java enum不太常用，但是作为一个重要的类型，会在很多开源项目中看到，总结一下。

    enum 用来保存一系列常量，如每周的日子，太阳系的行星。简单的列子如下：

    public enum Day {

       SUNDAY,MONDAY,TUESDAY,THURSDAY,WEDNESDAY,FRIDAY

    }

 

    public class EnumTest {

    Day day;

    public EnumTest(Day day) {

       this.day = day;

     }

    public void tellMeWhey() {

       switch (day) {

         case MONDAY:

              System.out.println("this is monday");

         case FRIDAY:

              System.out.println("friday i am in love");

         default:

              System.out.println("it is just a normal day");

        }

    }

}

 

    此外，enum在java中功能要强于其他语言，可以将enum 类似于class来使用，比如可以定义构建函数，定义方法等。例子如下：

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);    // in kilograms    private final double mass;    // in meters    private final double radius;    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));    }}