Java Enums

An enum is a special class for a fixed set of constants, like days of the week, order status or sizes. It is safer than plain numbers or strings, because the compiler checks the values.

Create and use an enum

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);
        System.out.println(today.name());
        System.out.println(today.ordinal());           // position, starts at 0
        System.out.println(Day.valueOf("FRIDAY"));

        for (Day d : Day.values()) {                   // all the constants
            System.out.print(d + " ");
        }
        System.out.println();
    }
}

Enum in switch

enum Level { LOW, MEDIUM, HIGH }

public class Sw {
    public static void main(String[] args) {
        Level level = Level.HIGH;
        String message = switch (level) {
            case LOW -> "All fine";
            case MEDIUM -> "Be careful";
            case HIGH -> "Danger";
        };
        System.out.println(message);
    }
}

When you use all the constants, a switch expression needs no default. If you add a new constant later, the compiler shows an error in every switch you must update.

Enum with fields, a constructor and methods

enum Size {
    SMALL(10), MEDIUM(20), LARGE(30);

    private final int price;

    Size(int price) {                  // constructor is always private
        this.price = price;
    }

    int getPrice() {
        return price;
    }
}

public class Fields {
    public static void main(String[] args) {
        for (Size s : Size.values()) {
            System.out.println(s + " costs " + s.getPrice());
        }
        System.out.println(Size.LARGE.compareTo(Size.SMALL));
    }
}

Enum with an abstract method

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

    abstract int apply(int a, int b);
}

public class Abs {
    public static void main(String[] args) {
        System.out.println(Operation.ADD.apply(3, 4));
        System.out.println(Operation.MULTIPLY.apply(3, 4));
    }
}

EnumMap and EnumSet

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

enum Color { RED, GREEN, BLUE }

public class Maps {
    public static void main(String[] args) {
        EnumMap<Color, Integer> stock = new EnumMap<>(Color.class);
        stock.put(Color.BLUE, 5);
        stock.put(Color.RED, 12);
        System.out.println(stock);                    // in the order of the enum

        EnumSet<Color> warm = EnumSet.of(Color.RED);
        System.out.println(warm);
        System.out.println(EnumSet.complementOf(warm));
    }
}

Facts

• Enums are classes. They can implement interfaces, but can not extend a class.
• The constants are objects, created once. You can compare them with ==.
• valueOf() throws an exception when the name does not exist.

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