Enum Types
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.
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 timefor example, the choices on a menu, command line flags, and so on.
Here is some code that shows you how to use the Day enum defined above:
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 static values 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 the Planet class example below iterates over all the planets in the solar system.
for (Planet p : Planet.values())
Note: All enums implicitly extend java.lang.Enum . Because a class can only extend one parent (see Declaring Classes), the Java language does not support multiple inheritance of state (see Multiple Inheritance of State, Implementation, and Type), and therefore an enum cannot extend anything else.
In the following example, 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.
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.
In addition to its properties and constructor, 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):
public 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); 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) < if (args.length != 1) < System.err.println("Usage: java Planet "); System.exit(-1); > 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)); > > If you run 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
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Enums
In 5.0, the Java™ programming language gets linguistic support for enumerated types. In their simplest form, these enums look just like their C, C++, and C# counterparts:
But appearances can be deceiving. Java programming language enums are far more powerful than their counterparts in other languages, which are little more than glorified integers. The new enum declaration defines a full-fledged class (dubbed an enum type). In addition to solving all the problems mentioned above, it allows you to add arbitrary methods and fields to an enum type, to implement arbitrary interfaces, and more. Enum types provide high-quality implementations of all the Object methods. They are Comparable and Serializable, and the serial form is designed to withstand arbitrary changes in the enum type.
Here is an example of a playing card class built atop a couple of simple enum types. The Card class is immutable, and only one instance of each Card is created, so it need not override equals or hashCode :
import java.util.*; public class Card < public enum Rank public enum Suit private final Rank rank; private final Suit suit; private Card(Rank rank, Suit suit) < this.rank = rank; this.suit = suit; >public Rank rank() < return rank; >public Suit suit() < return suit; >public String toString() < return rank + " of " + suit; >private static final List protoDeck = new ArrayList(); // Initialize prototype deck static < for (Suit suit : Suit.values()) for (Rank rank : Rank.values()) protoDeck.add(new Card(rank, suit)); >public static ArrayList newDeck() < return new ArrayList(protoDeck); // Return copy of prototype deck > >
The toString method for Card takes advantage of the toString methods for Rank and Suit . Note that the Card class is short (about 25 lines of code). If the typesafe enums ( Rank and Suit ) had been built by hand, each of them would have been significantly longer than the entire Card class.
The (private) constructor of Card takes two parameters, a Rank and a Suit . If you accidentally invoke the constructor with the parameters reversed, the compiler will politely inform you of your error. Contrast this to the int enum pattern, in which the program would fail at run time.
Note that each enum type has a static values method that returns an array containing all of the values of the enum type in the order they are declared. This method is commonly used in combination with the for-each loop to iterate over the values of an enumerated type.
The following example is a simple program called Deal that exercises Card . It reads two numbers from the command line, representing the number of hands to deal and the number of cards per hand. Then it creates a new deck of cards, shuffles it, and deals and prints the requested hands.
import java.util.*; public class Deal < public static void main(String args[]) < int numHands = Integer.parseInt(args[0]); int cardsPerHand = Integer.parseInt(args[1]); Listdeck = Card.newDeck(); Collections.shuffle(deck); for (int i=0; i < numHands; i++) System.out.println(deal(deck, cardsPerHand)); >public static ArrayList deal(List deck, int n) < int deckSize = deck.size(); ListhandView = deck.subList(deckSize-n, deckSize); ArrayList hand = new ArrayList(handView); handView.clear(); return hand; > > $ java Deal 4 5 [FOUR of HEARTS, NINE of DIAMONDS, QUEEN of SPADES, ACE of SPADES, NINE of SPADES] [DEUCE of HEARTS, EIGHT of SPADES, JACK of DIAMONDS, TEN of CLUBS, SEVEN of SPADES] [FIVE of HEARTS, FOUR of DIAMONDS, SIX of DIAMONDS, NINE of CLUBS, JACK of CLUBS] [SEVEN of HEARTS, SIX of CLUBS, DEUCE of DIAMONDS, THREE of SPADES, EIGHT of CLUBS]
Suppose you want to add data and behavior to an enum. For example consider the planets of the solar system. Each planet knows its mass and radius, and can calculate its surface gravity and the weight of an object on the planet. Here is how it looks:
public 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; >public double mass() < return mass; >public double radius() < return radius; >// universal gravitational constant (m3 kg-1 s-2) public static final double G = 6.67300E-11; public double surfaceGravity() < return G * mass / (radius * radius); >public double surfaceWeight(double otherMass) < return otherMass * surfaceGravity(); >>
The enum type Planet contains a constructor, and each enum constant is declared with parameters to be passed to the constructor when it is created.
