EnumMap and EnumSet

Key points

• EnumMap is a plain array indexed by ordinal(). No hashing, no collisions, no entry objects.
• It always iterates in enum declaration order, whatever order you inserted things in.
• EnumSet is a bit vector, usually one long, so contains is a single bit test.
• Factories read well: EnumSet.of, allOf, noneOf, range, complementOf.
• Both reject null, and both are dramatically smaller and faster than the hash-based equivalents.
• If the key type is an enum, these are the default choice rather than an optimisation.

Example

import java.util.*;

public class Main {
    enum Day { MON, TUE, WED, THU, FRI, SAT, SUN }

    public static void main(String[] args) {
        EnumMap<Day, String> plan = new EnumMap<>(Day.class);
        plan.put(Day.WED, "ship");
        plan.put(Day.MON, "plan");
        plan.put(Day.FRI, "review");
        System.out.println("EnumMap           : " + plan);
        System.out.println("inserted WED first, printed in declaration order");
        System.out.println("WED.ordinal()     : " + Day.WED.ordinal() + "   (that is its array slot)");

        EnumSet<Day> weekend = EnumSet.of(Day.SAT, Day.SUN);
        System.out.println("EnumSet.of        : " + weekend);
        System.out.println("complementOf      : " + EnumSet.complementOf(weekend));
        System.out.println("range(MON, WED)   : " + EnumSet.range(Day.MON, Day.WED));
        System.out.println("allOf size        : " + EnumSet.allOf(Day.class).size());
        System.out.println("noneOf            : " + EnumSet.noneOf(Day.class));
        System.out.println("contains(SAT)     : " + weekend.contains(Day.SAT) + "   (one bit test)");

        EnumSet<Day> busy = EnumSet.copyOf(plan.keySet());
        busy.removeAll(weekend);
        System.out.println("busy weekdays     : " + busy);
    }
}

Enum keys have their own collections, and they beat HashMap on both speed and memory.