Java Enum Types

之所以要定义Enum，原因很简单，只能取到预定义的数据，并且只能是其中的一个。

简单定义一个Enum：

public enum Day {    SUNDAY, MONDAY, TUESDAY, WEDNESDAY,    THURSDAY, FRIDAY, SATURDAY }

稍微复杂一点：

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);    //这里属性要和上面构造参数一一对应    private final double mass;   // in kilograms    private final double radius; // in meters    //这里的构造函数也是    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));    }}

Note: All enums implicitly extend java.lang.Enum. Because a class can only extend one parent (see Declaring Classes), the Java language does not support multiple inheritance of state (see Multiple Inheritance of State, Implementation, and Type), and therefore an enum cannot extend anything else.

Note: The constructor for an enum type must be package-private or private access. It automatically creates the constants that are defined at the beginning of the enum body. You cannot invoke an enum constructor yourself.也就是你不能自己new，Enum type不能被实例化。