Why did the developers prefer to use java.util.EnumSet instead of HashSet or TreeSet for enums?

Using EnumSet for enums is preferable to HashSet or TreeSet for several reasons:

1. Performance: EnumSet is internally implemented as a bit vector or array, making it very efficient in terms of memory and performance for add, remove, and contains operations. It is significantly faster than general Set implementations based on hash tables or trees.
2. Type safety: EnumSet guarantees that it can only contain elements of a specific enum type upon creation, preventing the addition of elements of other types.
3. Compactness: Thanks to the bit vector, EnumSet requires very little memory, especially for enums with a small number of constants.
4. Immutability (partially): Although EnumSet itself is not immutable, it can be wrapped in Collections.unmodifiableSet() to create an unmodifiable version that still benefits from EnumSet optimizations.
5. JVM support: The JVM has special support for optimizing enum and EnumSet operations.

Example usage:

// Enum example
enum Day {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY
}

// Creating an EnumSet
import java.util.EnumSet;

// Creating an empty EnumSet for Day
EnumSet<Day> weekend = EnumSet.noneOf(Day.class);
weekend.add(Day.SATURDAY);
weekend.add(Day.SUNDAY);

// Creating an EnumSet with a range
EnumSet<Day> weekdays = EnumSet.range(Day.MONDAY, Day.FRIDAY);

// Checking for an element
boolean isMondayWeekday = weekdays.contains(Day.MONDAY); // true

// Iterating over elements
for (Day day : weekend) {
    System.out.println(day); // Will print SATURDAY, then SUNDAY
}

Performance comparison (conceptual):

Operation	EnumSet	HashSet	TreeSet
add, remove	O(1) (for most)	Average O(1)	O(log n)
contains	O(1)	Average O(1)	O(log n)
Memory usage	Very low	Low/Moderate	Moderate/High