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):
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)); >$ 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 The idea of adding behavior to enum constants can be taken one step further. You can give each enum constant a different behavior for some method. One way to do this by switching on the enumeration constant. Here is an example with an enum whose constants represent the four basic arithmetic operations, and whose eval method performs the operation:
public enum Operation < PLUS, MINUS, TIMES, DIVIDE; // Do arithmetic op represented by this constant double eval(double x, double y)< switch(this) < case PLUS: return x + y; case MINUS: return x - y; case TIMES: return x * y; case DIVIDE: return x / y; >throw new AssertionError("Unknown op: " + this); > > This works fine, but it will not compile without the throw statement, which is not terribly pretty. Worse, you must remember to add a new case to the switch statement each time you add a new constant to Operation. If you forget, the eval method with fail, executing the aforementioned throw statement
There is another way give each enum constant a different behavior for some method that avoids these problems. You can declare the method abstract in the enum type and override it with a concrete method in each constant. Such methods are known as constant-specific methods. Here is the previous example redone using this technique:
public enum Operation < PLUS < double eval(double x, double y) < return x + y; >>, MINUS < double eval(double x, double y) < return x - y; >>, TIMES < double eval(double x, double y) < return x * y; >>, DIVIDE < double eval(double x, double y) < return x / y; >>; // Do arithmetic op represented by this constant abstract double eval(double x, double y); >
Here is a sample program that exercises the Operation class. It takes two operands from the command line, iterates over all the operations, and for each operation, performs the operation and prints the resulting equation:
public static void main(String args[]) < double x = Double.parseDouble(args[0]); double y = Double.parseDouble(args[1]); for (Operation op : Operation.values()) System.out.printf("%f %s %f = %f%n", x, op, y, op.eval(x, y)); >$ java Operation 4 2 4.000000 PLUS 2.000000 = 6.000000 4.000000 MINUS 2.000000 = 2.000000 4.000000 TIMES 2.000000 = 8.000000 4.000000 DIVIDE 2.000000 = 2.000000 Constant-specific methods are reasonably sophisticated, and many programmers will never need to use them, but it is nice to know that they are there if you need them.
Two classes have been added to java.util in support of enums: special-purpose Set and Map implementations called EnumSet and EnumMap . EnumSet is a high-performance Set implementation for enums. All of the members of an enum set must be of the same enum type. Internally, it is represented by a bit-vector, typically a single long . Enum sets support iteration over ranges of enum types. For example given the following enum declaration:
for (Day d : EnumSet.range(Day.MONDAY, Day.FRIDAY)) System.out.println(d);
EnumSet.of(Style.BOLD, Style.ITALIC)
Similarly, EnumMap is a high-performance Map implementation for use with enum keys, internally implemented as an array. Enum maps combine the richness and safety of the Map interface with speed approaching that of an array. If you want to map an enum to a value, you should always use an EnumMap in preference to an array.
The Card class, above, contains a static factory that returns a deck, but there is no way to get an individual card from its rank and suit. Merely exposing the constructor would destroy the singleton property (that only a single instance of each card is allowed to exist). Here is how to write a static factory that preserves the singleton property, using a nested EnumMap:
private static Map> table = new EnumMap>(Suit.class); static < for (Suit suit : Suit.values()) < MapsuitTable = new EnumMap(Rank.class); for (Rank rank : Rank.values()) suitTable.put(rank, new Card(rank, suit)); table.put(suit, suitTable); > > public static Card valueOf(Rank rank, Suit suit)
The EnumMap ( table ) maps each suit to an EnumMap that maps each rank to a card. The lookup performed by the valueOf method is internally implemented as two array accesses, but the code is much clearer and safer. In order to preserve the singleton property, it is imperative that the constructor invocation in the prototype deck initialization in Card be replaced by a call to the new static factory:
// Initialize prototype deck static < for (Suit suit : Suit.values()) for (Rank rank : Rank.values()) protoDeck.add(Card.valueOf(rank, suit)); >
It is also imperative that the initialization of table be placed above the initialization of protoDeck, as the latter depends on the former.
So when should you use enums? Any time you need a fixed set of constants. That includes natural enumerated types (like the planets, days of the week, and suits in a card deck) as well as other sets where you know all possible values at compile time, such as choices on a menu, rounding modes, command line flags, and the like. It is not necessary that the set of constants in an enum type stay fixed for all time. The feature was specifically designed to allow for binary compatible evolution of enum types.