我想采取一个现有的enum,并添加更多的元素,如下所示:
enum A {a,b,c}
enum B extends A {d}
/*B is {a,b,c,d}*/
这在Java中可行吗?
我想采取一个现有的enum,并添加更多的元素,如下所示:
enum A {a,b,c}
enum B extends A {d}
/*B is {a,b,c,d}*/
这在Java中可行吗?
当前回答
enum A {a,b,c}
enum B extends A {d}
/*B is {a,b,c,d}*/
可以写成:
public enum All {
a (ClassGroup.A,ClassGroup.B),
b (ClassGroup.A,ClassGroup.B),
c (ClassGroup.A,ClassGroup.B),
d (ClassGroup.B)
...
ClassGroup.B.getMembers()包含{a,b,c,d}
如何使用:假设我们想要这样的东西: 我们有事件,我们使用枚举。可以通过类似的处理对这些枚举进行分组。如果我们有多个元素的操作,那么有些事件开始操作,有些只是步骤,有些则结束操作。为了收集这样的操作和避免长开关情况,我们可以将它们分组,如示例所示,并使用:
if(myEvent.is(State_StatusGroup.START)) makeNewOperationObject()..
if(myEnum.is(State_StatusGroup.STEP)) makeSomeSeriousChanges()..
if(myEnum.is(State_StatusGroup.FINISH)) closeTransactionOrSomething()..
例子:
public enum AtmOperationStatus {
STARTED_BY_SERVER (State_StatusGroup.START),
SUCCESS (State_StatusGroup.FINISH),
FAIL_TOKEN_TIMEOUT (State_StatusGroup.FAIL,
State_StatusGroup.FINISH),
FAIL_NOT_COMPLETE (State_StatusGroup.FAIL,
State_StatusGroup.STEP),
FAIL_UNKNOWN (State_StatusGroup.FAIL,
State_StatusGroup.FINISH),
(...)
private AtmOperationStatus(StatusGroupInterface ... pList){
for (StatusGroupInterface group : pList){
group.addMember(this);
}
}
public boolean is(StatusGroupInterface with){
for (AtmOperationStatus eT : with.getMembers()){
if( eT .equals(this)) return true;
}
return false;
}
// Each group must implement this interface
private interface StatusGroupInterface{
EnumSet<AtmOperationStatus> getMembers();
void addMember(AtmOperationStatus pE);
}
// DEFINING GROUPS
public enum State_StatusGroup implements StatusGroupInterface{
START, STEP, FAIL, FINISH;
private List<AtmOperationStatus> members = new LinkedList<AtmOperationStatus>();
@Override
public EnumSet<AtmOperationStatus> getMembers() {
return EnumSet.copyOf(members);
}
@Override
public void addMember(AtmOperationStatus pE) {
members.add(pE);
}
static { // forcing initiation of dependent enum
try {
Class.forName(AtmOperationStatus.class.getName());
} catch (ClassNotFoundException ex) {
throw new RuntimeException("Class AtmEventType not found", ex);
}
}
}
}
//Some use of upper code:
if (p.getStatus().is(AtmOperationStatus.State_StatusGroup.FINISH)) {
//do something
}else if (p.getStatus().is(AtmOperationStatus.State_StatusGroup.START)) {
//do something
}
添加一些更高级的内容:
public enum AtmEventType {
USER_DEPOSIT (Status_EventsGroup.WITH_STATUS,
Authorization_EventsGroup.USER_AUTHORIZED,
ChangedMoneyAccountState_EventsGroup.CHANGED,
OperationType_EventsGroup.DEPOSIT,
ApplyTo_EventsGroup.CHANNEL),
SERVICE_DEPOSIT (Status_EventsGroup.WITH_STATUS,
Authorization_EventsGroup.TERMINAL_AUTHORIZATION,
ChangedMoneyAccountState_EventsGroup.CHANGED,
OperationType_EventsGroup.DEPOSIT,
ApplyTo_EventsGroup.CHANNEL),
DEVICE_MALFUNCTION (Status_EventsGroup.WITHOUT_STATUS,
Authorization_EventsGroup.TERMINAL_AUTHORIZATION,
ChangedMoneyAccountState_EventsGroup.DID_NOT_CHANGED,
ApplyTo_EventsGroup.DEVICE),
CONFIGURATION_4_C_CHANGED(Status_EventsGroup.WITHOUT_STATUS,
ApplyTo_EventsGroup.TERMINAL,
ChangedMoneyAccountState_EventsGroup.DID_NOT_CHANGED),
(...)
在上面,如果我们有一些失败(myEvent.is(State_StatusGroup.FAIL)),然后通过之前的事件迭代,我们可以很容易地检查我们是否必须通过以下方法恢复资金转移:
if(myEvent2.is(ChangedMoneyAccountState_EventsGroup.CHANGED)) rollBack()..
