blood.children = Blood.class;
            for (Blood b : Blood.values())
                b.p = blood;
            blood.p = orange;
        }
    }
    static {
        for (Orange o : Orange.values())
            o.p = orange;
        orange.children = Orange.class;
    }
}
public class EnumTree {
    static void visit(Class<?> clazz) {
        Object[] enumConstants = clazz.getEnumConstants();
        if (enumConstants[0] instanceof HasChildren) for (Object o : enumConstants)
            visit((HasChildren) o, clazz.getName());
    }
    static void visit(HasChildren hasChildren, String prefix) {
        if (hasChildren instanceof Enum) {
            System.out.println(prefix + ' ' + hasChildren + " (" + hasChildren.parent() + ")" + " (" + hasChildren.p()
                    + " " + ((Enum) hasChildren).getDeclaringClass());
            if (hasChildren.children() != null) for (Object o : hasChildren.children())
                visit((HasChildren) o, prefix + ' ' + hasChildren);
        } else
            System.out.println("other " + hasChildren.getClass());
    }
    static <E extends Enum<E> & HasChildren> Set<E> foo() {
        return null;
    }
    public static void main(String[] args) {
        //looks like p is the right parent
        System.out.println(Citrus.Orange.Navel.washinton);
        System.out.println(Citrus.Orange.Navel.washinton.getClass().getDeclaredClasses());
        System.out.println(Citrus.Orange.Navel.washinton.getClass().getEnclosingClass());
        System.out.println(Citrus.Orange.Navel.washinton.getClass().getDeclaringClass());
        visit(Citrus.lemon, "");
        System.out.println("----------------------");
        visit(Citrus.orange, "");
        System.out.println("----------------------");
        visit(Citrus.class);
        System.out.println("----------------------");
    }
}

Answer 3

public enum A {

    a1, 
    a2;

    public enum B {
        b1, 
        b2
    }
}

and so on...