Enum Types

参考Java的官方tutorial和Doc整理如下。

What is Enum

An enum type is a special data type.

It enables for a variable to be a set of predefined constants. Because they are constants, the names of an enum type's fields are in UPPERCASE letters.

For example, you can specify a days-of-the-week enum type as:

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

You can use enum by . (dot), example Day.MONDAY

Enum can be more powerful. The enum declaration defines a class (called an enum type). The enum class body can include methods and other fields.

For example, you can specify a planet enum type:

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);

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));
}
}