Java编程思想笔记——枚举类型
关键字enum可以将一组具名的值的有限集合创建为一种新的类型，而这些具名的值可以作为常规的程序组件使用。
基本enum特性
values()返回enum实例的数组，而且保持声明的顺序：
enum Shrubbery {GROUND, CRAWLING, HANGING}
public class EnumClass {
public static void main(String[] args) {
for (Shrubbery s : Shrubbery.values()) {
print(s + " ordinal: " + s.ordinal());
printnb(s.compareTo(Shrubbery.CRAWLING) + " ");
printnb(s.equals(Shrubbery.CRAWLING) + " ");
print(s == Shrubbery.CRAWLING);
print(s.getDeclaringClass());
print(s.name());
print("----------------------");
}
// Produce an enum value from a string name:
for (String s : "HANGING CRAWLING GROUND".split(" ")) {
Shrubbery shrub = Enum.valueOf(Shrubbery.class, s);
print(shrub);
}
}
} /*
GROUND ordinal: 0
-1 false false
class Shrubbery
GROUND
----------------------
CRAWLING ordinal: 1
0 true true
class Shrubbery
CRAWLING
----------------------
HANGING ordinal: 2
1 false false
class Shrubbery
HANGING
----------------------
HANGING
CRAWLING
GROUND
*/ordinal方法返回一个int值，这是每个enum实例在声明时的次序，从0开始。可以使用==来比较enum实例，编译器会自动提供equals和hashCode方法。Enum类实现了Comparable接口，所以它具有compareTo()方法。同时还实现了Serializable接口。
getDeclaringClass()获取其所属的enum类。
name()返回enum实例声明时的名字，与使用toString()效果相同。valueOf()实在Enum中定义的static方法，他根据给定的名字返回相应的enum实例，如果不存在会抛出异常。
将静态导入用于enum
public class Burrito {
Spiciness degree;
public Burrito(Spiciness degree) {
this.degree = degree;
}
@Override
public String toString() {
return "Burrito is " + degree;
}
public static void main(String[] args) {
System.out.println(new Burrito(NOT));
System.out.println(new Burrito(MEDIUM));
System.out.println(new Burrito(HOT));
}
} /*
Burrito is NOT
Burrito is MEDIUM
Burrito is HOT
*/使用static import能够将enum实例的标识符代入当前的命名空间，所以无需再用enum类型来修饰enum实例。唯一担心的是使用静态导入会不会导致代码令人难以理解。
向enum中添加新方法
除了不能继承自一个enum之外，基本上可以将enum看做一个常规类。也就是说，可以添加方法，甚至可以有main方法。
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());
}
}
} /*
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
*/必须在enum实例序列的最后添加一个分号。Java要求必须先定义enum实例，如果在实例之前定义任何方法或属性，编译时会报错。有意将构造器声明为private，但对于他的可访问性并没有什么影响，因为即使不声明为private，我们只能在enum内部使用其构造器创建enum实例。一旦enum的定义结束，编译器就不允许在使用其构造器来创建任何实例了。
覆盖enum的方法
public enum SpaceShip {
SCOUT, CARGO, TRANSPORT, CRUISER, BATTLESHIP, MOTHERSHIP;
@Override
public String toString() {
String id = name();
String lower = id.substring(1).toLowerCase();
return id.charAt(0) + lower;
}
public static void main(String[] args) {
for (SpaceShip s : values()) {
System.out.println(s);
}
}
} /*
Scout
Cargo
Transport
Cruiser
Battleship
Mothership
*/switch语句中的enum
enum Signal {
GREEN, YELLOW, RED,
}
public class TrafficLight {
Signal color = Signal.RED;
public void change() {
switch (color) {
// Note that you don't have to say Signal.RED
// in the case statement:
case RED:
color = Signal.GREEN;
break;
case GREEN:
color = Signal.YELLOW;
break;
case YELLOW:
color = Signal.RED;
break;
}
}
@Override
public String toString() {
return "The traffic light is " + color;
}
public static void main(String[] args) {
TrafficLight t = new TrafficLight();
for (int i = 0; i < 7; i++) {
print(t);
t.change();
}
}
} /*
The traffic light is RED
The traffic light is GREEN
The traffic light is YELLOW
The traffic light is RED
The traffic light is GREEN
The traffic light is YELLOW
The traffic light is RED
*/values()的神秘之处
enum类都继承自Enum类，但是Enum并没有values()方法。利用反射机制查看究竟：
enum Explore {HERE, THERE}
public class Reflection {
public static Set analyze(Class enumClass) {
print("----- Analyzing " + enumClass + " -----");
print("Interfaces:");
for (Type t : enumClass.getGenericInterfaces()) {
print(t);
}
print("Base: " + enumClass.getSuperclass());
print("Methods: ");
Set methods = new TreeSet();
for (Method m : enumClass.getMethods()) {
methods.add(m.getName());
}
print(methods);
return methods;
}
public static void main(String[] args) {
Set exploreMethods = analyze(Explore.class);
Set enumMethods = analyze(Enum.class);
print("Explore.containsAll(Enum)? " +
exploreMethods.containsAll(enumMethods));
printnb("Explore.removeAll(Enum): ");
exploreMethods.removeAll(enumMethods);
print(exploreMethods);
// Decompile the code for the enum:
