Enum Types in java

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本文详细介绍了Java中枚举类型的定义及使用方法，包括如何为枚举常量指定参数值，定义枚举构造器，以及如何实现枚举类型的方法来完成特定任务。通过两个实例，演示了枚举类型在实际应用中的灵活性。

java tutorial fourth edition 4.5

Enum Types

Each enum constant is declared with values for the mass and radius parameters. These values are passed to the constructor when the constant is created. Java requires that the constants be defined first, prior to any fields or methods. Also, when there are fields and methods, the list of enum constants must end with a semicolon.

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.

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),

  PLUTO   (1.27e+22, 1.137e6);



  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) {

    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));

  }

}

If you run Planet.class from the command line with an argument of 175, you get this output:

$ java Planet 175

Your weight on MERCURY is 66.107583

Your weight on VENUS is 158.374842

Your weight on EARTH is 175.000000

Your weight on MARS is 66.279007

Your weight on JUPITER is 442.847567

Your weight on SATURN is 186.552719

Your weight on URANUS is 158.397260

Your weight on NEPTUNE is 199.207413

Your weight on PLUTO is 11.703031

--------------------------------------------------------------------------------

practice :

public enum FC {
BOJAN ("Bojan", 27, 1990),
SILVA ("Henry", 14, 1977),
PUYOL ("Puyol", 5, 1978),
XAVI ("Xavi", 6, 1980),
INIESTA ("Iniesta", 8, 1984),
MESSI ("Messi", 19, 1987);

private String name;
private int num;
private int year;

FC(String name, int num, int year){
  this.name = name;
  this.num = num;
  this.year = year;
}

int getAge(){
  return 2010 - year;
}

public static void main(String[] args){
  for(FC fc : FC.values()){
   System.out.println("Name :" + fc.name +
   "/tNumber :" + fc.num +
   "/tAge :" + fc.getAge());
  }
}
}
--------------

Name :Bojan Number :27 Age :20
Name :Henry Number :14 Age :33
Name :Puyol Number :5 Age :32
Name :Xavi Number :6 Age :30
Name :Iniesta Number :8 Age :26
Name :Messi Number :19 Age :23