Item 33: Use EnumMap instead of ordinal indexing

1.  There is a very fast Map implementation designed for use with enum keys, known as java.util.EnumMap. EnumMap is comparable in speed to an ordinal-indexed array because it uses such an array internally. The EnumMap constructor takes the Class object of the key type: this is a bounded type token, which provides runtime generic type information.

 

2.  You may see an array of arrays indexed by ordinals used to represent a mapping from two enum values:

// Using ordinal() to index array of arrays - DON'T DO THIS!
public enum Phase { SOLID, LIQUID, GAS;
    public enum Transition {
        MELT, FREEZE, BOIL, CONDENSE, SUBLIME, DEPOSIT;
        // Rows indexed by src-ordinal, cols by dst-ordinal
        private static final Transition[][] TRANSITIONS = {
            { null, MELT, SUBLIME },
            { FREEZE, null, BOIL },
            { DEPOSIT, CONDENSE, null }
        };
        // Returns the phase transition from one phase to another
        public static Transition from(Phase src, Phase dst) {
            return TRANSITIONS[src.ordinal()][dst.ordinal()];
        }
    }
}

Again, you can do much better with EnumMap:

// Using a nested EnumMap to associate data with enum pairs
public enum Phase {
    SOLID, LIQUID, GAS;
    public enum Transition {
        MELT(SOLID, LIQUID), FREEZE(LIQUID, SOLID),
        BOIL(LIQUID, GAS), CONDENSE(GAS, LIQUID),
        SUBLIME(SOLID, GAS), DEPOSIT(GAS, SOLID);
        final Phase src;
        final Phase dst;
        Transition(Phase src, Phase dst) {
            this.src = src;
            this.dst = dst;
        } 
// Initialize the phase transition map
        private static final Map<Phase, Map<Phase,Transition>> m = new EnumMap<Phase, Map<Phase,Transition>>(Phase.class);
        static {
            for (Phase p : Phase.values())
                m.put(p,new EnumMap<Phase,Transition>(Phase.class));
            for (Transition trans : Transition.values())
                m.get(trans.src).put(trans.dst, trans);
        }
        public static Transition from(Phase src, Phase dst) {
            return m.get(src).get(dst);
        }
    }
}