OSExecute.command("javap Explore");
}
} /*
----- Analyzing class Explore -----
Interfaces:
Base: class java.lang.Enum
Methods:
[compareTo, equals, getClass, getDeclaringClass, hashCode, name, notify, notifyAll, ordinal, toString, valueOf, values, wait]
----- Analyzing class java.lang.Enum -----
Interfaces:
java.lang.Comparable
interface java.io.Serializable
Base: class java.lang.Object
Methods:
[compareTo, equals, getClass, getDeclaringClass, hashCode, name, notify, notifyAll, ordinal, toString, valueOf, wait]
Explore.containsAll(Enum)? true
Explore.removeAll(Enum): [values]
Compiled from "Reflection.java"
final class Explore extends java.lang.Enum{
public static final Explore HERE;
public static final Explore THERE;
public static final Explore[] values();
public static Explore valueOf(java.lang.String);
static {};
}
*/ values()是由编译器添加的static方法。同时创建Explore的过程中，编译器还添加了valueOf()方法。不是Enum类不是已经有valueOf()方法了吗，不过Enum中的ValueOf()方法需要两个参数，这个新增方法只需一个参数。由于Set只存储方法的名字，不考虑签名，所以removeAll只剩下values。
Explore被标记为final类，所以无法继承自enum。
由于values方法有编译器插入到enum定义中的static方法，所以enum向上转型为Enum，那么values就不可访问了，不过Class中有一个getEnumConstants方法，所以即便Enum接口中没有vlaues方法，仍然可以通过Class对象取得所有enum实例：
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);
}
}
} /*
HITHER
YON
*/甚至可以对不是枚举的类调用getEnumConstants方法：
public class NonEnum {
public static void main(String[] args) {
Class intClass = Integer.class;
try {
for (Object en : intClass.getEnumConstants()) {
System.out.println(en);
}
} catch (Exception e) {
System.out.println(e);
}
}
} /*
java.lang.NullPointerException
*/ 只不过，此时该方法返回null，所以当试图使用其返回的结果时会发生异常。
实现，而非继承
创建一个新的enum，可以同时实现一个或多个接口
enum CartoonCharacter implements Generator {
SLAPPY, SPANKY, PUNCHY, SILLY, BOUNCY, NUTTY, BOB;
private Random rand = new Random(47);
@Override
public CartoonCharacter next() {
return values()[rand.nextInt(values().length)];
}
}
public class EnumImplementation {
public static void printNext(Generator rg) {
System.out.print(rg.next() + ", ");
}
public static void main(String[] args) {
// Choose any instance:
CartoonCharacter cc = CartoonCharacter.BOB;
for (int i = 0; i < 10; i++) {
printNext(cc);
}
}
} /*
BOB, PUNCHY, BOB, SPANKY, NUTTY, PUNCHY, SLAPPY, NUTTY, NUTTY, SLAPPY,
*/ 这个结果有点奇怪，不过必须有一个enum实例才能调用其上的方法。现在，在任何接受Generator参数的方法中。例如printNext()，都可以使用CartoonCharacter。
随机选取
public class Enums {
private static Random rand = new Random(47L);
public Enums() {
}
public static > T random(Class ec) {
return (Enum)random((Object[])((Enum[])ec.getEnumConstants()));
}
public static T random(T[] values) {
return values[rand.nextInt(values.length)];
}
} 古怪的语法
enum Activity {
SITTING, LYING, STANDING, HOPPING,
RUNNING, DODGING, JUMPING, FALLING, FLYING
}
public class RandomTest {
public static void main(String[] args) {
for (int i = 0; i < 20; i++) {
System.out.print(Enums.random(Activity.class) + " ");
}
}
} /*
STANDING FLYING RUNNING STANDING RUNNING STANDING LYING DODGING SITTING RUNNING HOPPING HOPPING HOPPING RUNNING STANDING LYING FALLING RUNNING FLYING LYING
*/使用接口组织枚举
有事希望使用子类将一个enum中的元素进行分组。在一个接口内部创建实现该接口的枚举，以此将元素分组。
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 class TypeOfFood {
public static void main(String[] args) {
Food food = Appetizer.SALAD;
food = MainCourse.LASAGNE;
food = Dessert.GELATO;
food = Coffee.CAPPUCCINO;
}
}然而，当需要与一大堆类型打交道时，接口就不如enum好用了。
如果想创建一个枚举的枚举，可以创建一个新的enum，然后用其实例包装Food中的每一个enum类：
public enum Course {
APPETIZER(Food.Appetizer.class),
MAINCOURSE(Food.MainCourse.class),
DESSERT(Food.Dessert.class),
COFFEE(Food.Coffee.class);
private Food[] values;
private Course(Classextends Food> kind) {
values = kind.getEnumConstants();
}
public Food randomSelection() {
return Enums.random(values);
}
}通过getEnumConstants方法可以从该Class对象中取得某个Food子类的所有enum实例。这些实例在randomSelection被用到。因此，通过从每一个COurse实例中随机地选择一个Food可生成一份菜单：
public class Meal {
public static void main(String[] args) {
for(int i = 0; i < 5; i++) {
for(Course course : Course.values()) {
Food food = course.randomSelection();
System.out.println(food);
}
System.out.println("---");
}
}
} /*
