What is an EnumSet?

Because an EnumSet only ever stores values from one enum, each possible value can be represented by a single bit: bit set means the value is present, bit unset means it isn't. This bit-vector representation is what makes operations like add, remove, and contains so fast, and why the memory footprint is tiny regardless of how many values you add.

EnumSet doesn't have a public constructor — you create one through static factory methods:

• EnumSet.allOf(Class) — puts all the enum's values in the set.
• EnumSet.noneOf(Class) — gives you an empty set of the right enum type, to which you can add values one by one.
• EnumSet.complementOf(otherSet) — takes another EnumSet and returns the complement: every value of the enum that isn't in otherSet.

java
enum Day { MON, TUE, WED, THU, FRI, SAT, SUN }

EnumSet<Day> allDays = EnumSet.allOf(Day.class);
EnumSet<Day> weekend = EnumSet.of(Day.SAT, Day.SUN);
EnumSet<Day> weekdays = EnumSet.complementOf(weekend);

EnumSet<Day> empty = EnumSet.noneOf(Day.class);
empty.add(Day.MON);

An EnumSet is modifiable, so you can add and remove elements after creating it. But you cannot add null to it — attempting to do so throws a NullPointerException. This isn't a real limitation in practice: an EnumSet can only ever contain values from a specific enum, and enum constants can never be null, so there's nothing meaningful a null element would represent.

Reach for EnumSet whenever you need a set of enum values: it's faster and more compact than HashSet, and the factory methods (allOf, noneOf, complementOf) cover the common construction patterns without extra boilerplate.