Best practice to look up Java Enum

Question

We have a REST API where clients can supply parameters representing values defined on the server in Java Enums   So we can provide a descriptive error  we add this lookup method to each Enum   Seems like we re just copying code  bad    Is there a better practice     public enum MyEnum       A  B  C  D       public static MyEnum lookup String id            try               return MyEnum valueOf id             catch  IllegalArgumentException e                throw new RuntimeException  Invalid value for my enum blah blah      id                       Update  The default error message provided by valueOf     would be No enum const class a b c MyEnum BadValue   I would like to provide a more descriptive error from the API

User · Answer

You can use a static lookup map to avoid the exception and return a null  then throw as you d like   public enum Mammal       COW      MOUSE      OPOSSUM       private static Map lt String  Mammal gt  lookup                Arrays stream values                       collect Collectors toMap Enum  name  Function identity           public static Mammal getByName String name            return lookup get name

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We do all our enums like this when it comes to Rest Json etc  It has the advantage that the error is human readable and also gives you the accepted value list  We are using a custom method MyEnum fromString instead of MyEnum valueOf  hope it helps   public enum MyEnum        A  B  C  D       private static final Map lt String  MyEnum gt  NAME MAP   Stream of values                 collect Collectors toMap MyEnum  toString  Function identity           public static MyEnum fromString final String name            MyEnum myEnum   NAME MAP get name           if  null    myEnum                throw new IllegalArgumentException String format    s  has no corresponding value  Accepted values   s   name  Arrays asList values                         return myEnum            so for example if you call  MyEnum value   MyEnum fromString  X      you ll get an IllegalArgumentException with the following message       X  has no corresponding value  Accepted values   A  B  C  D    you can change the IllegalArgumentException to a custom one

User · Answer

The error message in IllegalArgumentException is already descriptive enough     Your method makes a generic exception out of a specific one with the same message simply reworded   A developer would prefer the specific exception type and can handle the case appropriately instead of trying to handle RuntimeException   If the intent is to make the message more user friendly  then references to values of enums is irrelevant to them anyway   Let the UI code determine what should be displayed to the user  and the UI developer would be better off with the IllegalArgumentException

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Apache Commons Lang 3 contais the class EnumUtils  If you aren t using Apache Commons in your projects  you re doing it wrong  You are reinventing the wheel   There s a dozen of cool methods that we could use without throws an Exception  For example      Gets the enum for the class  returning null if not found       This method differs from Enum valueOf in that it does not throw an   exceptionfor an invalid enum name and performs case insensitive   matching of the name    EnumUtils getEnumIgnoreCase SeasonEnum class  season

User · Answer

Probably you can implement generic static lookup method   Like so  public class LookupUtil      public static  lt E extends Enum lt E gt  gt  E lookup Class lt E gt  e  String id             try                      E result   Enum valueOf e  id           catch  IllegalArgumentException e                log error or something here           throw new RuntimeException              Invalid value for enum     e getSimpleName            id                  return result           Then you can  public enum MyEnum      static public MyEnum lookup String id           return LookupUtil lookup MyEnum class  id            or call explicitly utility class lookup method

User · Answer

If you want the lookup to be case insensitive you can loop through the values making it a little more friendly    public enum MyEnum      A  B  C  D         public static MyEnum lookup String id            boolean found   false          for MyEnum enum  values                if enum toString   equalsIgnoreCase id   found   true                      if  found  throw new RuntimeException  Invalid value for my enum     id

User · Answer

update  As GreenTurtle correctly remarked  the following is wrong    I would just write  boolean result   Arrays asList FooEnum values    contains  Foo      This is possibly less performant than catching a runtime exception  but makes for much cleaner code  Catching such exceptions is always a bad idea  since it is prone to misdiagnosis  What happens when the retrieval of the compared value itself causes an IllegalArgumentException   This would then be treaten like a non matching value for the enumerator

User · Answer

Guava also provides such function which will return an Optional if an enum cannot be found    Enums getIfPresent MyEnum class  id  toJavaUtil                orElseThrow   - gt  new RuntimeException  Invalid enum blah blah blah

User · Answer

Why do we have to write that 5 line code    public class EnumTest   public enum MyEnum       A  B  C  D      Test public void test   throws Exception       MyEnum valueOf  A      gives you A       this throws ILlegalargument without having to do any lookup     MyEnum valueOf  RADD

User · Answer

Looks like you have a bad practice here but not where you think   Catching an IllegalArgumentException to rethrow another RuntimeException with a clearer message might look like a good idea but it is not  Because it means you care about messages in your exceptions   If you care about messages in your exceptions  then it means that your user is somehow seeing your exceptions  This is bad   If you want to provide an explicit error message to your user  you should check the validity of the enum value when parsing user input and send the appropriate error message in the response if user input is incorrect   Something like      This code uses pure fantasy  you are warned  class MyApi          Return the 24-hour from a 12-hour and AM PM      void getHour24 Request request  Response response                   validate user input         int nTime12   1          try                       nTime12   Integer parseInt request getParam  hour12                 if  nTime12  lt   0    nTime12  gt  12                                 throw new NumberFormatException                                    catch  NumberFormatException e                         response setCode 400      Bad request             response setContent  time12 must be an integer between 1 and 12                return                     AMPM pm   null          try                       pm   AMPM lookup request getParam  pm                       catch  IllegalArgumentException e                         response setCode 400      Bad request             response setContent  pm must be one of     AMPM values                 return                     response setCode 200           switch  pm                         case AM                  response setContent nTime12                   break              case PM                  response setContent nTime12   12                   break                    return

[java] Best practice to look up Java Enum

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