SPRING_ROLLS
VINDALOO
FRUIT
DECAF_COFFEE
---
SOUP
VINDALOO
FRUIT
TEA
---
SALAD
BURRITO
FRUIT
TEA
---
SALAD
BURRITO
CREME_CARAMEL
LATTE
---
SOUP
BURRITO
TIRAMISU
ESPRESSO
---
*/将一个enum嵌套在另一个enum内：
enum SecurityCategory {
STOCK(Security.Stock.class), BOND(Security.Bond.class);
Security[] values;
SecurityCategory(Classextends Security> kind) {
values = kind.getEnumConstants();
}
interface Security {
enum Stock implements Security {SHORT, LONG, MARGIN}
enum Bond implements Security {MUNICIPAL, JUNK}
}
public Security randomSelection() {
return Enums.random(values);
}
public static void main(String[] args) {
for (int i = 0; i < 10; i++) {
SecurityCategory category =
Enums.random(SecurityCategory.class);
System.out.println(category + ": " +
category.randomSelection());
}
}
} /*
BOND: MUNICIPAL
BOND: MUNICIPAL
STOCK: MARGIN
STOCK: MARGIN
BOND: JUNK
STOCK: SHORT
STOCK: LONG
STOCK: LONG
BOND: MUNICIPAL
BOND: JUNK
*/Security接口的作用是将其所包含组合成一个公共类型，这一点是有必要的。然后SecurityCategory 才能将Security中的enum作为其构造器的参数使用，以起到组织的效果。
应用到Food：
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 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("---");
}
}
}使用EnumSet替代标志
Set是一种集合不能添加重复元素。enum也要求其成员是唯一的。
Java SE5引入了EnumSet，是为了通过enum创建一种替代品，以替代传统的基于int的“位标志”。这种标志可以用来表示某种开关信息，不过，使用这种标志，最终操作的只是一些bit。使用EnumSet的有点是，它在说明一个二进制位是否存在时，具有更好的表达能力，并且无需担心性能。
enum存放大楼中，警报器的安放位置：
public enum AlarmPoints {
STAIR1, STAIR2, LOBBY, OFFICE1, OFFICE2, OFFICE3,
OFFICE4, BATHROOM, UTILITY, KITCHEN
}然后，用EnumSet来跟踪警报器的状态：
public class EnumSets {
public static void main(String[] args) {
EnumSet points =
EnumSet.noneOf(AlarmPoints.class); // Empty set
points.add(BATHROOM);
print(points);
points.addAll(EnumSet.of(STAIR1, STAIR2, KITCHEN));
print(points);
points = EnumSet.allOf(AlarmPoints.class);
points.removeAll(EnumSet.of(STAIR1, STAIR2, KITCHEN));
print(points);
points.removeAll(EnumSet.range(OFFICE1, OFFICE4));
print(points);
points = EnumSet.complementOf(points);
print(points);
}
} /*
[BATHROOM]
[STAIR1, STAIR2, BATHROOM, KITCHEN]
[LOBBY, OFFICE1, OFFICE2, OFFICE3, OFFICE4, BATHROOM, UTILITY]
[LOBBY, BATHROOM, UTILITY]
[STAIR1, STAIR2, OFFICE1, OFFICE2, OFFICE3, OFFICE4, KITCHEN]
*/ 仔细研究EnumSet文档，会发现of()方法被重载了很多次，不但为可变数量参数进行了重载，而且为接受2至5个显式的参数的情况都进行了重载。这也从侧面表现了EnumSet对性能的关注。因为，其实只使用可变参书已经可以解决整个问题，但是对比显式的参数，会有一点性能损失。采用现在这种设计，当只使用2到5个参数调用of()方法时，可以调用重载过的方法，而当使用一个参数或多过5个参数时，调用的将是使用可变参数的of()方法。
EnumSet的基础是long，一个long值有64位，而一个enum实例只需一位bit表示其是否存在。也就是说，在不超过一个long的表达能力的情况下，EnumSet可以应用于最多不超过64个元素的enum。
public class BigEnumSet {
enum Big {
A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10,
A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21,
A22, A23, A24, A25, A26, A27, A28, A29, A30, A31, A32,
A33, A34, A35, A36, A37, A38, A39, A40, A41, A42, A43,
A44, A45, A46, A47, A48, A49, A50, A51, A52, A53, A54,
A55, A56, A57, A58, A59, A60, A61, A62, A63, A64, A65,
A66, A67, A68, A69, A70, A71, A72, A73, A74, A75
}
public static void main(String[] args) {
EnumSet bigEnumSet = EnumSet.allOf(Big.class);
System.out.println(bigEnumSet);
}
} /*
[A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22, A23, A24, A25, A26, A27, A28, A29, A30, A31, A32, A33, A34, A35, A36, A37, A38, A39, A40, A41, A42, A43, A44, A45, A46, A47, A48, A49, A50, A51, A52, A53, A54, A55, A56, A57, A58, A59, A60, A61, A62, A63, A64, A65, A66, A67, A68, A69, A70, A71, A72, A73, A74, A75]
*/显然，EnumSet可以应用于多过64个元素的enum。
使用EnumMap
EnumMap要求其中的键必须来自于一个enum。由于enum本身的限制，所以EnumMap内部是数组实现。因此EnumMap速度很快，可以放心进行查找操作。
interface Command {
void action();
}
public class EnumMaps {
public static void main(String[] args) {
EnumMap em = new EnumMap(AlarmPoints.class);
em.put(KITCHEN, new Command() {
public void action() {
print("Kitchen fire!");
}
});
em.put(BATHROOM, new Command() {
public void action() {
print("Bathroom alert!");
}
});
for (Map.Entry e : em.entrySet()) {
printnb(e.getKey() + ": ");
e.getValue().action();
}
try { // If there's no value for a particular key:
em.get(UTILITY).action();
} catch (Exception e) {
print(e);
}
}
} /*
BATHROOM: Bathroom alert!
