Enums

Syntax

enum EnumName {\n  CONSTANT_1, CONSTANT_2\n}

Examples

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

public class EnumExample {
    public static void main(String[] args) {
        Day today = Day.WEDNESDAY;

        if (today == Day.SATURDAY || today == Day.SUNDAY) {
            System.out.println("Weekend!");
        } else {
            System.out.println("Weekday");
        }
    }
}

enum TrafficLight { RED, YELLOW, GREEN }

public static String getAction(TrafficLight light) {
    switch (light) {
        case RED:
            return "Stop";
        case YELLOW:
            return "Slow down";
        case GREEN:
            return "Go";
        default:
            return "Unknown";
    }
}

System.out.println(getAction(TrafficLight.GREEN));  // Go

public enum Planet {
    MERCURY(3.3e23, 2.4e6),
    EARTH(5.97e24, 6.4e6),
    MARS(6.4e23, 3.4e6);

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

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

    public double surfaceGravity() {
        final double G = 6.67300E-11;
        return G * mass / (radius * radius);
    }
}

System.out.printf("Earth gravity: %.2f%n", Planet.EARTH.surfaceGravity());

enum Size { SMALL, MEDIUM, LARGE }

for (Size s : Size.values()) {  // iterate over every constant
    System.out.println(s.name() + " = " + s.ordinal());
}
// SMALL = 0
// MEDIUM = 1
// LARGE = 2

Size chosen = Size.valueOf("MEDIUM");  // parse from a String
System.out.println(chosen);

Best practices

• Use enums instead of plain int or String constants whenever a variable should only take one of a fixed, known set of values - it gives you compile-time safety
• Take advantage of switch statements with enums for exhaustive, readable branching on the possible values
• Add fields and methods to an enum when each constant needs associated data, rather than maintaining a separate lookup table
• Use ALL_CAPS naming for enum constants, following the same convention as other constants in Java

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation