Enum (enumeration) is a custom data type (a data type that can define its type). We can determine what it contains. When defining it, we give a set of values of this type in braces (similarly, e.g., an int has a set of allowed values - a range).

public enum Day {
    Mo, Tu, We, Th, Fr, Sa, Su
}                                                                         
                                    

public class Main {
    public static void main(String[] args) {
        System.out.println(Day.Tu); // just like typing int(5)

        Day number_of_a_day_of_the_week = Day.Mo;
        System.out.println(getDay(number_of_a_day_of_the_week));
    }

    private static String getDay(Day x) {
        return switch(x) {
            case Mo -> "Monday";
            case Tu -> "Tuesday";
            case We -> "Wednesday";
            case Th -> "Thursday";
            case Fr -> "Friday";
            case Sa -> "Saturday";
            case Su -> "Sunday";
        };
    }
}