Convert from enum ordinal to enum type

Question

I ve the enum type ReportTypeEnum that get passed between methods in all my classes but I then need to pass this on the URL so I use the ordinal method to get the int value  After I get it in my other JSP page  I need to convert it to back to an ReportTypeEnum so that I can continue passing it    How can I convert ordinal to the ReportTypeEnum   Using Java 6 SE

User · Answer

This is almost certainly a bad idea  Certainly if the ordinal is de-facto persisted  e g  because someone has bookmarked the URL  - it means that you must always preserve the enum ordering in future  which may not be obvious to code maintainers down the line   Why not encode the enum using myEnumValue name    and decode via ReportTypeEnum valueOf s   instead

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This is what I do on Android with Proguard   public enum SomeStatus       UNINITIALIZED  STATUS 1  RESERVED 1  STATUS 2  RESERVED 2  STATUS 3   do not change order      private static SomeStatus   values   null      public static SomeStatus fromInteger int i            if SomeStatus values    null                SomeStatus values   SomeStatus values                      if  i  lt  0  return SomeStatus values 0           if  i  gt   SomeStatus values length  return SomeStatus values 0           return SomeStatus values i             it s short and I don t need to worry about having an exception in Proguard

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Safety first  with Kotlin      Default to null EnumName values   getOrNull ordinal      Default to a value EnumName values   getOrElse ordinal    EnumName MyValue

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You can define a simple method like   public enum Alphabet      A B C D       public static Alphabet get int index           return Alphabet values   index             And use it like   System out println Alphabet get 2

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If I m going to be using values   a lot  enum Suit      Hearts  Diamonds  Spades  Clubs     public static final Suit values     values       Meanwhile wherever java  Suit suit   Suit values ordinal    If you want the array to be private  be my guest  private static final Suit values     values    public static Suit get int ordinal    return values ordinal           Suit suit   Suit get ordinal    Mind your array bounds

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public enum Suit implements java io Serializable  Comparable lt Suit gt     spades  hearts  diamonds  clubs    private static final Suit    lookup    Suit values      public Suit fromOrdinal int ordinal        if ordinal lt  1    ordinal gt  3  return null      return lookup value-1           the test class  public class MainTest       public static void main String   args            Suit d3   Suit diamonds          Suit d3Test   Suit fromOrdinal 2           if d3 equals d3Test                System out println  Susses             else System out println  Fails              I appreciate that you share with us if you have a more efficient code  My enum is huge and constantly called thousands of times

User · Answer

To convert an ordinal into its enum represantation you might want to do this   ReportTypeEnum value   ReportTypeEnum values   ordinal     Please notice the array bounds                       Note that every call to values   returns a newly cloned array which might impact performance in a negative way  You may want to cache the array if it s going to be called often    Code example on how to cache values       This answer was edited to include the feedback given inside the comments

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You could use a static lookup table   public enum Suit     spades  hearts  diamonds  clubs     private static final Map lt Integer  Suit gt  lookup   new HashMap lt Integer  Suit gt        static      int ordinal   0      for  Suit suit   EnumSet allOf Suit class           lookup put ordinal  suit         ordinal   1               public Suit fromOrdinal int ordinal        return lookup get ordinal

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This is what I use  I make no pretense that it s far less  efficient  than the simpler solutions above  What it does do is provide a much clearer exception message than  ArrayIndexOutOfBounds  when an invalid ordinal value is used in the solution above   It utilizes the fact that EnumSet javadoc specifies the iterator returns elements in their natural order  There s an assert if that s not correct   The JUnit4 Test demonstrates how it s used           convert ordinal to Enum     param clzz may not be null     param ordinal     return e with e ordinal       ordinal     throws IllegalArgumentException if ordinal out of range     public static  lt E extends Enum lt E gt   gt  E lookupEnum Class lt E gt  clzz  int ordinal        EnumSet lt E gt  set   EnumSet allOf clzz       if  ordinal  lt  set size              Iterator lt E gt  iter   set iterator            for  int i   0  i  lt  ordinal  i                  iter next                      E rval   iter next            assert rval ordinal      ordinal           return rval            throw new IllegalArgumentException  Invalid value     ordinal     for     clzz getName         must be  lt      set size         Test public void lookupTest          java util concurrent TimeUnit tu   lookupEnum TimeUnit class  3       System out println tu

User · Answer

So one way is to doExampleEnum valueOfOrdinal   ExampleEnum values   ordinal   which works and its easy  however  as mentioned before  ExampleEnum values   returns a new cloned array for every call  That can be unnecessarily expensive  We can solve that by caching the array like so ExampleEnum   values   values    It is also  dangerous  to allow our cached array to be modified  Someone could write ExampleEnum values 0    ExampleEnum type2  So I would make it private with an accessor method that does not do extra copying   private enum ExampleEnum      type0  type1  type2  type3      private static final ExampleEnum   values   values        public static ExampleEnum value int ord            return values ord             You would use ExampleEnum value ordinal  to get the enum value associated with ordinal

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Every enum has name    which gives a string with the name of enum member   Given enum Suit Heart  Spade  Club  Diamond   Suit Heart name   will give Heart   Every enum has a valueOf   method  which takes an enum type and a string  to perform the reverse operation   Enum valueOf Suit class   Heart   returns Suit Heart   Why anyone would use ordinals is beyond me  It may be nanoseconds faster  but it is not safe  if the enum members change  as another developer may not be aware some code is relying on ordinal values  especially in the JSP page cited in the question  network and database overhead completely dominates the time  not using an integer over a string

User · Answer

I agree with most people that using ordinal is probably a bad idea   I usually solve this problem by giving the enum a private constructor that can take for example a DB value then create a static fromDbValue function similar to the one in Jan s answer   public enum ReportTypeEnum       R1 1       R2 2       R3 3       R4 4       R5 5       R6 6       R7 7       R8 8        private static Logger log   LoggerFactory getLogger ReportEnumType class         private static Map lt Integer  ReportTypeEnum gt  lookup      private Integer dbValue       private ReportTypeEnum Integer dbValue            this dbValue   dbValue              static           try               ReportTypeEnum   vals   ReportTypeEnum values                lookup   new HashMap lt Integer  ReportTypeEnum gt  vals length                for  ReportTypeEnum  rpt  vals                  lookup put rpt getDbValue    rpt                       catch  Exception e                    Careful  if any exception is thrown out of a static block  the class                 won t be initialized              log error  Unexpected exception initializing     ReportTypeEnum class  e                         public static ReportTypeEnum fromDbValue Integer dbValue            return lookup get dbValue              public Integer getDbValue             return this dbValue             Now you can change the order without changing the lookup and vice versa

[java] Convert from enum ordinal to enum type

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