它可以用于:
包括关于处理逻辑的显式元数据,不需要记住 实现一些多继承 我们不想使用类结构,例如发送简短的状态消息
其他回答
枚举表示可能值的完整枚举。所以(毫无帮助的)答案是否定的。
举一个实际问题的例子,工作日,周末,工会,一周的天数。我们可以在“天-周”中定义所有的天,但这样就不能表示“工作日”和“周末-天”的特殊属性。
我们可以做的是,有三种枚举类型,并在weekdays/weekend-days和days-of-week之间进行映射。
public enum Weekday {
MON, TUE, WED, THU, FRI;
public DayOfWeek toDayOfWeek() { ... }
}
public enum WeekendDay {
SAT, SUN;
public DayOfWeek toDayOfWeek() { ... }
}
public enum DayOfWeek {
MON, TUE, WED, THU, FRI, SAT, SUN;
}
或者,我们可以为day-of-week提供一个开放式接口:
interface Day {
...
}
public enum Weekday implements Day {
MON, TUE, WED, THU, FRI;
}
public enum WeekendDay implements Day {
SAT, SUN;
}
或者我们可以结合这两种方法:
interface Day {
...
}
public enum Weekday implements Day {
MON, TUE, WED, THU, FRI;
public DayOfWeek toDayOfWeek() { ... }
}
public enum WeekendDay implements Day {
SAT, SUN;
public DayOfWeek toDayOfWeek() { ... }
}
public enum DayOfWeek {
MON, TUE, WED, THU, FRI, SAT, SUN;
public Day toDay() { ... }
}
对此,推荐的解决方案是可扩展枚举模式。
这涉及到创建一个接口,并在当前使用枚举的位置使用该接口。然后使枚举实现接口。通过添加扩展接口的附加enum/类,可以添加更多常量。大致是这样的:
public interface TrafficLights {
public abstract String getColour();
}
public enum StandardTrafficLights implements TrafficLights {
RED, YELLOW, GREEN;
public String getColour() {
return name();
}
}
public enum WeirdTrafficLights implements TrafficLights {
DOUBLE_RED;
public String getColour() {
return name();
}
}
注意,如果你想要TrafficLights.valueof(String)这样的东西,你必须自己实现它。
这就是我如何增强枚举继承模式运行时检查在静态初始化器。 BaseKind的#checkEnumExtender检查“extends”enum是否以完全相同的方式声明了基enum的所有值,以便#name()和#ordinal()保持完全兼容。
声明值仍然涉及复制粘贴,但如果有人在基类中添加或修改值而没有更新扩展值,则程序很快就会失败。
不同枚举相互扩展的常见行为:
public interface Kind {
/**
* Let's say we want some additional member.
*/
String description() ;
/**
* Standard {@code Enum} method.
*/
String name() ;
/**
* Standard {@code Enum} method.
*/
int ordinal() ;
}
基准enum,带有验证方法:
public enum BaseKind implements Kind {
FIRST( "First" ),
SECOND( "Second" ),
;
private final String description ;
public String description() {
return description ;
}
private BaseKind( final String description ) {
this.description = description ;
}
public static void checkEnumExtender(
final Kind[] baseValues,
final Kind[] extendingValues
) {
if( extendingValues.length < baseValues.length ) {
throw new IncorrectExtensionError( "Only " + extendingValues.length + " values against "
+ baseValues.length + " base values" ) ;
}
for( int i = 0 ; i < baseValues.length ; i ++ ) {
final Kind baseValue = baseValues[ i ] ;
final Kind extendingValue = extendingValues[ i ] ;
if( baseValue.ordinal() != extendingValue.ordinal() ) {
throw new IncorrectExtensionError( "Base ordinal " + baseValue.ordinal()
+ " doesn't match with " + extendingValue.ordinal() ) ;
}
if( ! baseValue.name().equals( extendingValue.name() ) ) {
throw new IncorrectExtensionError( "Base name[ " + i + "] " + baseValue.name()
+ " doesn't match with " + extendingValue.name() ) ;
}
if( ! baseValue.description().equals( extendingValue.description() ) ) {
throw new IncorrectExtensionError( "Description[ " + i + "] " + baseValue.description()
+ " doesn't match with " + extendingValue.description() ) ;
}
}
}
public static class IncorrectExtensionError extends Error {
public IncorrectExtensionError( final String s ) {
super( s ) ;
}
}
}
扩展示例:
public enum ExtendingKind implements Kind {
FIRST( BaseKind.FIRST ),
SECOND( BaseKind.SECOND ),
THIRD( "Third" ),
;
private final String description ;
public String description() {
return description ;
}
ExtendingKind( final BaseKind baseKind ) {
this.description = baseKind.description() ;
}
ExtendingKind( final String description ) {
this.description = description ;
}
}
enum A {a,b,c}
enum B extends A {d}
/*B is {a,b,c,d}*/
可以写成:
public enum All {
a (ClassGroup.A,ClassGroup.B),
b (ClassGroup.A,ClassGroup.B),
c (ClassGroup.A,ClassGroup.B),
d (ClassGroup.B)
...