KITCHEN: Kitchen fire!
java.lang.NullPointerException
*/ 与EnumSet一样，enum实例定义时的次序决定了其在EnumMap中的顺序。
main()方法的最后部分说明，enum的每一个实例作为一键，总是存在的。但是，如果没有为这个键调用put()方法来存入相应的值的话，其对应的值就是null。
与常量相关的方法（下节）相比，EnumMap有一个优点，那EnumMap允许改变值对象，而常量相关方法在编译期就被固定了。
在有多种类型的enum，而且他们之间存在互操作的情况下，可以用EnumMap实现多路分发（下下节）。
常量相关的方法
Java的enum有一个非常有趣的特性，即它允许程序员为enum实例编写方法，从而为每个enum实例赋予各自不同的行为。要实现常量相关的方法，需要为enum定义一个或多个abstract方法，然后为每个enum实例实现该抽象方法：
public enum ConstantSpecificMethod {
DATE_TIME {
String getInfo() {
return
DateFormat.getDateInstance().format(new Date());
}
},
CLASSPATH {
String getInfo() {
return System.getenv("CLASSPATH");
}
},
VERSION {
String getInfo() {
return System.getProperty("java.version");
}
};
abstract String getInfo();
public static void main(String[] args) {
for (ConstantSpecificMethod csm : values()) {
System.out.println(csm.getInfo());
}
}
}/*
2018-5-23
.;C:\Program Files\Java\jdk1.8.0_77\lib\dt.jar;C:\Program Files\Java\jdk1.8.0_77\lib\tools.jar;
1.8.0_77
*/通过相应的enum实例，可以调用其上的方法，称为表驱动的代码（tabled riven code）。enum似乎被当做其超类ConstantSpecificMethod来使用，在调用getInfo()方法时，体现出多态的行为。
然而，enum实例与类的相似之处也仅限于此了，并不能真的将enum实例作为一个类型使用：
enum LikeClasses {
WINKEN {
void behavior() {
print("Behavior1");
}
},
BLINKEN {
void behavior() {
print("Behavior2");
}
},
NOD {
void behavior() {
print("Behavior3");
}
};
abstract void behavior();
}
public class NotClasses {
// void f1(LikeClasses.WINKEN instance) {} // Nope
} /*
Compiled from "NotClasses.java"
abstract class LikeClasses extends java.lang.Enum{
public static final LikeClasses WINKEN;
public static final LikeClasses BLINKEN;
public static final LikeClasses NOD;
...
