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Enums |
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This pattern has many problems, such as:// int Enum Pattern - has severe problems! public static final int SEASON_WINTER = 0; public static final int SEASON_SPRING = 1; public static final int SEASON_SUMMER = 2; public static final int SEASON_FALL = 3;
int you can pass in any other int value where a season is required, or add two seasons together (which makes no sense).
SEASON_) to avoid collisions with other int enum types.
switch statements.
In Tiger, 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:
enum Season { WINTER, SPRING, SUMMER, FALL }
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 { DEUCE, THREE, FOUR, FIVE, SIX,
SEVEN, EIGHT, NINE, TEN, JACK, QUEEN, KING, ACE }
public enum Suit { CLUBS, DIAMONDS, HEARTS, SPADES }
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<Card> protoDeck = new ArrayList<Card>();
// Initialize prototype deck
static {
for (Suit suit : Suit.values())
for (Rank rank : Rank.values())
protoDeck.add(new Card(rank, suit));
}
public static ArrayList<Card> newDeck() {
return new ArrayList<Card>(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]);
List<Card> deck = Card.newDeck();
Collections.shuffle(deck);
for (int i=0; i < numHands; i++)
System.out.println(deal(deck, cardsPerHand));
}
public static ArrayList<Card> deal(List<Card> deck, int n) {
int deckSize = deck.size();
List<Card> handView = deck.subList(deckSize-n, deckSize);
ArrayList<Card> hand = new ArrayList<Card>(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;
}
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();
}
}
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:
enum Day { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY }
you can iterate over the weekdays. The EnumSet class
provides a static factory that makes it easy:
for (Day d : EnumSet.range(Day.MONDAY, Day.FRIDAY))
System.out.println(d);
Enum sets also provide a rich, typesafe replacement for traditional
bit flags:
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<Suit, Map<Rank, Card>> table =
new EnumMap<Suit, Map<Rank, Card>>(Suit.class);
static {
for (Suit suit : Suit.values()) {
Map<Rank, Card> suitTable = new EnumMap<Rank, Card>(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) {
return table.get(suit).get(rank);
}
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.
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