java enum用法

1. 基本用法

   enum Day {
   SUNDAY, MONDAY, TUESDAY, WENDSDAY, THURSDAY, FRIDAY, SATURDAY;
   }

   枚举是常量，所以应该用大写。

2. 枚举是对象

   枚举隐含地继承了java.lang.Enum，所以它具有java.lang.Enum的属性和方法。遍历枚举：

   public class Main {
   public static void main(String[] args) {
   for(Day day:Day.values()) {
   System.out.println(day);
   }
   }
   }

3. 枚举可以带字段和方法，以下示例来自官方The Java™ Tutorials

   public enum EnumDemo {
   AOBJECT("field one", "field two");

   private String field1;
   private String field2;

   EnumDemo(String val1, String val2){
   this.field1 = val1;
   this.field2 = val2;
   }

   public void printFields(){
   System.out.println(this.field1);
   System.out.println(this.field2);
   }

   public static void main(String[] args) {
   EnumDemo.AOBJECT.printFields();
   }
   }

   以下现实中的示例来自官方的Java Tutorial:

   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) {
   Double earthWeight = 120;
   for(Planet p: Planet.values()){
   System.out.println(p.surfaceGravity());
   System.out.println(p.surfaceWeight(earthWeight/EARTH.surfaceGravity()));
   }
   }
   }

4. 枚举是singleton，可以用枚举来构建一个Singleton

   public enum Singleton {
   INSTANCE(new String[]{"arg1", "arg2"});

   String[] myArgs;
   Singleton(String[] args){
   this.myArgs = args;
   }

   public static Singleton getInstance(){
   return INSTANCE;
   }

   public static void main(String[] args) {
   for(String arg : INSTANCE.myArgs)
   System.out.println(arg);
   }
   }