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Enums

An enum type is a special data type that enables for a variable to be a set of predefined constants. The variable must be equal to one of the values that have been predefined for it. Common examples include compass directions (values of NORTH, SOUTH, EAST, and WEST) and the days of the week.

Because they are constants, the names of an enum type's fields are in uppercase letters.

Defining an enum type

In the Java programming language, you define an enum type by using the enum keyword. For example, you would specify a days-of-the-week enum type as:

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

You should use enum types any time you need to represent a fixed set of constants. That includes natural enum types such as the planets in our solar system and data sets where you know all possible values at compile time - for example, the choices on a menu, command line flags, and so on.

Using an enum type

Here is some code that shows you how to use the DayOfWeek enum defined above:

public class Shop {
    public String getOpeningHours(DayOfWeek dayOfWeek) {
        switch (dayOfWeek) {
            case MONDAY:
            case TUESDAY:
            case WEDNESDAY:
            case THURSDAY:
            case FRIDAY:
                return "9am - 5pm";
            case SATURDAY:
                return "10am - 4pm"
            case SUNDAY:
                return "Closed.";
        }
    }
}

var shop = new Shop();
shop.getOpeningHours(DayOfWeek.WEDNESDAY);
// => "9am - 5pm"

Adding methods and fields

Java programming language enum types are much more powerful than their counterparts in other languages. The enum declaration defines a class (called an enum type). The enum class body can include methods and other fields:

public enum Rating {
    GREAT(5),
    GOOD(4),
    OK(3),
    BAD(2),
    TERRIBLE(1);

    private final int numberOfStars;

    Rating(int numberOfStars) {
        this.numberOfStars = numberOfStars;
    }

    public int getNumberOfStars() {
        return this.numberOfStars;
    }
}

Calling the getNumberOfStars method on a member of the Rating enum type:

Rating.GOOD.getNumberOfStars();
// => 4