Enums

An enum (enum) defines a fixed, named set of constants — for example, the days of the week, or the suits in a deck of cards. Java enums are type-safe: a variable of an enum type can only ever hold one of that enum's declared constants, and nothing else.

A basic enum

Java

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

public class Main {
    public static void main(String[] args) {
        Day today = Day.WEDNESDAY;
        System.out.println(today); // WEDNESDAY
    }
}

Warning

This is very different from the old C-style approach of using plain `int` constants (`int MONDAY = 0, TUESDAY = 1, ...`). With ints, nothing stops you from passing `42` or the wrong int constant where a day was expected — the compiler can't tell it's wrong. With a Java `enum`, the compiler enforces that only an actual `Day` constant can ever be assigned; invalid values simply cannot exist.

Enums are real classes, not just labeled ints

This is one of Java's most distinctive and powerful features compared to many other languages: an enum in Java is a genuine class. It can have fields, constructors, and methods, just like any other class — each constant is really a specific object of that class.

Planet.java — an enum with fields and a method

Java

enum Planet {
    MERCURY(3.303e+23, 2.4397e6),
    VENUS(4.869e+24, 6.0518e6),
    EARTH(5.976e+24, 6.37814e6);

    private final double mass;   // kilograms
    private final double radius; // meters

    // Every constant above calls this constructor with its own values
    Planet(double mass, double radius) {
        this.mass = mass;
        this.radius = radius;
    }

    static final double G = 6.67300E-11;

    double surfaceGravity() {
        return G * mass / (radius * radius);
    }
}

public class Main {
    public static void main(String[] args) {
        for (Planet p : Planet.values()) {
            System.out.printf("Gravity on %s: %.2f m/s^2%n", p, p.surfaceGravity());
        }
    }
}

Each constant — MERCURY, VENUS, EARTH — is a distinct object carrying its own mass and radius, constructed exactly once by the JVM when the Planet class is loaded, and surfaceGravity() is a completely ordinary instance method computed from that constant's own fields.

Using enums in a switch

switch on an enum

Java

Day today = Day.SATURDAY;

switch (today) {
    case SATURDAY:
    case SUNDAY:
        System.out.println("It's the weekend!");
        break;
    default:
        System.out.println("Back to work.");
}

Note that inside a switch on an enum, you write the bare constant name (SATURDAY), not Day.SATURDAY — the compiler already knows the type being switched on.

values() and valueOf()

Every enum automatically gets two useful static methods for free, generated by the compiler:

values() returns an array of every constant, in declaration order — used above to loop over all planets.
valueOf(String name) converts a matching string back into the enum constant, throwing IllegalArgumentException if no constant has that exact name.

values() and valueOf()

Java

Day[] allDays = Day.values();
System.out.println(allDays.length); // 7

Day parsed = Day.valueOf("FRIDAY");
System.out.println(parsed); // FRIDAY

Day bad = Day.valueOf("Friday"); // throws IllegalArgumentException — case-sensitive