Enum笔记
//enum是类，里面定义的类型是枚举实例，所以可以有构造函数
public enum OzWitch {
// Instances must be defined first, before methods:
WEST("Miss Gulch, aka the Wicked Witch of the West"),
NORTH("Glinda, the Good Witch of the North"),
EAST("Wicked Witch of the East, wearer of the Ruby " +
"Slippers, crushed by Dorothy’s house"),
SOUTH("Good by inference, but missing");
private String description;
// Constructor must be package or private access:
private OzWitch(String description) {
this.description = description;
}
public String getDescription() { return description; }
public static void main(String[] args) {
for(OzWitch witch : OzWitch.values())
print(witch + ": " + witch.getDescription());
}
}
======================================================
enum Search { HITHER, YON }
public class UpcastEnum {
public static void main(String[] args) {
Search[] vals = Search.values();
Enum e = Search.HITHER; // Upcast
// e.values(); // No values() in Enum
for(Enum en : e.getClass().getEnumConstants())
System.out.println(en);
}
}
========================================================
这里面的每个enum会被编译成一个类，类实现了Food接口，所以enum类的每个枚举实例也就可以是转换成Food了。
public enum Meal2 {
APPETIZER(Food.Appetizer.class),
MAINCOURSE(Food.MainCourse.class),
DESSERT(Food.Dessert.class),
COFFEE(Food.Coffee.class);
private Food[] values;
private Meal2(Class<? extends Food> kind) {
values = kind.getEnumConstants();
}
public interface Food {
enum Appetizer implements Food {
SALAD, SOUP, SPRING_ROLLS;
}
enum MainCourse implements Food {
LASAGNE, BURRITO, PAD_THAI,
LENTILS, HUMMOUS, VINDALOO;
}
enum Dessert implements Food {
TIRAMISU, GELATO, BLACK_FOREST_CAKE,
FRUIT, CREME_CARAMEL;
}
enum Coffee implements Food {
BLACK_COFFEE, DECAF_COFFEE, ESPRESSO,
LATTE, CAPPUCCINO, TEA, HERB_TEA;
}
}
public Food randomSelection() {
return Enums.random(values);
}
public static void main(String[] args) {
for(int i = 0; i < 5; i++) {
for(Meal2 meal : Meal2.values()) {
Food food = meal.randomSelection();
System.out.println(food);
}
System.out.println("---");
}
}
}
==========================================================
//由于枚举是final和static，所以在枚举实例里面实现的方法，只能调用static类型的变量和方法
public class CarWash {
public enum Cycle {
UNDERBODY {
void action() {
System.out.println("Spraying the underbody");
}
},
WHEELWASH {
void action() {
System.out.println("Washing the wheels");
}
},
PREWASH {
void action() {
System.out.println("Loosening the dirt");
}
},
BASIC {
void action() {
System.out.println("The basic wash");
}
},
HOTWAX {
void action() {
System.out.println("Applying hot wax");
}
},
RINSE {
void action() {
System.out.println("Rinsing");
}
},
BLOWDRY {
void action() {
System.out.println("Blowing dry");
}
};
abstract void action();
}
EnumSet<Cycle> cycles = EnumSet.of(Cycle.BASIC, Cycle.RINSE);
public void add(Cycle cycle) {
cycles.add(cycle);
}
public void washCar() {
for (Cycle c : cycles)
c.action();
}
public String toString() {
return cycles.toString();
}
public static void main(String[] args){
CarWash wash = new CarWash();
System.out.println(wash);
wash.washCar();
// Order of addition is unimportant:
wash.add(Cycle.BLOWDRY);
wash.add(Cycle.BLOWDRY); // Duplicates ignored
wash.add(Cycle.RINSE);
wash.add(Cycle.HOTWAX);
System.out.println(wash);
wash.washCar();
}
}
//或者可以覆盖方法：
public enum OverrideConstantSpecific {
NUT, BOLT,
WASHER {
void f() { print("Overridden method"); }
};
void f() { print("default behavior"); }
public static void main(String[] args) {
for(OverrideConstantSpecific ocs : values()) {
printnb(ocs + ": ");
ocs.f();
}
}
}
=======================================================
Multiple dispatching（这里只列出两种方法）：
这意思就是，当所有变量都不知道类型时，如何执行方法调用。
public class Enums {
private static Random rand = new Random(47);
public static <T extends Enum<T>> T random(Class<T> ec) {
return random(ec.getEnumConstants());
}
public static <T> T random(T[] values) {
return values[rand.nextInt(values.length)];
}
}
interface Competitor<T extends Competitor<T>> {
Outcome compete(T competitor);
}
public class RoShamBo {
public static <T extends Competitor<T>> void match(T a, T b) {
System.out.println(a + " vs. " + b + ": " + a.compete(b));
}
public static <T extends Enum<T> & Competitor<T>> void play(
Class<T> rsbClass, int size) {
for (int i = 0; i < size; i++)
match(Enums.random(rsbClass), Enums.random(rsbClass));
}
}
====================
方法一：
enum RoShamBo5 implements Competitor<RoShamBo5> {
PAPER, SCISSORS, ROCK;
static EnumMap<RoShamBo5, EnumMap<RoShamBo5, Outcome>> table = new EnumMap<RoShamBo5, EnumMap<RoShamBo5, Outcome>>(
RoShamBo5.class);
static {
for (RoShamBo5 it : RoShamBo5.values())
table.put(it, new EnumMap<RoShamBo5, Outcome>(RoShamBo5.class));
initRow(PAPER, DRAW, LOSE, WIN);
initRow(SCISSORS, WIN, DRAW, LOSE);
initRow(ROCK, LOSE, WIN, DRAW);
}
static void initRow(RoShamBo5 it, Outcome vPAPER, Outcome vSCISSORS,
Outcome vROCK) {
EnumMap<RoShamBo5, Outcome> row = RoShamBo5.table.get(it);
row.put(RoShamBo5.PAPER, vPAPER);
row.put(RoShamBo5.SCISSORS, vSCISSORS);
row.put(RoShamBo5.ROCK, vROCK);
}
public Outcome compete(RoShamBo5 it) {
return table.get(this).get(it);
}
public static void main(String[] args) {
RoShamBo.play(RoShamBo5.class, 20);
}
}
方法二：
enum RoShamBo6 implements Competitor<RoShamBo6> {
PAPER, SCISSORS, ROCK;
private static Outcome[][] table = {
{ DRAW, LOSE, WIN }, // PAPER
{ WIN, DRAW, LOSE }, // SCISSORS
{ LOSE, WIN, DRAW }, // ROCK
};
public Outcome compete(RoShamBo6 other) {
return table[this.ordinal()][other.ordinal()];
}
public static void main(String[] args) {
RoShamBo.play(RoShamBo6.class, 20);
}
}