Conveniently map between enum and int String

Question

When working with variables parameters that can only take a finite number of values  I try to always use Java s enum  as in   public enum BonusType     MONTHLY  YEARLY  ONE OFF     As long as I stay inside my code  that works fine  However  I often need to interface with other code that uses plain int  or String  values for the same purpose  or I need to read write from to a database where the data is stored as a number or string   In that case  I d like to have a convenient way to associate each enum value with a an integer  such that I can convert both ways  in other words  I need a  reversible enum     Going from enum to int is easy   public enum BonusType     public final int id     BonusType int id        this id   id        MONTHLY 1   YEARLY 2   ONE OFF 3       Then I can access the int value as BonusType x   MONTHLY  int id   x id    However  I can see no nice way for the reverse  i e  going from int to enum  Ideally  something like  BonusType bt   BonusType getById 2      The only solutions I could come up with are    Put a lookup method into the enum  which uses BonusType values   to fill a map  int -  enum   then caches that and uses it for lookups  Would work  but I d have to copy this method identically into each enum I use  -   Put the lookup method into a static utility class  Then I d only need one  lookup  method  but I d have to fiddle with reflection to get it to work for an arbitrary enum    Both methods seem terribly awkward for such a simple     problem    Any other ideas insights

User · Answer

Both the  ordinal   and values   i  are unstable since they are dependent to the order of enums  Thus if you change the order of enums or add delete some your program would break    Here is a simple yet effective method to map between enum and int    public enum Action       ROTATE RIGHT 0   ROTATE LEFT 1   RIGHT 2   LEFT 3   UP 4   DOWN 5        public final int id      Action int id            this id   id             public static Action get int id           for  Action a  Action values                  if  a id    id                  return a                    throw new IllegalArgumentException  Invalid id              Applying it for strings shouldn t be difficult

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Really great question  -  I used solution similar to Mr Ferguson s sometime ago  Our decompiled enum looks like this   final class BonusType extends Enum        private BonusType String s  int i  int id                super s  i           this id   id             public static BonusType   values                 BonusType abonustype            int i          BonusType abonustype1            System arraycopy abonustype   ENUM VALUES  0  abonustype1   new BonusType i   abonustype length   0  i           return abonustype1             public static BonusType valueOf String s                return  BonusType Enum valueOf BonusType  s              public static final BonusType MONTHLY      public static final BonusType YEARLY      public static final BonusType ONE OFF      public final int id      private static final BonusType ENUM VALUES         static                MONTHLY   new BonusType  MONTHLY   0  1           YEARLY   new BonusType  YEARLY   1  2           ONE OFF   new BonusType  ONE OFF   2  3           ENUM VALUES    new BonusType                 MONTHLY  YEARLY  ONE OFF                       Seeing this is apparent why ordinal   is unstable  It is the i in super s  i    I m also pessimistic that you can think of a more elegant solution than these you already enumerated  After all enums are classes as any final classes

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Int -- gt String    public enum Country        US  US  0       UK  UK  2       DE  DE  1         private static Map lt Integer  String gt  domainToCountryMapping       private String country      private int domain       private Country String country int domain           this country country toUpperCase            this domain domain             public String getCountry            return country              public static String getCountry String domain            if  domainToCountryMapping    null                initMapping                       if domainToCountryMapping get domain   null               return domainToCountryMapping get domain            else              return  US                          private static void initMapping              domainToCountryMapping   new HashMap lt Integer  String gt                 for  Country s   values                      domainToCountryMapping put s domain  s country

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In this code  for permanent and intense  search   have  memory or process for use  and I select memory  with converter array as index   I hope it s helpful  public enum Test   VALUE ONE 101   Im value one    VALUE TWO 215   Im value two    private final int number  private final byte   desc   private final static int   converter   new int 216   static      Test   st   values        for int i 0 i lt st length i             cv st i  number  i           Test int value  byte   description        this number   value      this desc   description       public int value         return this number    public byte   description        return this desc     public static String description int value        return values   converter rps   desc     public static Test fromValue int value   return values   converter rps

