枚举的好处：
1. 类型安全性
2.使用方便性
public class EnumDemo {
enum Color{
RED(3),BLUE(5),BLACK(8),YELLOW(13),GREEN(28);
private int colorValue;
private Color(int rv){
this.colorValue=rv;
}
private int getColorValue(){
return colorValue;
} private int value(){
return ordinal()+1;
}
}
public static void main(String[] args) {
for(Color s : Color.values()) {
//enum的values()返回一个数组，这里就是Seasons[]
System.out.println(s.value()+":"+s.name()+"="+s.getColorValue());
}
}
}
output：
1:RED=3
2:BLUE=5
3:BLACK=8
4:YELLOW=13
5:GREEN=28
其中，
/**
* Returns the ordinal of this enumeration constant (its position
* in its enum declaration, where the initial constant is assigned
* an ordinal of zero).
*
* Most programmers will have no use for this method. It is
* designed for use by sophisticated enum-based data structures, such
* as {@link java.util.EnumSet} and {@link java.util.EnumMap}.
*
* @return the ordinal of this enumeration constant
*/
public final int ordinal() {
return ordinal;
}
EnumMap是专门为枚举类型量身定做的Map实现。虽然使用其它的Map实现（如HashMap）也能完成枚举类型实例到值得映射，但是使用EnumMap会更加高效：它只能接收同一枚举类型的实例作为键值，并且由于枚举类型实例的数量相对固定并且有限，所以EnumMap使用数组来存放与枚举类型对应的值。这使得EnumMap的效率非常高。
import java.util.*;
public enum Phase {
SOLID, LIQUID, GAS;
public enum Transition {
MELT(SOLID, LIQUID), FREEZE(LIQUID, SOLID), BOIL(LIQUID, GAS), CONDENSE(
GAS, LIQUID), SUBLIME(SOLID, GAS), DEPOSIT(GAS, SOLID);
private final Phase src;
private final Phase dst;
Transition(Phase src, Phase dst) {
this.src = src;
this.dst = dst;
}
private static final Map<Phase, Map<Phase, Transition>> m = new EnumMap<Phase, Map<Phase, Transition>>(
Phase.class);
static {
for (Phase p : Phase.values())
m.put(p, new EnumMap<Phase, Transition>(Phase.class));
for (Transition trans : Transition.values())
m.get(trans.src).put(trans.dst, trans);
}
public static Transition from(Phase src, Phase dst) {
return m.get(src).get(dst);
}
}
public static void main(String[] args) {
for (Phase src : Phase.values())
for (Phase dst : Phase.values())
if (src != dst)
System.out.printf("%s to %s : %s %n", src, dst,
Transition.from(src, dst));
}
}
