用途
Enumeration 通常用在所謂的狀態值或者屬性，這些值的特性通常都是由多個固定值組成，且不允許工程師任意自行添加新的值。Ex: Database 有個欄位
is_valid ，設定只能是 0 或 1，在程式的寫作過程，就不允許工程師使用或修改成其他值(ex: 2)，以免造成資料庫資料的錯亂。要避免上述的情況發生，就是限制工程師，也因此就有了
Enumeration。最簡單的 Enumeration
Java 版
- Source
public enum DirectionJava { TOP, DOWN, LEFT, RIGHT; } javap -p
Compiled from "DirectionJava.java" public final class DirectionJava extends java.lang.Enum<DirectionJava> { public static final DirectionJava TOP; public static final DirectionJava DOWN; public static final DirectionJava LEFT; public static final DirectionJava RIGHT; private static final DirectionJava[] $VALUES; public static DirectionJava[] values(); public static DirectionJava valueOf(java.lang.String); private DirectionJava(); static {}; }
enum 這個 keyword 來宣告即可。從 Java Bytecode 可以發現，其實 enum 底層本身是個 final class, 也因此 enum 不能再繼承其他的 class (Java 是單一繼承)。 Constructor 設計成 private，所以工程師不能隨意自行產生值，只能使用內定的值或 null。Scala 版本
- Source
object DirectionScala extends Enumeration { val Top, Down, Left, Right = Value } scalap -private
object DirectionScala extends scala.Enumeration { def this() = { /* compiled code */ } val Top : scala.Enumeration.Value = { /* compiled code */ } val Down : scala.Enumeration.Value = { /* compiled code */ } val Left : scala.Enumeration.Value = { /* compiled code */ } val Right : scala.Enumeration.Value = { /* compiled code */ } }javap -p DirectionScala
Compiled from "DirectionScala.scala" public final class DirectionScala { public static scala.Enumeration$Value Right(); public static scala.Enumeration$Value Left(); public static scala.Enumeration$Value Down(); public static scala.Enumeration$Value Top(); public static scala.Enumeration$Value withName(java.lang.String); public static scala.Enumeration$Value apply(int); public static int maxId(); public static scala.Enumeration$ValueSet values(); public static java.lang.String toString(); }javap -p DirectionScala\$.class
public static final DirectionScala$ MODULE$; private final scala.Enumeration$Value Top; private final scala.Enumeration$Value Down; private final scala.Enumeration$Value Left; private final scala.Enumeration$Value Right; public static {}; public scala.Enumeration$Value Top(); public scala.Enumeration$Value Down(); public scala.Enumeration$Value Left(); public scala.Enumeration$Value Right(); private DirectionScala$();
Enumeration 跟 Java 的很類似，但 Scala 的內定值型別是 Enumeration.Values, 它本身是個 abstract class 在 Enumeration 內有一個實作的 class: protected class Val，如果需要自定 Enumeration 時，就會需要繼承這個 class。進階版
使用 Java 說明文件上的 Planet 範例。Java 版
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 <earth_weight>");
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));
}
}
Scala 版
Scala 實作的方式很多，網路上都找得到，以下是我自己寫的版本。object Planets extends Enumeration {
val G: Double = 6.67300E-11
final case class Planet private[Planets] (mass: Double, radius: Double) extends Val {
def surfaceGravity(): Double = G * mass / (radius * radius)
def surfaceWeight(otherMass: Double): Double = otherMass * surfaceGravity()
}
val Mercury = Planet(3.303e+23, 2.4397e6)
val Venus = Planet(4.869e+24, 6.0518e6)
val Earth = Planet(5.976e+24, 6.37814e6)
val Mars = Planet(6.421e+23, 3.3972e6)
val Jupiter = Planet(1.9e+27, 7.1492e7)
val Saturn = Planet(5.688e+26, 6.0268e7)
val Uranus = Planet(8.686e+25, 2.5559e7)
val Neptune = Planet(1.024e+26, 2.4746e7)
def main(args: Array[String]) {
require(args.length == 1, "Usage: java Planet <earth_weight>")
val earthWeight = args(0).toDouble
val mass = earthWeight / Planets.Earth.surfaceGravity
Planets.values.foreach(p => println("Your weight on %s is %f%n".format(p, p.asInstanceOf[Planet].surfaceWeight(mass))))
}
}
Enumeration.Val，由於 Scala 的 values 沒有 generic 的功能，所以需要做轉型，不過 Enumeration 的 values 功能很少用，所以不用太在意。