ClassGroup.B.getMembers()包含{a,b,c,d}
如何使用:假设我们想要这样的东西: 我们有事件,我们使用枚举。可以通过类似的处理对这些枚举进行分组。如果我们有多个元素的操作,那么有些事件开始操作,有些只是步骤,有些则结束操作。为了收集这样的操作和避免长开关情况,我们可以将它们分组,如示例所示,并使用:
if(myEvent.is(State_StatusGroup.START)) makeNewOperationObject()..
if(myEnum.is(State_StatusGroup.STEP)) makeSomeSeriousChanges()..
if(myEnum.is(State_StatusGroup.FINISH)) closeTransactionOrSomething()..
例子:
public enum AtmOperationStatus {
STARTED_BY_SERVER (State_StatusGroup.START),
SUCCESS (State_StatusGroup.FINISH),
FAIL_TOKEN_TIMEOUT (State_StatusGroup.FAIL,
State_StatusGroup.FINISH),
FAIL_NOT_COMPLETE (State_StatusGroup.FAIL,
State_StatusGroup.STEP),
FAIL_UNKNOWN (State_StatusGroup.FAIL,
State_StatusGroup.FINISH),
(...)
private AtmOperationStatus(StatusGroupInterface ... pList){
for (StatusGroupInterface group : pList){
group.addMember(this);
}
}
public boolean is(StatusGroupInterface with){
for (AtmOperationStatus eT : with.getMembers()){
if( eT .equals(this)) return true;
}
return false;
}
// Each group must implement this interface
private interface StatusGroupInterface{
EnumSet<AtmOperationStatus> getMembers();
void addMember(AtmOperationStatus pE);
}
// DEFINING GROUPS
public enum State_StatusGroup implements StatusGroupInterface{
START, STEP, FAIL, FINISH;
private List<AtmOperationStatus> members = new LinkedList<AtmOperationStatus>();
@Override
public EnumSet<AtmOperationStatus> getMembers() {
return EnumSet.copyOf(members);
}
@Override
public void addMember(AtmOperationStatus pE) {
members.add(pE);
}
static { // forcing initiation of dependent enum
try {
Class.forName(AtmOperationStatus.class.getName());
} catch (ClassNotFoundException ex) {
throw new RuntimeException("Class AtmEventType not found", ex);
}
}
}
}
//Some use of upper code:
if (p.getStatus().is(AtmOperationStatus.State_StatusGroup.FINISH)) {
//do something
}else if (p.getStatus().is(AtmOperationStatus.State_StatusGroup.START)) {
//do something
}
添加一些更高级的内容:
public enum AtmEventType {
USER_DEPOSIT (Status_EventsGroup.WITH_STATUS,
Authorization_EventsGroup.USER_AUTHORIZED,
ChangedMoneyAccountState_EventsGroup.CHANGED,
OperationType_EventsGroup.DEPOSIT,
ApplyTo_EventsGroup.CHANNEL),
SERVICE_DEPOSIT (Status_EventsGroup.WITH_STATUS,
Authorization_EventsGroup.TERMINAL_AUTHORIZATION,
ChangedMoneyAccountState_EventsGroup.CHANGED,
OperationType_EventsGroup.DEPOSIT,
ApplyTo_EventsGroup.CHANNEL),
DEVICE_MALFUNCTION (Status_EventsGroup.WITHOUT_STATUS,
Authorization_EventsGroup.TERMINAL_AUTHORIZATION,
ChangedMoneyAccountState_EventsGroup.DID_NOT_CHANGED,
ApplyTo_EventsGroup.DEVICE),
CONFIGURATION_4_C_CHANGED(Status_EventsGroup.WITHOUT_STATUS,
ApplyTo_EventsGroup.TERMINAL,
ChangedMoneyAccountState_EventsGroup.DID_NOT_CHANGED),
(...)
在上面,如果我们有一些失败(myEvent.is(State_StatusGroup.FAIL)),然后通过之前的事件迭代,我们可以很容易地检查我们是否必须通过以下方法恢复资金转移:
if(myEvent2.is(ChangedMoneyAccountState_EventsGroup.CHANGED)) rollBack()..
它可以用于:
包括关于处理逻辑的显式元数据,不需要记住 实现一些多继承 我们不想使用类结构,例如发送简短的状态消息
为了帮助理解为什么在语言实现级别上扩展Enum是不合理的,请考虑如果将扩展Enum的实例传递给只理解基本Enum的例程会发生什么。编译器承诺覆盖所有情况的开关实际上并不覆盖那些扩展的Enum值。
这进一步强调了Java Enum的值不是像C那样的整数,例如:要使用Java Enum作为数组索引,你必须显式地要求它的ordinal()成员,要给Java Enum一个任意的整数值,你必须为它添加一个显式的字段并引用命名的成员。
这不是对OP的愿望的评论,只是关于为什么Java永远不会做到这一点。