java enmu 使用说明

    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 <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));
    }
}