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
基于@Tom Hawtin - tackline的回答，我们增加了开关支持，
interface Day<T> {
...
T valueOf();
}
public enum Weekday implements Day<Weekday> {
MON, TUE, WED, THU, FRI;
Weekday valueOf(){
return valueOf(name());
}
}
public enum WeekendDay implements Day<WeekendDay> {
SAT, SUN;
WeekendDay valueOf(){
return valueOf(name());
}
}
Day<Weekday> wds = Weekday.MON;
Day<WeekendDay> wends = WeekendDay.SUN;
switch(wds.valueOf()){
case MON:
case TUE:
case WED:
case THU:
case FRI:
}
switch(wends.valueOf()){
case SAT:
case SUN:
}
其他回答
如果你错过了，在Joshua Bloch的书“Effective Java，第二版”中有一章。
第6章-枚举和注释 项目34:用接口模拟可扩展枚举
结论是:
A minor disadvantage of the use of interfaces to emulate extensible enums is those implementations cannot be inherited from one enum type to another. In the case of our Operation example, the logic to store and retrieve the symbol associated with an operation is duplicated in BasicOperation and ExtendedOperation. In this case, it doesn’t matter because very little code is duplicated. If there were a a larger amount of shared functionality, you could encapsulate it in a helper class or a static helper method to eliminate the code duplication.
总之，虽然不能编写可扩展枚举类型，但可以编写 通过编写接口来模拟实现的基本枚举类型 接口。这允许客户端编写自己实现的枚举 接口。这些枚举可以用于基本枚举类型所在的任何地方 假设api是根据接口编写的。
我的编码方式如下:
// enum A { a, b, c }
static final Set<Short> enumA = new LinkedHashSet<>(Arrays.asList(new Short[]{'a','b','c'}));
// enum B extends A { d }
static final Set<Short> enumB = new LinkedHashSet<>(enumA);
static {
enumB.add((short) 'd');
// If you have to add more elements:
// enumB.addAll(Arrays.asList(new Short[]{ 'e', 'f', 'g', '♯', '♭' }));
}
LinkedHashSet既提供了每个条目只存在一次，又提供了它们的顺序保留。如果顺序无关紧要，则可以使用HashSet。以下代码在Java中是不可行的:
for (A a : B.values()) { // enum B extends A { d }
switch (a) {
case a:
case b:
case c:
System.out.println("Value is: " + a.toString());
break;
default:
throw new IllegalStateException("This should never happen.");
}
}
代码可以这样写:
for (Short a : enumB) {
switch (a) {
case 'a':
case 'b':
case 'c':
System.out.println("Value is: " + new String(Character.toChars(a)));
break;
default:
throw new IllegalStateException("This should never happen.");
}
}
从Java 7开始，你甚至可以用String做同样的事情:
// enum A { BACKWARDS, FOREWARDS, STANDING }
static final Set<String> enumA = new LinkedHashSet<>(Arrays.asList(new String[] {
"BACKWARDS", "FOREWARDS", "STANDING" }));
// enum B extends A { JUMP }
static final Set<String> enumB = new LinkedHashSet<>(enumA);
static {
enumB.add("JUMP");
}
使用enum替换:
for (String a : enumB) {
switch (a) {
case "BACKWARDS":
case "FOREWARDS":
case "STANDING":
System.out.println("Value is: " + a);
break;
default:
throw new IllegalStateException("This should never happen.");
}
}
我希望我的一个同事的这个优雅的解决方案能在这篇长文章中看到，我想分享这种继承接口方法的方法。
请注意，我们在这里使用自定义异常，除非您将其替换为您的异常，否则此代码将无法编译。
文档内容很广泛，我希望大多数人都能理解。
每个子类枚举都需要实现的接口。
public interface Parameter {
/**
* Retrieve the parameters name.
*
* @return the name of the parameter
*/
String getName();
/**
* Retrieve the parameters type.
