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。

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 ;
  }

}

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());
  }
}

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。

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

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。