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.

Answer 2

Alternatively, one can go somehow along the lines of the following idea:

public enum SomeEnum {   A, B, C;    public Optional<SomeEnum> next() {     switch (this) {       case A: return Optional.of(B);       case B: return Optional.of(C);       // any other case can NOT be mapped!       default: return Optional.empty();   } } 

Notes:

1. In contrast to the other answer, this way does some implicit mapping; instead of relying on ordinal(). Of course that means more code; but it also forces the author to consider what it means to add new constants or remove existing ones. When relying on ordinal, your implicit assumption is that the order is based on the order used for the enum constant declaration. So when somebody comes back 6 months later and has to add a new constant, he has to understand that the new constant Y needs X, Y, Z ... instead of just appending X, Z, Y!
2. There might be situations where it doesn't make any sense for the "last" enum constant to have the "first" as successor. Think of T-Shirt sizes for examples. XXL.next() is for sure not XS. For such situations, using Optional is the more appropriate answer.