Enums: Types With a Closed Set

An enum is a type with a fixed, named set of values — compile-time protection against impossible states:

enum OrderStatus { PENDING, PAID, SHIPPED, CANCELLED }

OrderStatus s = OrderStatus.PAID;

switch over an enum is exhaustive-checked: add a status later and every switch without it fails to compile — the compiler becomes your checklist.

boolean isFinal = switch (s) {
    case PAID, SHIPPED -> false;      // multiple labels share an arm
    case CANCELLED -> true;
    case PENDING -> false;
};

Enums are full classes: they can carry fields, constructors, and methods — each constant runs the constructor once:

enum Planet {
    MERCURY(0.39), EARTH(1.0), MARS(1.52);   // semicolon after the list!

    private final double auFromSun;
    Planet(double au) { this.auFromSun = au; }
    public double au() { return auFromSun; }
}

Every enum automatically gives you values() (all constants in order), valueOf("PAID") (parse; throws on unknown), name(), and ordinal() (declaration position — avoid using it for logic; reordering constants should never change behavior). Comparing enum constants uses == safely: they are singletons, and this is the one place == on objects is idiomatic.

Use an enum whenever a variable's legal values are a closed list — status, season, difficulty, compass direction. It replaces int codes (what is 3?) and stringly-typed checks (typo-safe at compile time).

Next: records — data with no ceremony.