java - What's the best way to implement `next` and `previous` on an enum type?

Question

Suppose I have an enum:

enum E {
    A, B, C;
}

As shown in this answer by lucasmo, enum values are stored in a static array in the order that they are initialized, and you can later retrieve (a clone of) this array with E.values().

Now suppose I want to implement E#getNext and E#getPrevious such that all of the following expressions evaluate to true:

E.A.getNext() == E.B
E.B.getNext() == E.C
E.C.getNext() == E.A

E.A.getPrevious() == E.C
E.B.getPrevious() == E.A
E.C.getPrevious() == E.B

My current implementation for getNext is the following:

public E getNext() {
    E[] e = E.values();
    int i = 0;
    for (; e[i] != this; i++)
        ;
    i++;
    i %= e.length;
    return e[i];
}

and a similar method for getPrevious.

However, this code seems cumbersome at best (e.g., "empty" for loop, arguable abuse of a counter variable, and potentially erroneous at worst (thinking reflection, possibly).

What would be the best way to implement getNext and getPrevious methods for enum types in Java 7?

---

NOTE: I do not intend this question to be subjective. My request for the "best" implementation is shorthand for asking for the implementation that is the fastest, cleanest, and most maintainable.

Answer 1

Try this:

public static enum A { 
    X, Y, Z;
    private static A[] vals = values();
    public A next()
    {
        return vals[(this.ordinal()+1) % vals.length];
    }

Implementation of previous() is left as an exercise, but recall that in Java, the modulo a % b can return a negative number.

EDIT: As suggested, make a private static copy of the values() array to avoid array copying each time next() or previous() is called.