An enum type is a type whose fields consist of a fixed set of constants. 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.
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: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.public enum Day { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY }Here is some code that shows you how to use the
Day
enum defined above:The output is:public class EnumTest { Day day; public EnumTest(Day day) { this.day = day; } public void tellItLikeItIs() { switch (day) { case MONDAY: System.out.println("Mondays are bad."); break; case FRIDAY: System.out.println("Fridays are better."); break; case SATURDAY: case SUNDAY: System.out.println("Weekends are best."); break; default: System.out.println("Midweek days are so-so."); break; } } public static void main(String[] args) { EnumTest firstDay = new EnumTest(Day.MONDAY); firstDay.tellItLikeItIs(); EnumTest thirdDay = new EnumTest(Day.WEDNESDAY); thirdDay.tellItLikeItIs(); EnumTest fifthDay = new EnumTest(Day.FRIDAY); fifthDay.tellItLikeItIs(); EnumTest sixthDay = new EnumTest(Day.SATURDAY); sixthDay.tellItLikeItIs(); EnumTest seventhDay = new EnumTest(Day.SUNDAY); seventhDay.tellItLikeItIs(); } }Mondays are bad. Midweek days are so-so. Fridays are better. Weekends are best. Weekends are best.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. The compiler automatically adds some special methods when it creates an enum. For example, they have a staticvalues
method that returns an array containing all of the values of the enum in the order they are declared. This method is commonly used in combination with the for-each construct to iterate over the values of an enum type. For example, this code from thePlanet
class example below iterates over all the planets in the solar system.for (Planet p : Planet.values()) { System.out.printf("Your weight on %s is %f%n", p, p.surfaceWeight(mass)); }In the following example,
Note: All enums implicitly extendjava.lang.Enum
. Since Java does not support multiple inheritance, an enum cannot extend anything else.Planet
is an enum type that represents the planets in the solar system. They are defined with constant mass and radius properties.Each enum constant is declared with values for the mass and radius parameters. These values are passed to the constructor when the constant is created. Java requires that the constants be defined first, prior to any fields or methods. Also, when there are fields and methods, the list of enum constants must end with a semicolon.
In addition to its properties and constructor,
Note: The constructor for an enum type must be package-private or private access. It automatically creates the constants that are defined at the beginning of the enum body. You cannot invoke an enum constructor yourself.Planet
has methods that allow you to retrieve the surface gravity and weight of an object on each planet. Here is a sample program that takes your weight on earth (in any unit) and calculates and prints your weight on all of the planets (in the same unit):If you runpublic enum Planet { MERCURY (3.303e+23, 2.4397e6), VENUS (4.869e+24, 6.0518e6), EARTH (5.976e+24, 6.37814e6), MARS (6.421e+23, 3.3972e6), JUPITER (1.9e+27, 7.1492e7), SATURN (5.688e+26, 6.0268e7), URANUS (8.686e+25, 2.5559e7), NEPTUNE (1.024e+26, 2.4746e7), PLUTO (1.27e+22, 1.137e6); private final double mass; // in kilograms private final double radius; // in meters Planet(double mass, double radius) { this.mass = mass; this.radius = radius; } private double mass() { return mass; } private double radius() { return radius; } // universal gravitational constant (m3 kg-1 s-2) public static final double G = 6.67300E-11; double surfaceGravity() { return G * mass / (radius * radius); } double surfaceWeight(double otherMass) { return otherMass * surfaceGravity(); } public static void main(String[] args) { double earthWeight = Double.parseDouble(args[0]); double mass = earthWeight/EARTH.surfaceGravity(); for (Planet p : Planet.values()) System.out.printf("Your weight on %s is %f%n", p, p.surfaceWeight(mass)); } }Planet.class
from the command line with an argument of 175, you get this output:$ java Planet 175 Your weight on MERCURY is 66.107583 Your weight on VENUS is 158.374842 Your weight on EARTH is 175.000000 Your weight on MARS is 66.279007 Your weight on JUPITER is 442.847567 Your weight on SATURN is 186.552719 Your weight on URANUS is 158.397260 Your weight on NEPTUNE is 199.207413 Your weight on PLUTO is 11.703031