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For the sake of completeness  here is a generic approach to retrieve enum values by index from any enum type  My intention was to make the method look and feel like Enum valueOf Class  String   Fyi  i copied this method from here   Index related issues  already discussed in depth here  still apply          Returns the   link Enum  instance for a given ordinal     This method is the index based alternative    to   link Enum valueOf Class  String    which    requires the name of an instance          param  lt E gt  the enum type     param type the enum class object     param ordinal the index of the enum instance     throws IndexOutOfBoundsException if ordinal  lt  0    ordinal  gt   enums length     return the enum instance with the given ordinal     public static  lt E extends Enum lt E gt  gt  E valueOf Class lt E gt  type  int ordinal        Preconditions checkNotNull type   Type        final E   enums   type getEnumConstants        Preconditions checkElementIndex ordinal  enums length   ordinal        return enums ordinal

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org apache commons lang enums ValuedEnum   To save me writing loads of boilerplate code or duplicating code for each Enum  I used Apache Commons Lang s ValuedEnum instead   Definition   public class NRPEPacketType extends ValuedEnum           public static final NRPEPacketType TYPE QUERY   new NRPEPacketType   TYPE QUERY   1       public static final NRPEPacketType TYPE RESPONSE   new NRPEPacketType   TYPE RESPONSE   2        protected NRPEPacketType String name  int value            super name  value             Usage   int -  ValuedEnum   NRPEPacketType packetType      NRPEPacketType  EnumUtils getEnum NRPEPacketType class  1

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If you have a class Car  public class Car       private Color externalColor      And the property Color is a class   Data public class Color       private Integer id      private String name      And you want to convert Color to an Enum  public class CarDTO       private ColorEnum externalColor      Simply add a method in Color class to convert Color in ColorEnum   Data public class Color       private Integer id      private String name       public ColorEnum getEnum            ColorEnum getById id             and inside ColorEnum implements the method getById    public enum ColorEnum           public static ColorEnum getById int id            for ColorEnum e   values                  if e id  id                   return e                      Now you can use a classMap  private MapperFactory factory   new DefaultMapperFactory Builder   build        factory classMap Car class  CarDTO class       fieldAToB  externalColor enum   externalColor        byDefault        register        CarDTO dto   mapper map car  CarDTO class

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given   public enum BonusType     MONTHLY 0   YEARLY 1   ONE OFF 2     BonusType bonus   YEARLY   System out println bonus Ordinal           bonus   Output  1 YEARLY

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Seems the answer s  to this question are outdated with the release of Java 8    Don t use ordinal as ordinal is unstable if persisted outside the JVM such as a database  It is relatively easy to create a static map with the key values      public enum AccessLevel     PRIVATE  private   0     PUBLIC  public   1     DEFAULT  default   2      AccessLevel final String name  final int value        this name   name      this value   value         private final String name    private final int value     public String getName         return name         public int getValue         return value         static final Map lt String  AccessLevel gt  names   Arrays stream AccessLevel values           collect Collectors toMap AccessLevel  getName  Function identity        static final Map lt Integer  AccessLevel gt  values   Arrays stream AccessLevel values           collect Collectors toMap AccessLevel  getValue  Function identity         public static AccessLevel fromName final String name        return names get name          public static AccessLevel fromValue final int value        return values get value