*
* @return the {@link Class} according to the type of the parameter
*/
Class<?> getType();
/**
* Matches the given string with this parameters value pattern (if applicable). This helps to find
* out if the given string is a syntactically valid candidate for this parameters value.
*
* @param valueStr <i>optional</i> - the string to check for
* @return <code>true</code> in case this parameter has no pattern defined or the given string
* matches the defined one, <code>false</code> in case <code>valueStr</code> is
* <code>null</code> or an existing pattern is not matched
*/
boolean match(final String valueStr);
/**
* This method works as {@link #match(String)} but throws an exception if not matched.
*
* @param valueStr <i>optional</i> - the string to check for
* @throws ArgumentException with code
* <dl>
* <dt>PARAM_MISSED</dt>
* <dd>if <code>valueStr</code> is <code>null</code></dd>
* <dt>PARAM_BAD</dt>
* <dd>if pattern is not matched</dd>
* </dl>
*/
void matchEx(final String valueStr) throws ArgumentException;
/**
* Parses a value for this parameter from the given string. This method honors the parameters data
* type and potentially other criteria defining a valid value (e.g. a pattern).
*
* @param valueStr <i>optional</i> - the string to parse the parameter value from
* @return the parameter value according to the parameters type (see {@link #getType()}) or
* <code>null</code> in case <code>valueStr</code> was <code>null</code>.
* @throws ArgumentException in case <code>valueStr</code> is not parsable as a value for this
* parameter.
*/
Object parse(final String valueStr) throws ArgumentException;
/**
* Converts the given value to its external form as it is accepted by {@link #parse(String)}. For
* most (ordinary) parameters this is simply a call to {@link String#valueOf(Object)}. In case the
* parameter types {@link Object#toString()} method does not return the external form (e.g. for
* enumerations), this method has to be implemented accordingly.
*
* @param value <i>mandatory</i> - the parameters value
* @return the external form of the parameters value, never <code>null</code>
* @throws InternalServiceException in case the given <code>value</code> does not match
* {@link #getType()}
*/
String toString(final Object value) throws InternalServiceException;
}
实现ENUM基类。
public enum Parameters implements Parameter {
/**
* ANY ENUM VALUE
*/
VALUE(new ParameterImpl<String>("VALUE", String.class, "[A-Za-z]{3,10}"));
/**
* The parameter wrapped by this enum constant.
*/
private Parameter param;
/**
* Constructor.
*
* @param param <i>mandatory</i> - the value for {@link #param}
*/
private Parameters(final Parameter param) {
this.param = param;
}
/**
* {@inheritDoc}
*/
@Override
public String getName() {
return this.param.getName();
}
/**
* {@inheritDoc}
*/
@Override
public Class<?> getType() {
return this.param.getType();
}
/**
* {@inheritDoc}
*/
@Override
public boolean match(final String valueStr) {
return this.param.match(valueStr);
}
/**
* {@inheritDoc}
*/
@Override
public void matchEx(final String valueStr) {
this.param.matchEx(valueStr);
}
/**
* {@inheritDoc}
*/
@Override
public Object parse(final String valueStr) throws ArgumentException {
return this.param.parse(valueStr);
}
/**
* {@inheritDoc}
*/
@Override
public String toString(final Object value) throws InternalServiceException {
return this.param.toString(value);
}
}
继承自基类的子类ENUM。
public enum ExtendedParameters implements Parameter {
/**
* ANY ENUM VALUE
*/
VALUE(my.package.name.VALUE);
/**
* EXTENDED ENUM VALUE
*/
EXTENDED_VALUE(new ParameterImpl<String>("EXTENDED_VALUE", String.class, "[0-9A-Za-z_.-]{1,20}"));
/**
* The parameter wrapped by this enum constant.
*/
private Parameter param;
/**
* Constructor.
