enum

// Basic enum definition.
enum Season {
    SPRING, SUMMER, AUTUMN, WINTER
}

// Enum definition with fields and methods.
enum Planet {
    MERCURY(3.303e+23, 2.4397e6),
    VENUS  (4.869e+24, 6.0518e6);

    private final double mass;
    private final double radius;

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

    double surfaceGravity() { return 6.67430e-11 * mass / (radius * radius); }
}

// Common methods.
Season[] all  = Season.values();          // Returns all enum constants as an array.
Season s      = Season.valueOf("SPRING"); // Returns the enum constant with the given name.
int idx       = Season.SUMMER.ordinal();  // Returns the zero-based declaration index.
String name   = Season.SUMMER.name();     // Returns the declaration name as a string.

// Basic enum usage.
enum Day { MON, TUE, WED, THU, FRI, SAT, SUN }

Day today = Day.WED;
System.out.println(today);           // Prints "WED".
System.out.println(today.ordinal()); // Prints "2".
System.out.println(today.name());    // Prints "WED".

// Using an enum in a switch statement.
switch (today) {
    case SAT: case SUN:
        System.out.println("Weekend.");
        break;
    default:
        System.out.println("Weekday.");
}

// Iterating with values().
for (Day d : Day.values()) {
    System.out.print(d + " "); // Prints "MON TUE WED THU FRI SAT SUN ".
}

// Using an enum with fields and methods.
enum Status {
    PENDING("Pending"),
    ACTIVE("Active"),
    CLOSED("Closed");

    private final String label;
    Status(String label) { this.label = label; }
    String getLabel() { return label; }
}

System.out.println(Status.ACTIVE.getLabel()); // Prints "Active".

// Getting an enum constant from a string with valueOf().
Status s = Status.valueOf("CLOSED");
System.out.println(s.getLabel()); // Prints "Closed".

Enums are more type-safe than plain constants, preventing invalid assignments at compile time. You can add fields and methods to attach data and behavior to each enum constant.

Avoid using ordinal() values for database storage or other persistence — adding or reordering constants will break existing data. Use name() or a dedicated numeric field instead.

For immutable data classes, see 'record'. For details on access modifiers, see 'public / private / protected / static / final'.