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A very clean usage example of reverse Enum   Step 1 Define an interface EnumConverter  public interface EnumConverter  lt E extends Enum lt E gt   amp  EnumConverter lt E gt  gt        public String convert        E convert String pKey       Step 2  Create a class name ReverseEnumMap  import java util HashMap  import java util Map   public class ReverseEnumMap lt V extends Enum lt V gt   amp  EnumConverter lt V gt  gt        private Map lt String  V gt  map   new HashMap lt String  V gt          public ReverseEnumMap Class lt V gt  valueType            for  V v   valueType getEnumConstants                  map put v convert    v                        public V get String pKey            return map get pKey             Step 3  Go to you Enum class and implement it with EnumConverter lt ContentType gt  and of course override interface methods  You also need to initialize a static ReverseEnumMap   public enum ContentType implements EnumConverter lt ContentType gt        VIDEO  Video    GAME  Game    TEST  Test    IMAGE  Image         private static ReverseEnumMap lt ContentType gt  map   new ReverseEnumMap lt ContentType gt  ContentType class        private final String mName       ContentType String pName            this mName   pName             String value             return this mName              Override     public String convert             return this mName              Override     public ContentType convert String pKey            return map get pKey             Step 4  Now create a Communication class file and call it s new method to convert an Enum to String and String to Enum  I have just put main method for explanation purpose    public class Communication lt E extends Enum lt E gt   amp  EnumConverter lt E gt  gt        private final E enumSample       public Communication E enumSample            this enumSample   enumSample             public String resolveEnumToStringValue E e            return e convert               public E resolveStringEnumConstant String pName            return enumSample convert pName            Should not put main method here    just for explanation purpose       public static void main String    are            Communication lt ContentType gt  comm   new Communication lt ContentType gt  ContentType GAME           comm resolveEnumToStringValue ContentType GAME     return Game         comm resolveStringEnumConstant  Game      return GAME  Enum            Click for for complete explanation

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http   www javaspecialists co za archive Issue113 html  The solution starts out similar to yours with an int value as part of the enum definition   He then goes on to create a generics-based lookup utility   public class ReverseEnumMap lt V extends Enum lt V gt   amp  EnumConverter gt        private Map lt Byte  V gt  map   new HashMap lt Byte  V gt         public ReverseEnumMap Class lt V gt  valueType            for  V v   valueType getEnumConstants                  map put v convert    v                        public V get byte num            return map get num             This solution is nice and doesn t require  fiddling with reflection  because it s based on the fact that all enum types implicitly inherit the Enum interface

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I m not sure if it s the same in Java  but enum types in C are automatically mapped to integers as well so you can use either the type or integer to access it  Have you tried simply accessing it with integer yet

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I needed something different because I wanted to use a generic approach  I m reading the enum to and from byte arrays  This is where I come up with   public interface EnumConverter       public Number convert         public class ByteArrayConverter    SuppressWarnings  unchecked   public static Enum lt   gt  convertToEnum byte   values  Class lt   gt  fieldType  NumberSystem numberSystem  throws InvalidDataException       if  values    null    values length    0            final String message    The values parameter must contain the value           throw new IllegalArgumentException message              if   dtoFieldType isEnum              final String message    dtoFieldType must be an Enum            throw new IllegalArgumentException message              if   EnumConverter class isAssignableFrom fieldType             final String message    fieldType must implement the EnumConverter interface            throw new IllegalArgumentException message              Enum lt   gt  result   null      Integer enumValue    Integer  convertToType values  Integer class  numberSystem      Our enum s use Integer or Byte for the value field       for  Object enumConstant   fieldType getEnumConstants              Number ev     EnumConverter  enumConstant  convert             if  enumValue equals ev                 result    Enum lt   gt   enumConstant              break                       if  result    null            throw new EnumConstantNotPresentException  Class lt   extends Enum gt   fieldType  enumValue toString                return result     public static byte   convertEnumToBytes Enum lt   gt  value  int requiredLength  NumberSystem numberSystem  throws InvalidDataException       if    value instanceof EnumConverter             final String message    dtoFieldType must implement the EnumConverter interface            throw new IllegalArgumentException message              Number enumValue     EnumConverter  value  convert        byte   result   convertToBytes enumValue  requiredLength  numberSystem       return result     public static Object convertToType byte   values  Class lt   gt  type  NumberSystem numberSystem  throws InvalidDataException          some logic to convert the byte array supplied by the values param to an Object     public static byte   convertToBytes Object value  int requiredLength  NumberSystem numberSystem  throws InvalidDataException          some logic to convert the Object supplied by the value  param to a byte array        Example of enum s     public enum EnumIntegerMock implements EnumConverter       VALUE0 0   VALUE1 1   VALUE2 2        private final int value       private EnumIntegerMock int value            this value   value         public Integer convert         return value         public enum EnumByteMock implements EnumConverter       VALUE0 0   VALUE1 1   VALUE2 2        private final byte value       private EnumByteMock int value            this value    byte  value             public Byte convert             return value

