What capabilities do Java enums have beyond simple named constants?

Java enums are full-fledged classes that happen to have a fixed number of instances. This makes them far more powerful than C-style enums and eliminates entire categories of bugs that arise from using int constants.

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

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

    Planet(double mass, double radius) { // constructor
        this.mass   = mass;
        this.radius = radius;
    }

    static final double G = 6.67300E-11;

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

double earthWeight = 75.0;
double mass = earthWeight / Planet.EARTH.surfaceGravity();
for (Planet p : Planet.values()) {
    System.out.printf("Weight on %s = %6.2f%n", p, p.surfaceWeight(mass));
}

// Abstract methods — each constant provides its own implementation
enum Operation {
    PLUS("+ ") { @Override double apply(double x, double y) { return x + y; } },
    MINUS("-") { @Override double apply(double x, double y) { return x - y; } };

    private final String symbol;
    Operation(String symbol) { this.symbol = symbol; }
    abstract double apply(double x, double y);
    @Override public String toString() { return symbol; }
}

// Enum with switch (Java 21 — pattern matching works on enums)
int priority = switch (severity) {
    case CRITICAL -> 1;
    case HIGH     -> 2;
    case MEDIUM   -> 3;
    case LOW      -> 4;
};

// EnumSet and EnumMap — fast, memory-efficient specialisations
EnumSet workdays = EnumSet.range(MONDAY, FRIDAY);
EnumMap weights = new EnumMap<>(Planet.class);