*
* @param param <i>mandatory</i> - the value for {@link #param}
*/
private Parameters(final Parameter param) {
this.param = param;
}
/**
* {@inheritDoc}
*/
@Override
public String getName() {
return this.param.getName();
}
/**
* {@inheritDoc}
*/
@Override
public Class<?> getType() {
return this.param.getType();
}
/**
* {@inheritDoc}
*/
@Override
public boolean match(final String valueStr) {
return this.param.match(valueStr);
}
/**
* {@inheritDoc}
*/
@Override
public void matchEx(final String valueStr) {
this.param.matchEx(valueStr);
}
/**
* {@inheritDoc}
*/
@Override
public Object parse(final String valueStr) throws ArgumentException {
return this.param.parse(valueStr);
}
/**
* {@inheritDoc}
*/
@Override
public String toString(final Object value) throws InternalServiceException {
return this.param.toString(value);
}
}
最后使用泛型ParameterImpl添加一些实用程序。
public class ParameterImpl<T> implements Parameter {
/**
* The default pattern for numeric (integer, long) parameters.
*/
private static final Pattern NUMBER_PATTERN = Pattern.compile("[0-9]+");
/**
* The default pattern for parameters of type boolean.
*/
private static final Pattern BOOLEAN_PATTERN = Pattern.compile("0|1|true|false");
/**
* The name of the parameter, never <code>null</code>.
*/
private final String name;
/**
* The data type of the parameter.
*/
private final Class<T> type;
/**
* The validation pattern for the parameters values. This may be <code>null</code>.
*/
private final Pattern validator;
/**
* Shortcut constructor without <code>validatorPattern</code>.
*
* @param name <i>mandatory</i> - the value for {@link #name}
* @param type <i>mandatory</i> - the value for {@link #type}
*/
public ParameterImpl(final String name, final Class<T> type) {
this(name, type, null);
}
/**
* Constructor.
*
* @param name <i>mandatory</i> - the value for {@link #name}
* @param type <i>mandatory</i> - the value for {@link #type}
* @param validatorPattern - <i>optional</i> - the pattern for {@link #validator}
* <dl>
* <dt style="margin-top:0.25cm;"><i>Note:</i>
* <dd>The default validation patterns {@link #NUMBER_PATTERN} or
* {@link #BOOLEAN_PATTERN} are applied accordingly.
* </dl>
*/
public ParameterImpl(final String name, final Class<T> type, final String validatorPattern) {
this.name = name;
this.type = type;
if (null != validatorPattern) {
this.validator = Pattern.compile(validatorPattern);
} else if (Integer.class == this.type || Long.class == this.type) {
this.validator = NUMBER_PATTERN;
} else if (Boolean.class == this.type) {
this.validator = BOOLEAN_PATTERN;
} else {
this.validator = null;
}
}
/**
* {@inheritDoc}
*/
@Override
public boolean match(final String valueStr) {
if (null == valueStr) {
return false;
}
if (null != this.validator) {
final Matcher matcher = this.validator.matcher(valueStr);
return matcher.matches();
}
return true;
}
/**
* {@inheritDoc}
*/
@Override
public void matchEx(final String valueStr) throws ArgumentException {
if (false == this.match(valueStr)) {
if (null == valueStr) {
throw ArgumentException.createEx(ErrorCode.PARAM_MISSED, "The value must not be null",
this.name);
}
throw ArgumentException.createEx(ErrorCode.PARAM_BAD, "The value must match the pattern: "
+ this.validator.pattern(), this.name);
}
}
/**
* Parse the parameters value from the given string value according to {@link #type}. Additional
* the value is checked by {@link #matchEx(String)}.
*
* @param valueStr <i>optional</i> - the string value to parse the value from
* @return the parsed value, may be <code>null</code>
* @throws ArgumentException in case the parameter:
* <ul>
* <li>does not {@link #matchEx(String)} the {@link #validator}</li>
* <li>cannot be parsed according to {@link #type}</li>
* </ul>
* @throws InternalServiceException in case the type {@link #type} cannot be handled. This is a
* programming error.