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Use an interface to show it who s boss   public interface SleskeEnum       int id         SleskeEnum   getValues        public enum BonusType implements SleskeEnum       MONTHLY 1   YEARLY 2   ONE OFF 3      public final int id     BonusType int id        this id   id         public SleskeEnum   getValues         return values           public int id     return id         public class Utils      public static SleskeEnum getById SleskeEnum type  int id          for SleskeEnum t   type getValues              if t id      id  return t        throw new IllegalArgumentException  BonusType does not accept id     id          public static void main String   args           BonusType shouldBeMonthly    BonusType getById BonusType MONTHLY 1         System out println shouldBeMonthly    BonusType MONTHLY          BonusType shouldBeMonthly2    BonusType getById BonusType MONTHLY 1         System out println shouldBeMonthly2    BonusType YEARLY          BonusType shouldBeYearly    BonusType getById BonusType MONTHLY 2         System out println shouldBeYearly     BonusType YEARLY          BonusType shouldBeOneOff    BonusType getById BonusType MONTHLY 3         System out println shouldBeOneOff    BonusType ONE OFF          BonusType shouldException    BonusType getById BonusType MONTHLY 4           And the result   C  Documents and Settings user My Documents gt java Utils true false true true Exception in thread  main  java lang IllegalArgumentException  BonusType does not accept id 4         at Utils getById Utils java 6          at Utils main Utils java 23   C  Documents and Settings user My Documents gt

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You could perhaps use something like  interface EnumWithId       public int getId         enum Foo implements EnumWithId               That would reduce the need for reflection in your utility class

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I found this on the web  it was very helpful and simple to implement  This solution was NOT made by me  http   www ajaxonomy com 2007 java making-the-most-of-java-50-enum-tricks  public enum Status    WAITING 0    READY 1    SKIPPED -1    COMPLETED 5     private static final Map lt Integer Status gt  lookup          new HashMap lt Integer Status gt       static         for Status s   EnumSet allOf Status class              lookup put s getCode    s        private int code    private Status int code          this code   code       public int getCode     return code      public static Status get int code           return lookup get code

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Just because the accepted answer is not self contained   Support code   public interface EnumWithCode lt E extends Enum lt E gt   amp  EnumWithCode lt E gt  gt         public Integer getCode         E fromCode Integer code       public class EnumWithCodeMap lt V extends Enum lt V gt   amp  EnumWithCode lt V gt  gt         private final HashMap lt Integer  V gt   map   new HashMap lt Integer  V gt          public EnumWithCodeMap Class lt V gt  valueType            for  V v   valueType getEnumConstants                  map put v getCode    v              public V get Integer num            return  map get num             Example of use   public enum State implements EnumWithCode lt State gt        NOT STARTED 0   STARTED 1   ENDED 2        private static final EnumWithCodeMap lt State gt  map   new EnumWithCodeMap lt State gt               State class        private final int code       private State int code            this code   code              Override     public Integer getCode             return code              Override     public State fromCode Integer code            return map get code

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enum   int  yourEnum ordinal       int   enum  EnumType values   someInt      String   enum  EnumType valueOf yourString      enum   String  yourEnum name       A side-note As you correctly point out  the ordinal   may be  unstable  from version to version  This is the exact reason why I always store constants as strings in my databases   Actually  when using MySql  I store them as MySql enums

[java] Conveniently map between enum and int / String

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