Saturday, 8 June 2013

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

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

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.

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