Enums

An enum is a fixed, named set of constant values — safer than using plain Strings or numbers to represent a closed set of options, since the compiler rejects anything not in the list.

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

Day today = Day.WEDNESDAY;

String label = switch (today) {
    case SATURDAY, SUNDAY -> "Weekend";
    default -> "Weekday";
};
System.out.println(label); // Weekday

An enum can also carry fields and methods, making it more powerful than a plain named constant:

enum Planet {
    MERCURY(3.3e23), EARTH(5.9e24);

    final double mass;

    Planet(double mass) {
        this.mass = mass;
    }
}

Common mistakes

• Using a String ("MONDAY") to represent a fixed set of options instead of an enum — a typo like "MONDDAY" compiles fine and fails silently at runtime; an invalid enum constant simply won't compile.