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));

    }

}