*/f1()方法中，编译器不允许将一个enum实例当作class类型。因为每个enum元素都是一个LikeClasses类型的static final实例。同时，由于它们是static实例，无法访问外部类的非static元素或方法，所以对内部的enum的实例而言，其行为与一般的内部类并不相同。
再看一个更有趣的例子。每个顾客在洗车时，都有一个选择菜单，每个选择对应一个不同的动作。可以将一个常量相关的方法关联到一个选择上，再使用一个EnumSet来保存客户的选择：
public class CarWash {
public enum Cycle {
UNDERBODY {
void action() { print("Spraying the underbody"); }
},
WHEELWASH {
void action() { print("Washing the wheels"); }
},
PREWASH {
void action() { print("Loosening the dirt"); }
},
BASIC {
void action() { print("The basic wash"); }
},
HOTWAX {
void action() { print("Applying hot wax"); }
},
RINSE {
void action() { print("Rinsing"); }
},
BLOWDRY {
void action() { print("Blowing dry"); }
};
abstract void action();
}
EnumSet 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();
print(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);
print(wash);
wash.washCar();
}
} /*
[BASIC, RINSE]
The basic wash
Rinsing
[BASIC, HOTWAX, RINSE, BLOWDRY]
The basic wash
Applying hot wax
Rinsing
Blowing dry
*/ 与使用匿名内部类相比较，定义常量相关方法的语法更高效、简洁。向EnumSet添加 enum实例的顺序并不重要，因为其输出的次序决定于enum实例定义时的次序。
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();
}
}
} /*
NUT: default behavior
BOLT: default behavior
WASHER: Overridden method
*/虽然enum有些限制，但是一般而言，还是可以将其看做是类。
使用enum的职责链
职责链设计模式，以多种不同的方式来解决一个问题，然后将它们链接在一起。当请求到来时，他遍历这个链，直到链中的某个解决方案能够处理该请求。
class Mail {
// The NO's lower the probability of random selection:
enum GeneralDelivery {
YES, NO1, NO2, NO3, NO4, NO5
}
enum Scannability {UNSCANNABLE, YES1, YES2, YES3, YES4}
enum Readability {ILLEGIBLE, YES1, YES2, YES3, YES4}
enum Address {INCORRECT, OK1, OK2, OK3, OK4, OK5, OK6}
enum ReturnAddress {MISSING, OK1, OK2, OK3, OK4, OK5}
GeneralDelivery generalDelivery;
Scannability scannability;
Readability readability;
Address address;
ReturnAddress returnAddress;
static long counter = 0;
long id = counter++;
public String toString() {
return "Mail " + id;
}
public String details() {
return toString() +
", General Delivery: " + generalDelivery +
", Address Scanability: " + scannability +
", Address Readability: " + readability +
", Address Address: " + address +
", Return address: " + returnAddress;
}
// Generate test Mail:
public static Mail randomMail() {
Mail m = new Mail();
m.generalDelivery = Enums.random(GeneralDelivery.class);
m.scannability = Enums.random(Scannability.class);
m.readability = Enums.random(Readability.class);
m.address = Enums.random(Address.class);
m.returnAddress = Enums.random(ReturnAddress.class);
return m;
}
public static Iterable generator(final int count) {
return new Iterable() {
int n = count;
public Iterator iterator() {
return new Iterator() {
public boolean hasNext() {
return n-- > 0;
}
public Mail next() {
return randomMail();
}
public void remove() { // Not implemented
throw new UnsupportedOperationException();
}
};
}
};
}
}
public class PostOffice {
enum MailHandler {
GENERAL_DELIVERY {
boolean handle(Mail m) {
switch (m.generalDelivery) {
case YES:
print("Using general delivery for " + m);
return true;
default:
return false;
}
}
},
MACHINE_SCAN {
boolean handle(Mail m) {
switch (m.scannability) {
case UNSCANNABLE:
return false;
default:
switch (m.address) {
case INCORRECT:
return false;
default:
print("Delivering " + m + " automatically");
return true;
}
}
}
},
VISUAL_INSPECTION {
boolean handle(Mail m) {
switch (m.readability) {
case ILLEGIBLE:
return false;
default:
switch (m.address) {
case INCORRECT:
return false;
default:
print("Delivering " + m + " normally");
return true;
}
}
}
},
RETURN_TO_SENDER {
boolean handle(Mail m) {
switch (m.returnAddress) {
case MISSING:
return false;
default:
print("Returning " + m + " to sender");
return true;
}
}
};
abstract boolean handle(Mail m);
}
static void handle(Mail m) {
for (MailHandler handler : MailHandler.values()) {
if (handler.handle(m)) {
return;
}
}
print(m + " is a dead letter");
}
public static void main(String[] args) {
for (Mail mail : Mail.generator(10)) {
print(mail.details());
handle(mail);
print("*****");
}
}
} /*
