Java Enum 枚举类型

从官网上看到的例子，在代码中加以注释，方便学习 

http://docs.oracle.com/javase/tutorial/java/javaOO/enum.html

package pan.java.EnumDemo; 
//enum 等价与class文件 
public enum Day { 
//枚举值的声明必须在 fields methosd 之前 
SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY;

//可以定义fields
public int a=2;

//可以定义 methods
public void show(){
    System.out.println(a);
}

//enum 的构造器必须是  package-private(什么都不写就是package-private) or private access  

Day(){

}
}

package pan.java.EnumDemo;

public class EnumTest { 
Day day;

public EnumTest(Day day) {
    this.day = day;
}

public void tellItLikeItIs() {
    switch (day) {
        case MONDAY:
            System.out.println("Mondays are bad.");
            break;

        case FRIDAY:
            System.out.println("Fridays are better.");
            break;

        case SATURDAY: case SUNDAY:
            System.out.println("Weekends are best.");
            break;

        default:
            System.out.println("Midweek days are so-so.");
            break;
    }
}

public static void main(String[] args) {
    EnumTest firstDay = new EnumTest(Day.MONDAY);
    firstDay.tellItLikeItIs();
    EnumTest thirdDay = new EnumTest(Day.WEDNESDAY);
    thirdDay.tellItLikeItIs();
    EnumTest fifthDay = new EnumTest(Day.FRIDAY);
    fifthDay.tellItLikeItIs();
    EnumTest sixthDay = new EnumTest(Day.SATURDAY);
    sixthDay.tellItLikeItIs();
    EnumTest seventhDay = new EnumTest(Day.SUNDAY);
    seventhDay.tellItLikeItIs();

    //特殊方法 values() 返回enum数组
    for(Day d:Day.values()){
        System.out.println("Day.values() : "+d.toString());
    }

    //不能实例化 enum 类 那还要构造函数干嘛？
    //Day dd=new Day();

    //特殊方法 values() 返回enum数组
    Day[] day =Day.values();
    for(int x=0;x<=day.length-1;x++){
        System.out.println("Day.values() : "+day[x]);
    }

    //所有的enum 都隐式的继承了 java.lang.Enum

}
}

package pan.java.EnumDemo;

public enum Planet { 
//这样使用 enum 的构造函数 
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) {
    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));
}
}