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
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。
其他回答
我建议你采取另一种方式。
与其扩展现有的枚举，不如创建一个更大的枚举，并创建它的一个子集。 例如，如果你有一个叫PET的枚举，你想把它扩展到ANIMAL，你应该这样做:
public enum ANIMAL {
WOLF,CAT, DOG
}
EnumSet<ANIMAL> pets = EnumSet.of(ANIMAL.CAT, ANIMAL.DOG);
注意，pets不是一个不可变的集合，您可能会使用Guava或Java9来提高安全性。
我自己也有同样的问题，我想把我的观点发表出来。我认为这样做有几个激励因素:
您希望有一些相关的枚举代码，但在不同的类中。在我的例子中，我有一个基类，在一个相关的枚举中定义了几个代码。在以后的某一天(今天!)我想为基类提供一些新功能，这也意味着枚举的新代码。 派生类既支持基类的枚举，也支持它自己的枚举。没有重复的enum值!如何为子类创建一个枚举，包括父类的枚举及其新值。
使用接口并不能真正解决问题:您可能会意外地获得重复的enum值。不可取的。
我最终只是组合了枚举:这确保了不会有任何重复的值，但代价是与相关类的绑定不那么紧密。但是，我认为重复的问题是我主要担心的……
这是一种方法，我发现如何扩展一个枚举到其他枚举，是一个非常直接的方法:
假设你有一个包含公共常量的枚举:
public interface ICommonInterface {
String getName();
}
public enum CommonEnum implements ICommonInterface {
P_EDITABLE("editable"),
P_ACTIVE("active"),
P_ID("id");
private final String name;
EnumCriteriaComun(String name) {
name= name;
}
@Override
public String getName() {
return this.name;
}
}
然后你可以尝试这样做一个手动扩展:
public enum SubEnum implements ICommonInterface {
P_EDITABLE(CommonEnum.P_EDITABLE ),
P_ACTIVE(CommonEnum.P_ACTIVE),
P_ID(CommonEnum.P_ID),
P_NEW_CONSTANT("new_constant");
private final String name;
EnumCriteriaComun(CommonEnum commonEnum) {
name= commonEnum.name;
}
EnumCriteriaComun(String name) {
name= name;
}
@Override
public String getName() {
return this.name;
}
}
当然，每次你需要扩展一个常量时，你都必须修改你的SubEnum文件。
实际上，ENUM只是编译器生成的一个常规类。生成的类扩展了java.lang.Enum。不能扩展生成的类的技术原因是生成的类是最终的。在这个主题中讨论了它是最终的概念原因。但我将在讨论中加入机制。
下面是一个测试枚举:
public enum TEST {
ONE, TWO, THREE;
}
从javap得到的代码:
public final class TEST extends java.lang.Enum<TEST> {
public static final TEST ONE;
public static final TEST TWO;
public static final TEST THREE;
static {};
public static TEST[] values();
public static TEST valueOf(java.lang.String);
}
可以想象，您可以自己键入这个类，并删除“final”。但是编译器阻止你扩展“java.lang”。直接枚举”。你可以决定不扩展java.lang。Enum，但是这样你的类和它的派生类就不会是java.lang.Enum…这对你来说可能并不重要!
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());
}
}
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