Mail 0, General Delivery: NO2, Address Scanability: UNSCANNABLE, Address Readability: YES3, Address Address: OK1, Return address: OK1
Delivering Mail 0 normally
*****
Mail 1, General Delivery: NO5, Address Scanability: YES3, Address Readability: ILLEGIBLE, Address Address: OK5, Return address: OK1
Delivering Mail 1 automatically
*****
Mail 2, General Delivery: YES, Address Scanability: YES3, Address Readability: YES1, Address Address: OK1, Return address: OK5
Using general delivery for Mail 2
*****
Mail 3, General Delivery: NO4, Address Scanability: YES3, Address Readability: YES1, Address Address: INCORRECT, Return address: OK4
Returning Mail 3 to sender
*****
Mail 4, General Delivery: NO4, Address Scanability: UNSCANNABLE, Address Readability: YES1, Address Address: INCORRECT, Return address: OK2
Returning Mail 4 to sender
*****
Mail 5, General Delivery: NO3, Address Scanability: YES1, Address Readability: ILLEGIBLE, Address Address: OK4, Return address: OK2
Delivering Mail 5 automatically
*****
Mail 6, General Delivery: YES, Address Scanability: YES4, Address Readability: ILLEGIBLE, Address Address: OK4, Return address: OK4
Using general delivery for Mail 6
*****
Mail 7, General Delivery: YES, Address Scanability: YES3, Address Readability: YES4, Address Address: OK2, Return address: MISSING
Using general delivery for Mail 7
*****
Mail 8, General Delivery: NO3, Address Scanability: YES1, Address Readability: YES3, Address Address: INCORRECT, Return address: MISSING
Mail 8 is a dead letter
*****
Mail 9, General Delivery: NO1, Address Scanability: UNSCANNABLE, Address Readability: YES2, Address Address: OK1, Return address: OK4
Delivering Mail 9 normally
*****
*/ 职责链有enum MailHandler实现，而enum实现的次序决定了各个解决策略在应用时的次序。对每一封邮件，都要按此顺序仓尝试每个解决策略，直到其中一个能够成功处理该邮件，如果所有策略都失败了，那么该邮件将被判定为一封死信。
使用enum的状态机
枚举类型非常适合用来创建状态机。一个状态机可以具有有限个特定的状态，它通常根据输入，从一个状态转移到下一个状态，不过也有可能存在瞬时状态，而一旦任务执行结束，状态机就会立刻离开瞬时状态。
每个状态都具有某些可接受的输入，不同的输入会使状态机从当前状态转移到不同的新的状态。由于enum对其实例有严格限制，非常适合用来表现不同的状态和输入。一般而言，每个状态都具有一些相关的输出。
自动售货机：
public enum Input {
NICKEL(5), DIME(10), QUARTER(25), DOLLAR(100),
TOOTHPASTE(200), CHIPS(75), SODA(100), SOAP(50),
ABORT_TRANSACTION {
public int amount() { // Disallow
throw new RuntimeException("ABORT.amount()");
}
},
STOP { // This must be the last instance.
public int amount() { // Disallow
throw new RuntimeException("SHUT_DOWN.amount()");
}
};
int value; // In cents
Input(int value) {
this.value = value;
}
Input() {
}
int amount() {
return value;
}
static Random rand = new Random(47);
public static Input randomSelection() {
// Don't include STOP:
return values()[rand.nextInt(values().length - 1)];
}
}两个特殊Input实例调用amount()方法并不合适，会有异常抛出（在接口内定义一个方法，然后在调用该方法的某个实现时就会抛出异常）。这似乎有点奇怪，但由于enum的限制，不得不采用这种方式。
VendingMachine对输入的第一反应是将其归类为Category enum的某一个enum实例，这可以通过swithc实现：
//{Args: VendingMachineInput.txt}
QUARTER; QUARTER; QUARTER; CHIPS;
DOLLAR; DOLLAR; TOOTHPASTE;
QUARTER; DIME; ABORT_TRANSACTION;
QUARTER; DIME; SODA;
QUARTER; DIME; NICKEL; SODA;
ABORT_TRANSACTION;
STOP;
enum Category {
MONEY(NICKEL, DIME, QUARTER, DOLLAR),
ITEM_SELECTION(TOOTHPASTE, CHIPS, SODA, SOAP),
QUIT_TRANSACTION(ABORT_TRANSACTION),
SHUT_DOWN(STOP);
private Input[] values;
Category(Input... types) {
values = types;
}
private static EnumMap categories =
new EnumMap(Input.class);
static {
for (Category c : Category.class.getEnumConstants()) {
for (Input type : c.values) {
categories.put(type, c);
}
}
}
public static Category categorize(Input input) {
return categories.get(input);
}
}
public class VendingMachine {
private static State state = State.RESTING;
private static int amount = 0;
private static Input selection = null;
enum StateDuration {TRANSIENT} // Tagging enum
enum State {
RESTING {
void next(Input input) {
switch (Category.categorize(input)) {
case MONEY:
amount += input.amount();
state = ADDING_MONEY;
break;
case SHUT_DOWN:
state = TERMINAL;
default:
}
}
},
ADDING_MONEY {
void next(Input input) {
switch (Category.categorize(input)) {
case MONEY:
amount += input.amount();
break;
case ITEM_SELECTION:
selection = input;
