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);
       }
   }