Enums

An enum (enumeration) is a type with a fixed set of named values: the days of the week, order statuses, card suits. Using an enum instead of strings or numbers makes invalid values impossible and lets the compiler check your switches. Java enums are full classes, so they can also carry data and behaviour.

A basic enum

JAVA

enum Status { PENDING, SHIPPED, DELIVERED, CANCELLED }

public class Main {
    public static void main(String[] args) {
        Status s = Status.SHIPPED;
        System.out.println(s);
        System.out.println(s == Status.SHIPPED);
        System.out.println(s.name() + " is number " + s.ordinal());
        System.out.println(Status.valueOf("DELIVERED"));
        for (Status each : Status.values()) {
            System.out.print(each + " ");
        }
        System.out.println();
    }
}

Output

SHIPPED
true
SHIPPED is number 1
DELIVERED
PENDING SHIPPED DELIVERED CANCELLED

Enum constants are conventionally in capitals. Compare them with == (each constant is a single object). values() lists them all, and valueOf converts text to a constant — throwing IllegalArgumentException for unknown names.

Why not strings?

With String status, nothing stops "shiped" or "Delivered " slipping in. With Status status, only the four valid values exist — typos become compile errors, and IDEs can autocomplete.

Enums in switch

JAVA

enum Status { PENDING, SHIPPED, DELIVERED, CANCELLED }

public class Main {
    static String message(Status s) {
        return switch (s) {
            case PENDING -> "We're preparing your order.";
            case SHIPPED -> "Your order is on its way.";
            case DELIVERED -> "Delivered. Enjoy!";
            case CANCELLED -> "This order was cancelled.";
        };
    }

    public static void main(String[] args) {
        System.out.println(message(Status.PENDING));
    }
}

Output

We're preparing your order.

A switch expression over an enum needs no default when every constant is covered. If you later add a constant, the compiler flags every switch that doesn't handle it.

Fields, constructors and methods

JAVA

enum Size {
    SMALL("S", 250), MEDIUM("M", 350), LARGE("L", 500);

    private final String code;
    private final int millilitres;

    Size(String code, int millilitres) {
        this.code = code;
        this.millilitres = millilitres;
    }

    String code() { return code; }

    double priceFrom(double basePerLitre) {
        return basePerLitre * millilitres / 1000;
    }
}

public class Main {
    public static void main(String[] args) {
        for (Size s : Size.values()) {
            System.out.println("%s (%s): %.2f".formatted(s, s.code(), s.priceFrom(8.0)));
        }
    }
}

Output

SMALL (S): 2.00
MEDIUM (M): 2.80
LARGE (L): 4.00

Each constant can pass values to a (private) constructor, and the enum can have fields and methods like any class.

Behaviour per constant

JAVA

enum Operation {
    ADD("+") { int apply(int a, int b) { return a + b; } },
    SUBTRACT("-") { int apply(int a, int b) { return a - b; } },
    MULTIPLY("*") { int apply(int a, int b) { return a * b; } };

    final String symbol;
    Operation(String symbol) { this.symbol = symbol; }
    abstract int apply(int a, int b);
}

public class Main {
    public static void main(String[] args) {
        for (Operation op : Operation.values()) {
            System.out.println("6 " + op.symbol + " 3 = " + op.apply(6, 3));
        }
    }
}

Output

6 + 3 = 9
6 - 3 = 3
6 * 3 = 18

EnumMap and EnumSet

JAVA

import java.util.EnumMap;
import java.util.EnumSet;
import java.util.Map;

enum Day { MON, TUE, WED, THU, FRI, SAT, SUN }

public class Main {
    public static void main(String[] args) {
        EnumSet<Day> weekend = EnumSet.of(Day.SAT, Day.SUN);
        EnumSet<Day> workdays = EnumSet.complementOf(weekend);
        System.out.println(workdays);

        Map<Day, Integer> hours = new EnumMap<>(Day.class);
        hours.put(Day.MON, 8);
        hours.put(Day.FRI, 6);
        System.out.println(hours);
    }
}

Output

[MON, TUE, WED, THU, FRI]
{MON=8, FRI=6}

Specialised collections for enum keys are compact and fast, and keep the declaration order.

Try it yourself

Model a traffic light that knows its next state and how long it lasts.

enum Light {
    RED(30), GREEN(25), YELLOW(5);

    final int seconds;
    Light(int seconds) { this.seconds = seconds; }

    Light next() {
        return switch (this) {
            case RED -> GREEN;
            case GREEN -> YELLOW;
            case YELLOW -> RED;
        };
    }
}

public class Main {
    public static void main(String[] args) {
        Light light = Light.RED;
        int elapsed = 0;
        for (int i = 0; i < 4; i++) {
            System.out.println("t=" + elapsed + "s " + light);
            elapsed += light.seconds;
            light = light.next();
        }
    }
}

Edit the code, copy it into Main.java and run java Main.java (Java 25+). How to set up Java

Output

t=0s RED
t=30s GREEN
t=55s YELLOW
t=60s RED

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