if (amount < selection.amount())
print("Insufficient money for " + selection);
else state = DISPENSING;
break;
case QUIT_TRANSACTION:
state = GIVING_CHANGE;
break;
case SHUT_DOWN:
state = TERMINAL;
default:
}
}
},
DISPENSING(StateDuration.TRANSIENT) {
void next() {
print("here is your " + selection);
amount -= selection.amount();
state = GIVING_CHANGE;
}
},
GIVING_CHANGE(StateDuration.TRANSIENT) {
void next() {
if (amount > 0) {
print("Your change: " + amount);
amount = 0;
}
state = RESTING;
}
},
TERMINAL {
void output() {
print("Halted");
}
};
private boolean isTransient = false;
State() {
}
State(StateDuration trans) {
isTransient = true;
}
void next(Input input) {
throw new RuntimeException("Only call " +
"next(Input input) for non-transient states");
}
void next() {
throw new RuntimeException("Only call next() for " +
"StateDuration.TRANSIENT states");
}
void output() {
print(amount);
}
}
static void run(Generator gen) {
while (state != State.TERMINAL) {
state.next(gen.next());
while (state.isTransient)
state.next();
state.output();
}
}
public static void main(String[] args) {
Generator gen = new RandomInputGenerator();
if (args.length == 1)
gen = new FileInputGenerator(args[0]);
run(gen);
}
}
// For a basic sanity check:
class RandomInputGenerator implements Generator<Input> {
public Input next() {
return Input.randomSelection();
}
}
// Create Inputs from a file of ';'-separated strings:
class FileInputGenerator implements Generator<Input> {
private Iterator input;
public FileInputGenerator(String fileName) {
input = new TextFile(fileName, ";").iterator();
}
public Input next() {
if (!input.hasNext())
return null;
return Enum.valueOf(Input.class, input.next().trim());
}
} /*
25
50
75
here is your CHIPS
0
100
200
here is your TOOTHPASTE
0
25
35
Your change: 35
0
25
35
Insufficient money for SODA
35
60
70
75
Insufficient money for SODA
75
Your change: 75
0
Halted
*/ 对于每一个State，都需要在输入动作的基本分类中进行查找：用户塞入钞票、选择货物、操作被取消以及机器停止。然而在这些基本分类之下，有可能塞入不同钞票，选择不同货物。Categoryenum将不同类型的Input进行分组，因而，可以使用categorize()方法为switch语句生成恰当的Cateroy实例。并且，该方法使用的EnumMap确保了在其中进行查询时的效率与安全。
每种状态的不同之处，以及对于输入的不同响应，其中还有两个瞬时状态，在run方法中，状态机等待着下一个Input，并一直在各个状态中移动，直到它不在处于瞬时状态。
多路分发
Java只支持单路分发，也就是说，如果要执行的操作包含了不止一个类型未知的对象时，那么Java的动态绑定机制只能处理其中一个类型。所以必须自己来判定其他的类型，从而实现自己的动态绑定行为。
为了达到效果，需要与多个方法一同工作：因为每个分发都需要一个方法调用。在下面实现了石头剪刀布对应的方法是compete和eval，二者都是同一个类型的成员，可以产生三种Outcome实例中的一个作为结果：
public enum Outcome { WIN, LOSE, DRAW }
interface Item {
Outcome compete(Item it);
Outcome eval(Paper p);
Outcome eval(Scissors s);
Outcome eval(Rock r);
}
class Paper implements Item {
public Outcome compete(Item it) { return it.eval(this); }
public Outcome eval(Paper p) { return DRAW; }
public Outcome eval(Scissors s) { return WIN; }
public Outcome eval(Rock r) { return LOSE; }
public String toString() { return "Paper"; }
}
class Scissors implements Item {
public Outcome compete(Item it) { return it.eval(this); }
public Outcome eval(Paper p) { return LOSE; }
public Outcome eval(Scissors s) { return DRAW; }
public Outcome eval(Rock r) { return WIN; }
public String toString() { return "Scissors"; }
}
class Rock implements Item {
public Outcome compete(Item it) { return it.eval(this); }
public Outcome eval(Paper p) { return WIN; }
public Outcome eval(Scissors s) { return LOSE; }
public Outcome eval(Rock r) { return DRAW; }
public String toString() { return "Rock"; }
}
public class RoShamBo1 {
static final int SIZE = 20;
private static Random rand = new Random(47);
public static Item newItem() {
switch(rand.nextInt(3)) {
default:
case 0: return new Scissors();
case 1: return new Paper();
case 2: return new Rock();
}
}
public static void match(Item a, Item b) {
System.out.println(
a + " vs. " + b + ": " + a.compete(b));
}
public static void main(String[] args) {
for(int i = 0; i < SIZE; i++)
match(newItem(), newItem());
}
} /*
Rock vs. Rock: DRAW
Paper vs. Rock: WIN
Paper vs. Rock: WIN
Paper vs. Rock: WIN
Scissors vs. Paper: WIN
Scissors vs. Scissors: DRAW
Scissors vs. Paper: WIN
Rock vs. Paper: LOSE
Paper vs. Paper: DRAW