*/
@Override
public T parse(final String valueStr) throws ArgumentException, InternalServiceException {
if (null == valueStr) {
return null;
}
this.matchEx(valueStr);
if (String.class == this.type) {
return this.type.cast(valueStr);
}
if (Boolean.class == this.type) {
return this.type.cast(Boolean.valueOf(("1".equals(valueStr)) || Boolean.valueOf(valueStr)));
}
try {
if (Integer.class == this.type) {
return this.type.cast(Integer.valueOf(valueStr));
}
if (Long.class == this.type) {
return this.type.cast(Long.valueOf(valueStr));
}
} catch (final NumberFormatException e) {
throw ArgumentException.createEx(ErrorCode.PARAM_BAD, "The value cannot be parsed as "
+ this.type.getSimpleName().toLowerCase() + ".", this.name);
}
return this.parseOther(valueStr);
}
/**
* Field access for {@link #name}.
*
* @return the value of {@link #name}.
*/
@Override
public String getName() {
return this.name;
}
/**
* Field access for {@link #type}.
*
* @return the value of {@link #type}.
*/
@Override
public Class<T> getType() {
return this.type;
}
/**
* {@inheritDoc}
*/
@Override
public final String toString(final Object value) throws InternalServiceException {
if (false == this.type.isAssignableFrom(value.getClass())) {
throw new InternalServiceException(ErrorCode.PANIC,
"Parameter.toString(): Bad type of value. Expected {0} but is {1}.", this.type.getName(),
value.getClass().getName());
}
if (String.class == this.type || Integer.class == this.type || Long.class == this.type) {
return String.valueOf(value);
}
if (Boolean.class == this.type) {
return Boolean.TRUE.equals(value) ? "1" : "0";
}
return this.toStringOther(value);
}
/**
* Parse parameter values of other (non standard types). This method is called by
* {@link #parse(String)} in case {@link #type} is none of the supported standard types (currently
* String, Boolean, Integer and Long). It is intended for extensions.
* <dl>
* <dt style="margin-top:0.25cm;"><i>Note:</i>
* <dd>This default implementation always throws an InternalServiceException.
* </dl>
*
* @param valueStr <i>mandatory</i> - the string value to parse the value from
* @return the parsed value, may be <code>null</code>
* @throws ArgumentException in case the parameter cannot be parsed according to {@link #type}
* @throws InternalServiceException in case the type {@link #type} cannot be handled. This is a
* programming error.
*/
protected T parseOther(final String valueStr) throws ArgumentException, InternalServiceException {
throw new InternalServiceException(ErrorCode.PANIC,
"ParameterImpl.parseOther(): Unsupported parameter type: " + this.type.getName());
}
/**
* Convert the values of other (non standard types) to their external form. This method is called
* by {@link #toString(Object)} in case {@link #type} is none of the supported standard types
* (currently String, Boolean, Integer and Long). It is intended for extensions.
* <dl>
* <dt style="margin-top:0.25cm;"><i>Note:</i>
* <dd>This default implementation always throws an InternalServiceException.
* </dl>
*
* @param value <i>mandatory</i> - the parameters value
* @return the external form of the parameters value, never <code>null</code>
* @throws InternalServiceException in case the given <code>value</code> does not match
* {@link #getClass()}
*/
protected String toStringOther(final Object value) throws InternalServiceException {
throw new InternalServiceException(ErrorCode.PANIC,
"ParameterImpl.toStringOther(): Unsupported parameter type: " + this.type.getName());
}
}
不，在Java中不能这样做。除此之外，d可能是A的一个实例(考虑到“extends”的正常思想)，但只知道A的用户不会知道它——这就违背了枚举是一组已知值的观点。
如果你能告诉我们更多关于你想如何使用它，我们可能会建议其他解决方案。
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。
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