Rock vs. Paper: LOSE
Paper vs. Scissors: LOSE
Paper vs. Scissors: LOSE
Rock vs. Scissors: WIN
Rock vs. Paper: LOSE
Paper vs. Rock: WIN
Scissors vs. Paper: WIN
Paper vs. Scissors: LOSE
Paper vs. Scissors: LOSE
Paper vs. Scissors: LOSE
Paper vs. Scissors: LOSE
*/通过调用Item.compete方法开始两路分发。要判定a的类型，分发机制会在a的实际类型的compete内部起到分发作用。compete方法通过调用eval来为另一个类型实现第二次分发。将自身做为参数调用eval，能够调用重载过的eval方法。
使用enum分发
使用构造器初始化每个enum实例，并以一组结果作为参数，这二者放在一快，形成了类似查询表的结构：
public enum RoShamBo2 implements Competitor {
PAPER(DRAW, LOSE, WIN),
SCISSORS(WIN, DRAW, LOSE),
ROCK(LOSE, WIN, DRAW);
private Outcome vPAPER, vSCISSORS, vROCK;
RoShamBo2(Outcome paper,Outcome scissors,Outcome rock) {
this.vPAPER = paper;
this.vSCISSORS = scissors;
this.vROCK = rock;
}
@Override
public Outcome compete(RoShamBo2 it) {
switch(it) {
default:
case PAPER: return vPAPER;
case SCISSORS: return vSCISSORS;
case ROCK: return vROCK;
}
}
public static void main(String[] args) {
RoShamBo.play(RoShamBo2.class, 20);
}
} /*
ROCK vs. ROCK: DRAW
SCISSORS vs. ROCK: LOSE
SCISSORS vs. ROCK: LOSE
SCISSORS vs. ROCK: LOSE
PAPER vs. SCISSORS: LOSE
PAPER vs. PAPER: DRAW
PAPER vs. SCISSORS: LOSE
ROCK vs. SCISSORS: WIN
SCISSORS vs. SCISSORS: DRAW
ROCK vs. SCISSORS: WIN
SCISSORS vs. PAPER: WIN
SCISSORS vs. PAPER: WIN
ROCK vs. PAPER: LOSE
ROCK vs. SCISSORS: WIN
SCISSORS vs. ROCK: LOSE
PAPER vs. SCISSORS: LOSE
SCISSORS vs. PAPER: WIN
SCISSORS vs. PAPER: WIN
SCISSORS vs. PAPER: WIN
SCISSORS vs. PAPER: WIN
*/ 在compete方法中，一旦两种类型都被确定了，那么唯一的操作就是返回结果Outcome。只有第一次分发是实际方法调用，第二个分发使用的是switch，不过这样做是安全的，因为enum限制了swithc语句的选择分支。
public interface Competitor<T extends Competitor<T>> {
Outcome compete(T competitor);
}
public class RoShamBo {
public static >
void match(T a, T b) {
System.out.println(
a + " vs. " + b + ": " + a.compete(b));
}
public static & Competitor>
void play(Class rsbClass, int size) {
for (int i = 0; i < size; i++) {
match(Enums.random(rsbClass), Enums.random(rsbClass));
}
}
} play()方法没有将类型参数T做为返回值类型，因此，似乎应该在Class中使用通配符来代替上面的参数声明。然而，通配符不能扩展多个基类，所以必须采用以上的表达式。
使用常量相关的方法
常量相关的方法允许为每个enum实例提供方法的不同实现：
public enum RoShamBo3 implements Competitor {
PAPER {
public Outcome compete(RoShamBo3 it) {
switch(it) {
default: // To placate the compiler
case PAPER: return DRAW;
case SCISSORS: return LOSE;
case ROCK: return WIN;
}
}
},
SCISSORS {
public Outcome compete(RoShamBo3 it) {
switch(it) {
default:
case PAPER: return WIN;
case SCISSORS: return DRAW;
case ROCK: return LOSE;
}
}
},
ROCK {
public Outcome compete(RoShamBo3 it) {
switch(it) {
default:
case PAPER: return LOSE;
case SCISSORS: return WIN;
case ROCK: return DRAW;
}
}
};
public abstract Outcome compete(RoShamBo3 it);
public static void main(String[] args) {
RoShamBo.play(RoShamBo3.class, 20);
}
} 如果采用RoShamBo2的解决方案，那么在添加一个新的类型时，只需更小的代码：
public enum RoShamBo4 implements Competitor {
ROCK {
public Outcome compete(RoShamBo4 opponent) {
return compete(SCISSORS, opponent);
}
},
SCISSORS {
public Outcome compete(RoShamBo4 opponent) {
return compete(PAPER, opponent);
}
},
PAPER {
public Outcome compete(RoShamBo4 opponent) {
return compete(ROCK, opponent);
}
};
Outcome compete(RoShamBo4 loser, RoShamBo4 opponent) {
return ((opponent == this) ? Outcome.DRAW
: ((opponent == loser) ? Outcome.WIN
: Outcome.LOSE));
}
public static void main(String[] args) {
RoShamBo.play(RoShamBo4.class, 20);
}
} 其中，具有两个参数的compete()方法执行第二个分发，该方法执行一系列的比较，其行为类似swithc语句。这个版本程序简短，但难以理解。对于一个大型系统而言，难以理解的代码将导致整个系统不够健壮。
使用EnumMap分发
使用EnumMap能够实现真正的两路分发。
enum RoShamBo5 implements Competitor {
PAPER, SCISSORS, ROCK;
static EnumMap>
table = new EnumMap>(RoShamBo5.class);
static {
for (RoShamBo5 it : RoShamBo5.values()) {
table.put(it,
new EnumMap(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 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);
}
} 该程序在一个static子句中初始化EnumMap对象，具体见表格似的initRow()方法调用。
使用二维数组
enum RoShamBo6 implements Competitor {
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);
}
} 每个enum实例都有一个固定值（基于声明次序），并且可以通过ordinal()方法取到该值。不过由于使用的数组，不太安全。如果使用大型数组，可能会不小心使用错误的尺寸，而且，如果测试不能覆盖所有的可能性，有些错误就会发生。